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WORKS TRANSLATED BY 
WILLIAM T. HALL 


PUBLISHED BY 
JOHN WILEY & SONS, Inc. 


F, P. TREADWELL’S ANALYTICAL CHEMISTRY 
In Two VoLuMEs. 


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ANALYTICAL CHEMISTRY. 


BY 


F. P. TREADWELL, -Px.D., 
Professor of Analytical Chemistry in the Polytechnic Institute of Zurich. 


AUTHORIZED TRANSLATION FROM THE GERMAN 
BY 


WILLIAM T. HALL, S.B., 
Assistant Professor of Chemistry, Massachusetts Institute of Technology. 


Votume II. 
QUANTITATIVE ANALYSIS. 


THIRD EDITION, REVISED AND ENLARGED 
TOTAL ISSUE, TEN THOUSAND 


NEW YORK (3.00220. 

JOHN WILEY & SONS, Inc. . 

Lonpon: CHAPMAN & HALL, - Liwrep 
1914 















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TRANSLATOR’S NOTE. 





Tus translation has been made from the second German 
edition, but Professor Treadwell has kindly indicated quite a 
number of changes which he intends to make in the third edition. 
Since it has been my aim not so much to prepare an exact literal 
translation as to publish a book which will be useful to English- 
speaking students, I am under great‘obligations to several of my 
friends and colleagues for suggesting certain other changes. That 
part of the proof relating to Gravimetric Analysis has been care- 
fully read and criticised by Professor Henry Fay, that relating 
to Volumetric Analysis by Professor F. Jewett Moore, and Pro- 
fessor Augustus H. Gill has twice read the chapter on Gas Analysis. 
I have also received valuable assistance in reading the proof from 
Messrs. R. S. Williams, F. R. Kneeland and J. R. Odell, all of the 
Massachusetts Institute of Technology. I am indebted to Mr. 
A. R. Jackson, of Winthrop, for several drawings. 

! WiuuraM T.. Hat. 


MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 
April, 1904. 





In preparing the second edition, the text has been compared 
with the fourth German edition, and certain additions have 
been made which are not found in the German text. 

Weds 
March, 1910. 





The third edition has been compared with the fifth German 
edition. In the preparation of dilute acids the volume of con- 
centrated acid is always referred to the volume of water added. 
Thus sulphuric acid (1:4) signifies one volume of concentrated 
acid diluted with four volumes of water. With solutions of 
solids, the first number signifies the weight in grams of the 
solid and the second the volume of the.solvent in cubic centi- 


meters. — We TE HE 
June, 1911. “ 


YCON7%A4 





‘TABLE OF CONTENTS. 





INTRODUCTION. 

; PAGE 
Gravimetric and Volumetric Analysis. ............cc ce ceee ee eeee ees 1 
RG MAES STOOL ANOIVNOR or ee iia ea eal Os lee ok, 6 ed eS eh 2 
MIME fare ee ho oER pia e Sav ee de titlars AM Clae Ho bale CER oN ERS 6 
menueuon of Wenig to Vacue oo oe ek es PA HO. LE Oe 13 
I AE WOMAN 0k a ua pag weg PME Daf Flgebey cS er ae ale «ae Gulbis 15 
Filtration and Washing of Precipitates................ 0.00.02. ee eee 18 
Dryme and Igniting of Precipitates. 2... 0... 2 oe eee ee 21 
MrMnaLAY C CMI nce an ds os tie ise) Gye v eesti J aces Wes 30 
Drying Substances in Currents of Gases. ... 2.1.0.0... cece eee e eee 33 
Preparation of the Substance for Analysis. ................2.. 020005 35 
ME NCMIUMAUAOIE 5 55 oy p Sous stieeceht neice ERE RECUR USS REL e 8 Vine kes 35 

PART. I: 
GRAVIMETRIC DETERMINATION OF THE METALS. 
Group V (ALKALIEs). 
NETI e ort aie og Betyg x cred y Aes a ORs Vales Oe 38 
NNN ge ih a renal as hs ec hehe ioe eeoe Bo Ep hs aie ean Oe ee Bee 43 
Separation of Potassium from Sodium................ 05.0... 0.00 ee 43, 50 
NR ee a Maya sige lavas « nc siege obi He OME See Oe ea ee te PR _ 53 
Determination of Lithium, Potassium, and Sodium.................... 53 
NN a ge i a ey ss aly eaves os dw wine CEO Rade ve ete 57 
ea hi alate hee on vigk DREN Ok HA Ge SS ei ck ee oe ¥en 65 
Separation of Magnesium from the Alkalies.............00e cee eeees «68 
Group IV (ALKALINE EARTHS). 

NN eae ig NE Wele ti Me ok Rs ate os BOS Ada ee ee es we 70 
NNN eC he ore ee eg Wer ito Sec GRU Gace, Sim opteachcd «siege 72 
RG ee i ee eeu weeny coi Beet tne ae BM utala ee Fees 74 
Separation of Calcium from Magnesium...............  ....-.++.+. 16 


Vi TABLE OF CONTENTS. 


PAGE 
Separation of Strontium from Magnesium............... Sea ees 78 
Separation of Barium from Magnesium. ......................00c eee 79 
Separation of the Alkaline Earths from One Another................. 79 
Grovp III. 

POUT fo oes 2k Sie eg ie cae Sees ek 82 
BIDE os ie ow oie ote oe nde ncn Wem Jolie a wnt 9 aesol oA Pee em e ee 87 
Thaniiine oo. 8 DAG bel ee ee ee 100 
COROT i occ a ee ee ne I oe wa ORG ee BS 102 
CP ppeaA RR Se a eg ie es wien wivocelp > wie ve ee'e! 0 civ eo 0 peer 106 
Separation of Group III from Group IV........................ 107, 147 
Separation of fron from Aluminium: 3. 2005.03). ee ee 107 
Separation of Iron, Aluminium, and Phosphoric Acid. ................ 111 
Separation of Iron from. Chromium: 30.0505 3. eS Bes 113 
Separation of Aluminium from Chromium........................... 114 
Separation of Iron from. Titanium... 82 a ee 114 
Separation of Aluminium from Titanium.................. aaa, 116 
Separation of Uranium from Iron and Aluminium.................... 119 
WEATIGATIORO os SS aS wes NS Reis ele poeta y Sls noe Se ois eae 120 
WNaokced a5 2s ge cies ese iaiel ee hee etn gto oh EOE ST Petes ee en eae 129 
Gobalt. ix ores vs ae ceca heete ls Gite seen e yee nee ene 138 
DING oss. pee Beaks Nis edo UAe eles eco dee wpa eee a at Same See aoe seca Ree ae ge 140 

Separation of Manganese, Nickel, Cobalt, and Zinc from the Alkaline 
fOr) Demat li ena oemiriseg ata SE” WALL Dea aes atgu Ne MODS aaa 147 

Separation of the Bivalent from the Other Metals of the Ammonium 
Sulphide :Gapupy 6))5 a ea oe Behe oie ee ae aeeae le 149 
Separation of Zine from Nickel, Cobalt, and Manganese.............. 156 
Separation of Manganese from Nickel and Cobalt.................... 161 
Separation of Cobalt, from Nickel... 0) s.. 2.250%... oa eee ees aie 161 
Separation of Nickel from Zine so. oes ts cases eee eae eae ee 165 
Separation of Nickel from Manganese................... Set OE PES 165 
‘Separation of Nickel from Tron. oo 6c es. Sova ae es ae ee 166 
Removal of Ferric Chloride by Ether... .............2 0 cee ceeceeees 167 

Grovp II. 
(a) Sulpho-Bases. 

1 gti See ee er ORCN ES, SON RS ray ores phe Oe, Oh 168 
cL.” *, Rh ieyn Gree enemy INE Mis reer ysi rp Pe RE or mS 174 
PRBOUGR ko oe sa ss so hes tm os a Ree en ee na 179 
CBE 5 oes ain oe don ae 5 2 aie weed mee Oe ee ae Ae ec ae 182 
CeO fe en 8 hk 0 oa og Ne ee eas eee 189 


TABLE OF CONTENTS. Vii 

PAGE 

BGR Ga Eee aoc ec ig CE D e oe SRLIORY ial oe ROSS il EEE PED 193 
Separation of the Sulpho-Bases from One Another.................... 194 


(b) Sulpho-Acids. 


RS RAO GR RC INN Coc GROR MADE ros ee Oe A PRA Te Oa noe eee 205 
NINN i ocr ey ae re Vey eye AUS UN We «ci negle dap hae 218 
i ec ee sg ikige Baia ee cy Eee Bea ete EMR Brel ROT Sa. asin wie 228 
Separation of Arsenic, Antimony, and Tin from Members of the Ammo- 
nium Sulphide Group...... ee Ek ere re wie re aN Wael a a. ete So. O86 
Separation of Arsenic, Antimony, and Tin from Mercury, Lead, Copper, 
SMa MME ESIEMINEN ELE oo se eee cist ak Phe Kia aye pom pbs 235 
IE WIN CROMER a gn Tee cca Ao pia k, alates hhh 0, 00'S wire vieh's 236 
Separation of the Svlpho-Acids from One Another................... 241 
NMR OR AOCMPTINE. UCN 00 R 0 og gene oo whe erotics a ileid a haeues 252 
(ES SRE RRER OSS ESIRESE A GIR Stipe ia ONE ieee ae Be CREO yp Pon a Tc Aen PRR enaee IAS 257 
RE ones ey nes el ele ag atey yiate A eae § siege st athew ere 268 
Sermerntion Of Gold (ron © stimu 2 6 oe iw Sele cao ws gb wee 271 
mea Umi DLC MMCTOMal: © MAGI 625. ohy os ee elds Aated.s aminceieiintia 272 
STIR 5 Sa wie oxy Sh fe Heres ows Ie Nye Opie a sor Sew g are 277 
IE rt MC a ee Be eh STG aac wee me ewe es 279 
Separation of Selenium and Tellurium from the Metals of Groups III, 
ee Ae ne ratte Vea tpi ures has 279 
Separation of Selenium and Tellurium from Metals of Group II....... - 280 
Separation of Selenium from Tellurium..........................0.. 252 © 
MMI oy Corey Oo Ee he! he Sele poids ip a hsniera x dee wa min oe “284 
MRS A ag at era copie poten s weg Care ed cr te tei gcdig Kore War ee re 288 | 
Separation of Molybdenum from Tungsten... .. Sida eh Sol gu ea eae rap 293 
Analysis of Wolframite. <2... 266 eee. st UBS 7 a leah oe hae ON RB 296 
mentee OL RE @STCTL SOMO | 5 be one ee Os a aha Shek tk hale Nice ees 298 
eer ecnon) OF L Glor trOty EM og. i ek eae See eee « 300 
Separation of Tungstic Acid from Silica....................0-.2.00.. 302 
MMRAIE 8 Hees Dee ae oe ee eh le wr Atte eae Sy be ee 303 
Separation of Vanadium from Arsenic Acid......................0-0. 306 
Separation of Vanadium from Phosphoric Acid...................... 307 
Separation of Vanadium from Molybdenum.....................0... 308 
AME V MRR cs es ce es er er Pe, 308 
- Determination of Vanadium and Chromium in Iron Ores and Rocks.... 310 
Determination of Vanadium and Chromium in Pig Iron............... 312 
Determination of Vanadium, Molybdenum, Chromium, and Nickel in 
UO he Re ee eth ELT PEPER EREN ERNE LEO U Coneea eed tes 313 
Grovp I. 


Vill TABLE OF CONTENTS. 


GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Group I.* 
PAGE 
Pi yerouh lores ACG) 65 le ioc sein ea a ete Vike eke sie wha teena see 320 
Aiaslyais Of an Eisoltible Chlorite = si a wep ne pe ee 323 
Pres CMMGFING ok es ero aa he a Le FE ee 324 
Chiorine ‘in Organic Compounds: © sso. oo eek ete Pee 325 
FivGroDromiie AGIs. fo oy ees ee eas bao ne ee RE) aa ane 329 
Hydriodic Acid: e303 Or se ORS eee ees 330 
Separation of the Halogens from One Another....................... 331 
Hydroovanio Acids 345 04 isk tas ee ane eee ee 337 
Determination of Hydrocyanic Acid in the Presence of Halogen Hydride 339 
Sulphoevanie Acia: 9505555 ac scenic aie a ek ess ov a nae a 339 
Determination of Sulphocyanic and Hydrocyanic Acids............... 342 
Determination of Sulphocyanic Acid and Halogen Hydrides........... 342 
Hydroterrocyanic Acid 3 e555... S825). Sia eee bes aes <n ee 342 
Hydroterricyanie Amd 6555 ee a ak ole ie ee ee 344 
Fy pobhloraus: Acid 9595s oo ke ace bee ce oe oe oe eee 344 
Group II. 
Nitrous Acide. ise PAE ae eS ie ee 344 
Hydrosulphuric: Acid, Bigs 556 ks sie eke ek itis oe Ss ke Se ee 347 
Analysis: of ‘Tetrahedrite 0-66 25.705 a she aoe bn a eee 359 
A COMO ARE 2 CER A RES ee 371 
Cryanie AGIs 6 koe it Ee et a eek ae Gs ee 371 
Determination of Cyanic, Hydrocyanic, and Carbonic Acids........... 371 
Hypophosphorous Acid (2S eee es PG aa pe Sep een 372 
Group III. 
Sulphurous Acid 2:66 0s'. soe. so ehh pee ee eo wee 373 
Selenous and Tellurous Acid8.)3) 62 a 5 Gs ie 374 
Phosphorous Acid 2 5 5 Ose eeea ne dees mea ee ae a 374 
Carhomic AGG: ie 2S ES Sees Ue 1 a ee 375 
Determination of Carbon... 2. edited kes cose epee eee ee 398 
Determination of Carbon and Hydrogen in Organic Substances........ 414 
Dumas Method for Determining Nitrogen. ...............:0000eeeuee 422 
Bs CNC Be, Cs 1 Ie Deerrot- crprepet ence renter ear tea hh Pe See Se imc Ls 427 
Borie Hews bie ca thee wie eae ee GR a ho ene a eee 428 
DEAVOO AO. oo ie ESS ee ee Tl a eras Re Sains td eee 433 
PT RPtArIG AGIG 6s iocig SAN oe 80 EA CR ae Se Ee ee 433 
Meta-.and Pyrophosphoric Acids... 65s. peice meee Oe 433 
Tie AGHA 8s fe Ee REIS a A ee re ee 48 





* For the Division of Acids into Groups ef. Vol. I. 


TABLE OF CONTENTS. 1 


Grove IV. 
; PAGE 

eee ae Es Mprea CgpGe | a lt kane MORE NGL aE pa ey a eI Diag or ers ts Lowen ean 434 
Determination of Phosphorus and Silicon in Iron and Steel...... WAS ais 440 
Separation of Phosphoric Acid from the Metals. .................. »... 448 
ONLINE PRE 75s a oh A IA ee ek Ce OD uha a Sicee ae 450 

Group V. : 
Per ie ADH oo, ie Nc eee as Ee Mogg Ving Sala on 3 451 
Roba Ave ee es Ne ue cesta aia ele hae adie otbice ow vaca es 460 
MOVE PON eke ecru CLT 6 ik ee ale CE Le E TEN Cole Oalea een es 462 

Group VI. 
MURS ONE oc s ag oda aca fukk ge emake te Se cage a ae wa ps, CRs 464 
MUOIEIONND PU Se a at cea wo gine Vig RAMON TC CREM Moa com es ges s 471 
Separation of Phosphoric and Hydrofluoric Acids. ................... 474 
Separation of Fluorine from the Metals. ... 2.0.6.0... 0... ce cece eee ee 481 
Separation of Fluorine from the Acids... 2... ck ee cee 482 
PMNS FLUE 1 ails Pie Lien sare v5. ecw ey VAT Cw bie we hee aces 483 

Group VII. 
(ESTAS IES RGR ME SR RSIS AHPC) SG RE DKS eM NMG ACER IOUS Abeer eS ay ge ON gS epee  rORE N  2 485 
MEUM OE PRION oe radical Gago x x ca doc Rk GIT ej CR eed «saw Stee es 491 
Determination of Zirconium and Sulphur in Rocks................... 505 
CEM CORE PON ah 8) yon no Seo Se eas NRE GAd Mele U vincore) 0 ajo te whats 509 
Determination of Thorium in Monazite............. 0.2... e eee eens 510 
Determination of Water in Silicates... 0.0... cece cece cee eeeeee eens 512 

PART: II. 
VOLUMETRIC ANALYSIS. 
PRMMaNe AHLPUOD GEES ere Lose o ea eevee eae ae eel as Cae s 514 
Normal Volume and Normal Temperature. ..................0.-245. 516 
Cara tii- Or OMTITIIG: FARMS oS BSUS ROOD SiS ae tee selsietde ee eves 522 
SE 9 OST ERIE 1 eating Ge pie eats bur a arty Jos Pea aa es Ee 524 
SAERLION OF UPOIIER. ee beeen ed lata oes ese Pee end aes 527 
PRCA L POUT TIOINS tO aa aAe bisa egw ieneecwwesie AUER AE eee ol eae 530 
I, ACIDIMETRY AND ALKALIMETRY. 

RCM rate on utah Natt e e a  Cig Garg oh dg a ore mele A Were boat wait ns 538 
EBL OYA ay aaah oe Cy leat cca iy 'gly atetalas Wes: 4 «bo tees nse 558 


Ra Onin Sala b-eiay nee Pk tue NR ea eeidia Se we gio ee o's oe 571 


x TABLE OF CONTENTS. 


II. OXIDATION AND REDUCTION METHODS. 


Permanganate: Methods is 6 OUR TS Oy PERG A ONS Bee Ped eAk be bea 
Potassium: Dichromate: Methods: (21S at eis bose heb ce als 
ROIIMOTEY << ois CER WE ee le Oe ee vader ee FE pea 

Maduction- Methods ss 6c Ser ae eRe se ewe eee evan er 


III. PRECIPITATION ANALYSES. 


Determination of Silver................ a a eae eae aus 
Determination al Halowenss 3 seek oe eos ee haa 
Determination of Cranogen | oe ea eee ae 
Determination of Sulphocyanic Acid. . 2.2.0... 6 ec eee ee ec wees 
Deeminationof Sulphuric Avid 06.05 oes es Snes cine a ee 
Determination of Phosphoric Acid... ..........c cece cece eee eeceeees 
Pabamination of Nickel: 532550 R ae eis eee si ee ee 
Dehwmmination of ‘Copper... oo 6. Sawn hose ae i eh a 
Determmation of Leads.) 630... 6. cheeses bea aad es Cua 


PART III. 


GAS ANALYSIS: 


The Collection and Confinement of Gas Samples..................... 
Calibrating Gas Measuring Instruments......... ETE eer eet ues 
Pariioation of. Mereury = :-<55.0.5 9 5k oo Oe an ee 
Determination of Carbon Dioxide: 3.35). 200 Oe Re 
TSU OOS Seer ia evs Salone evil oe in inten Rs igh rhc tate UA ISTR 
BOR BORNG oe i oie ute aa a, a is dre ta gee ee 
Poet yhemne 255 oe ey Re I oan nN ara 
Separation of the Heavy Hydrocarbons...................b cece cece 


Analysis of [lluminating and Producer Gas....................-+.4-5 
‘Pechnical Gas Analysis: 0:0 65000. o aw alae Bae et cy eee ee 
Method. of Hempel os 3057 ass as PAG ee so tas gy Paisa Sate pe Mea 
Method ‘of :Winkler-Dennie - 22055 2 08s Sei oa ke kee ee 
Orsat'e Apparatus: 0.5.3. S55 Se eRe ee ee Se ae ppkyer ae: 
Bunte’s Apparatus 2)605 552s SoA 2 SE GEG er a ene ree eine 
Analysis of Gases which are Absorbed by Water..................... 
Nites Omeae ee ere eat tahun Ooh eek coors 
Nitese: Oxsihe oS os a ae AIOE Ee Ss eae 


TABLE OF CONTENTS. . xi 


MON rN To ania erg en a ar ep elie Wey apie ttaly winis PIN ok Fane Cemimiess 806 
A Oar CIE RIS oe aos Lee ens od ved ys ci wgriae 808 
ORION ae Sia APG PN Set at ad eo i a iO i UE SR ce og ag ee a Bee 808 
NECN PON acca At ei Sacks Cee bey Fo hata ReN eos Csi tis Vela ds 814 
STE ORM Ce ree ye wee gitie y's abi ewe Ain Pein ie ele erat 6 815 
Prydrogen Sulphide i 5i5 ie aes hao Pere eee te ee tes 816 
IN ee te tree ara hile oe gis atime: See alaleale ope oer dw wee 818 
Romie VONMMOITIC WL@UIIOGG. | cr eee ties abet es Ae eno cn aes et 822 
Determination of Ammonia in Ammonium Salts..................... 822 
Determination of Nitrous and Nitric Acids. ............... ey EE 825 
BEvarogels erties NICCNOUR ie es ces nciciee Oe eke rea wies wcees 826 
Standardization of Permanganate Solutions......................... 827 
Determination of Cerium in Soluble Salts........................... 828 
(SN oon 2” peas RSLS sy ae Cia hat i eR nt ae aOR a es psd valent $28 
aerate OF PIGORIIE cc. os ocr eae elk Die oes ete eens 829 
Determination of Vapor in Gas Mixtures..............44, Cetswuere one 
APPENDIX. 
SMe AIPUVIRY CR RONG oe roa eek Ue ramen aca keueWe aks 838 
Specific Gravity of Alkalies....... Ry Sane aN PeeWee ee ae Sica 840 
Tension of Aqueous Vapor......,............00.- Meee.) whee ok 842 
enRer NIE © OMAEMIAUEIT NAAR one sg tee hs eyed cee gis Saas Se ee 845. 
Tables for Calculating AMAlyees a Dice ee oe ahs 847 
THESTNALIOTAL ACOUNG VW CIDNUR) ios be ochre woah bec wee oc oere ee 849 
HR OR Gc IOIOAS POULOT oe hare ole baie Oc ata Vee Pen PRT 850° 
MMEMPUT NN oe gic Fe nol eS hy 4 OS aerate Sew ee Hay 1 ERENT Dy Ay 854 
APPENDIX I. 
The Influence of Fine Grinding on Composition.................00005 859 
The Use of Cupferron in Quantitative Analysis.................00005. 860 
nee UNINL Ol. Ss LONINMN gees CO ech vee bw is wccicu vikwe Gee eetecs. 862 
The Determination of Titanium Dioxide in Titanium-Iron Ore......... 866 
Determination of Sulphur in Pyrite: Fusion with Sodium Peroxide..... 870 
A Rapid Method for the Determination of Sulphur in Steel........... 872 
BUNNIES CMMURONUN ciel s os ete ge eons Cec nle Ohare Rae cee eee  ciawss 873 
The Determination of Chromium in Steel...............00000 Gieges 876 
Oxidation by the Barba Method................. i disso Gin ate Reawics ... 876 
Oxidation by Sodium Bismuthate...... WEE ov kee Ree eae Pree eet Deets 878 


InpEX.:... seeecseeeoer eee ee eeeereeesees ees eeee es @eeesee Peeps eevesee es @eeseeans eae 881 





QUANTITATIVE ANALYSIS. 


INTRODUCTION, 


Tue purpose of a quantitative analysis is to determine the 
quantity of the constituents present in a compound or in a mix- 
ture. The methods to be employed depend upon the nature of 
the substances to be determined, so that in every case a qualitative 
analysis should precede the quantitative one. In quantitative analy- 
sis we distinguish two essentially different methods of procedure: 


I. Gravimetric Anatysis (Analysis by Weight). 
Il. Vouumetric ANAztysis (Analysis by Volume). 


In the case of gravimetric analysis we separate the component 
to be determined from a solution in the form of an insoluble com- 
pound of known chemical composition, and then determine the 
weignt of this compound; from this we can calculate the amount 
of the substance present. 

Suppose, for example, that we have for analysis a sample . 
of barium chloride. The amount of barium present can be 
determined by dissolving a weighed amount of the chloride in 
water, precipitating the barium from the solution by the addi- 
tion of sulphuric acid and weighing the insoluble barium sulphate 
formed. 

If we start with a grams of barium chloride and obtain p grams 
of barium sulphate, the amount of barium present may be calcu- 
lated as follows: 

BaSO4:Ba=p:s. 


Ba : els ‘ ‘ 
s = aso, P =weight of barium in a gm. of barium chloride. 


2 INTRODUCTION. 


It is, however, customary to express the results in percentages} 
therefore in this case we have 


Ba 
““Baso, ? 100: 


t=O Gg Pet cent. barium. 


In the case of volumetric analysis the constituents are not 
weighed, but they are determined by measuring the amounts of 
reagents of known strength which react with them. 

Suppose that we have a sample of caustic soda which contains 
some sodium chloride as an impurity and that we desire to know how 
much caustic soda there is in 100 gms. of the mixture. A portion 
of the substance weighing a gms. is dissolved in water, some methyl 
orange is added and hydrochloric acid of known strength is then 
run into the solution from a burette until the alkali is just 
neutralized, this point being reached when the yellow color of 
the solution changes to pink. If ¢ ¢.c. of hydrochloric acid 
were necessary, of which 1 ¢.c. contained exactly a gms. of 
HCl, it is evident that to neutralize the caustic soda contained 
in a gms. of the mixture a-t gms. of HCl were used up, and it 
follows: 


HCl: NaOH =a:t:s 


s= OF a-t=ems. NaOH in a gms. of the mixture; in 100 gms, 
NaOH 
a: ey t= 100: 2 
Lex ee Sis per cent NaOH. 





We will first consider 


A. GRAVIMETRIC MEtTHops. 
These are divided into | 
(a) Direct Analyses, 
(b) Indirect Analyses. 
In the case of a direct analysis the substance to be determined 


is separated from the solution in the form of an insoluble com- 
pound and weighed. 


GRAVIMETRIC METHODS. 3 


The determination of barium in barium chloride was an example 
of a direct analysis. 

The indirect method depends upon the fact that when two or more 
substances are made to undergo the same chemical treatment they ex- 
perience a relatively different change of weight. 

For example, suppose that we have a mixture of the chlorides 
of sodium and of potassium and desire to determine the relative 
amounts of each of the two substances in the mixture. For this 
purpose a portion of the mixture (@ gms.) is weighed, dissolved 
in water, the chlorine precipitated as silver chloride, and the 
weight of the latter determined (p gms.). From these data it 
is possible to calculate the amount of sodium chloride and of potas- 
sium chloride that was present in the mixture. 

If we let x represent the amount of the sodium chloride, y 
the amount of the potassium chloride, @ the amount of silver 
chloride formed from zx gms. of sodium chloride, and # the 
amount of silver chloride formed from y gms. of potassium 
chloride, then 


NaCl KCl 
x+y=a 

AgCl AgCl 
a+ B=p. 


We have, therefore, two equations with apparently four un- 
known quantities, but @ and ? can be expressed in terms of & 
and y: : 





NaCl: AgCl=2:a@ KCl: AgCl=y:8 
_AgCl y Bess 
~ NaCl ~ kel 4 
AgCl _, AgCl 


Nac 22¢ 5eeqp however, are known quantities; they are simply 
the quotients of the molecular weights in co 
If we designate by m the faced’ NaCl es | and by 2 the frac- 


AcCl 


Kel » we have 


tion 


r+ y =a 
mMx+ny=p 


4 - INTRODUCTION. 


and from this we can calculate 





_ “ and Y=a—2x 


or 





nN 
if eee "a. 
m—-nN P m—-nN 





All indirect analyses may be calculated by means of this last 
general equation. 








In the above example ' 
_ AgCl 143.34 | _AgCl 143.34 | 
m—NaCl SB400 et RL Cae 
and : 
m—n=0.5297. 


If these values are substituted in the above equations we 


obtain 
: — £=1.888-p—3.628-a. 


Consequently, in order to determine the amount of sodium 
chloride in the original mixture it is only necessary to determine 
the values a and p, then multiply them by the coefficients 3.628 
and 1.888 respectively, and subtract the first product from the 
last. 

Although this method appears so simple and attractive on paper, 
impossible values are often obtained in practice, so that itis always 
necessary to be very cautious about using an indirect: method. 

The experimental errors which are unavoidable in such an 
analysis are multiplied by the value of the coefficients; thus in 
the above case the actual error in the determination of the weight 
a is multiplied by 3.63... and the error in determining the weight 
of the silver chloride (p) is multiplied by 1.89... 

It is clear, therefore, that an indirect analysis becomes more 
accurate in proportion as the coefficients are small and when the 
error in determining @ and 7 is slight. | 

In the above example the coefficients are relatively small and 
consequently good results are to be expected, and experiment 
shows this to be the case. 

Example: A mixture weighing 0.5480 gm. (a) and consisting of 


GRAVIMETRIC METHODS. 5 


0.4966 gm. sodium chloride (x) and 0.0514 gm. potassium chloride 
(y) yielded 1.3161 gm. of silver chloride (p), but from the values 
of a and p we can calculate those of x and y: 


x=1.888- 1.3161 —3.628-0.5480 
-=0.4963 gm. sodium chloride; 
y=0.0517 gm. potassium chloride. 


The calculated values, therefore, show 


99.92 per cent. of the true value for the sodium chloride, 
100.6 per cent. of the true value for the potassium chloride. 


Although the above results are satisfactory, it must be borne 
in mind that the analysis was carried out with chemically pure 
substances. If this were not so, as is usually the case in prac- 
tice, the results would be far less accurate. 

The same analysis may be performed in a much more simple 
manner than as above described, by weighing the mixture of the © 
chlorides in a platinum crucible, then changing them to sulphates 
(by treatment with sulphuric acid and evaporating off the excess 
of the latter) and again weighing. In this case the actual experi- 


mental error is slight and excellent results might be expected. 
We have 








NaCl KCl 
“+y= 
Na,SO, K,SO, 
a nd 2 
_Na,SO, BBO 
BNacr he Pore Y 
ae fe K,S0, | 
m—n-==0.0464 
Now 
(1) t+y=a 
(2) mz+ny=p 
and ° 


1 n 
Oo aa maa 


6 INTRODUCTION. 


Substituting the values for m and n in equation (8 we obtain 
¢=21.547-p—25.181a. 


In this case the coefficients are very large, so that the analytical 
error is multiplied enormously in the calculation, so much so that 
it is impossible to obtain even approximate values except when 
the mixture is composed of about equal parts of the two chlorides. 

Example: In a mixture containing about equal parts of the 


two salts there was found . 


99.64 per cent. of the sodium chloride present; 
100.76 per cent. of the potassium chloride present. 


In a mixture containing considerable sodium chloride and Sitth 
potassium chloride there was found 


(a) 95.0 per cent. of the sodium chloride present; 

, . 148.0 per cent. of the potassium chloride present. 

(b) 96.8 per cent. of the sodium chloride present; 
129.9 per cent. of the potassium chloride present. 


The values obtained are, therefore, worthless. : 

In the case of a direct analysis the small unavoidable errors 
exert a much less influence upon the result, so that a direct deter- 
mination should always be preferred. 

Only in those cases where a direct method is unknown should one 
resort to an indirect analysis! 


OPERATIONS. 


The principal operations of quantitative analysis are those 
of weighing, filtration, and the washing, drying, and ignition of 
precipitates. 

Weighing. 

The balance, as used for purposes of quantitative chemical 
analysis, is shown in Fig. 1. 

It consists of a horizontal lever with two arms of equal 
length, and in order to be serviceable it must be accurate and 
sensitive. | 

It fulfils the first condition if 

(1) The arms of the lever are equally long; 


WEIGHING. y 


(2) The point of support (the fulcrum) lies above the centre 
of gravity; 

(3) The fulcrum (a knife-edge) and the knife-edges from which 
the pans are suspended lie in the same plane and are parallel 
to one another. 

The balance is more sensitive the greater the displacement 
of the position of equilibrium brought about by the addition of 
a small weight, e.g. one milligram. 














Fig. 1. 


The sensitiveness, or sensibility, may be expressed by the 
equation: : 
Pp: 
tan a *¥= on 
- in which 7 is the weight added, J the length of the balance-arm, 
q the weight of the beam, and d the distance between the centre 
of gravity and the point of support. 

The sensitiveness of the balance is greater, therefore, the 
heavier the weight added, the longer the beam, the lighter the 
beam, and the shorter the distance between the centre of gravity 
and the point of support. 





* q is the angle through which the pointer moves on the addition of the 
small weight. 


$ . _ INTRODUCTION. 


For convenience in determining the position of the balance, 
a pointer is fastened to the beam which, when the equilibrium is 
established, rests at the zero of a scale on an ivory plate below. 

The object to be weighed is placed upon the left secale-pan and 
the weights upon the right pan; the beam is lowered and the 
balance set in slight motion, by producing, with the hand, a 
gentle draft of air upon one of the pans. If the correct weight 
has been added, the pointer will swing to the same number 
of scale divisions to the right of the zero that it does to the left, 
provided that it does so when there is nothing in either 
scale-pan, which is usually not the case. It is to be noted that 
when the zero-point of the balance (i.e., the point of the scale 
at which the pointer rests when the balance is in equilibrium 
with nothing in either scale-pan) coincides with the zero of the seale, 
it may change during the course of the day, so that disregard of 
this fact may lead to a considerable error. 

The cause of the displacement of the zero-point is that the 
first condition for the accuracy of a balance is not fulfilled. On 
account of unequal warming the arms become of unequal length. 

In order that accurate weighings may be obtained, it is necessary 
to make them independent of any inequality in the lengths of the 
arms, which can readily be done, as the following consideration 
will show. In the case of a lever, equilibrium takes place when 
the statical moments are equal, 

By statical moment is understood the product of the force 
into the length of the lever-arm, and the length of the lever-arm 
is the perpendicular distance from the axis of revolution (the | 
fulcrum) to the line of action of the force. 

If an object, whose weight Q (Fig. 2) is to be ascertained, is 
placed upon the left balance-pan and equilibrium is established 
(the pointer rests at zero) by putting weights enaunaes to P 
gms. in the right balance-pan, then 


(1) Ql=Fi.,. 
If now the object Q is placed on the right-hand balance-pan and 


the balance again brought to the state of equilibrium by placing 
weights upon the left-hand balance-pan, in this case the weights 


WEIGHING. 9 


will not as a rule amount to P gms., but to P, gms. Since, how- 
ever, equilibrium has been reached, we have 


(2) Ql, =Pl. 


ne 








Y 

__———— | 

0 7 
Fig. 2, 











If equation 1 is multiplied by equation 2, we obtain 


Ql, = P, Pll, 
@=P,P 
Q=VP,P. . 


The true weight is obtained. therefore, by taking the geometric 
mean of the two values. For practical purposes, however, it is 
sufficiently accurate to take the arithmetical mean, in-which case . 
the true weight of the object would be: 


Desist 


a 


This method of obtaining the true weight independent of the 
lengths of the balance-arms is known as that of double weighing. 

The same end is obtained by Borda’s method oj substitution. 

According to this method the object to be weighed ‘Q) is coun- 
terbalanced (tared) by means of shot, sand, weights, etc., the 
object Q is then removed and equilibrium with the tare is again 
established by placing weights upon the scale-pan. We have, 
then, as a result of the first weighing, 


Ql=Tl, 


Io INTRODUCTION. 


and from the second weighing, 


PiIaTG 

from which it follows: 
Ql=Pl 
V=P. 


The latter method is used ‘chiefly in weighing large objects. 

For ordinary analytical work the weighing is made by the 
method of swings. 

First of all the zero-point of the balance is determined by setting 
the balance in motion (without any load in either pan), observing 


L i 











mi Fal 

Fig. 3. 
and recording the turning-points, or extreme positions, of the 
pointer on the scale of an uneven number of swings (say five*) 
and taking the mean of the readings. In order to give the same 
algebraic sign to all the observed readings it is best to number 
the divisions on thescale from left to right from 0 to 20 so that the 
zero-point in case both balance-arms were of equal length would 
be numbered 10. 

The next thing to be determined is the sensitiveness of the 
balance for the object to be weighed. For this purpose the object 
is placed in the left-hand balance-pan, and by placing weights in 
the right-hand pan equilibrium is established as nearly as possible 





* The first two swings are inaccurate on account of the jar in shutting 
the balance-door, etc., so that they are disregarded. 


WEIGHING. II 


and the point of rest of the pointer on the scale is determined as 
above. An additional weight of 1 mgm. is added, or removed if 
the object was too light before, and the point of rest is again deter- 
mined. 

The difference (d) between this and the previous point of rest 
gives the sensitiveness of the balance. Assuming the zero-point to 
lie at 10.22, the first point of rest, obtained with a load of 19.723 
gms., to be at 9.80, and the point of rest with a load of 1 mgm. less 
(i.e., with a load of 19.722 gms.) to lie at 12.32, then the sensitive- 
ness of the balance will amount to 12.32 —9.80 =2.52 scale divisions. 

As the zero-point of the balance was at 10.22 and the point 
of rest with a load of 19.723 gms. was 9.80, it follows that the 
object was lighter than the weights in the right-hand pan, and 
in fact the excess of weights in the pan was sufficient to move 
the point of rest 10.22—9.80=0.42 divisions on the scale. This 
amount can be calculated from the determination of the sensitive- 
ness of the balance as follows: 

Since 2.52 of the scale divisions correspond to 1 mgm., then 0.42 
of the scale divisions correspond to the weight which must be sub- 
tracted from 19.723 gms. in order to obtain the true weight; there- 
fore 


2.52:1=0.42:2% 


0.42 


pat WE 





=0.17 mgm., or about 0.2 mgm. 


The true weight of the body in air is consequently 
19.723 — 0.0002 = 19.7228 * gms. 
In making a weighing one should always accustom himself to 


note the observations methodically, as follows: 
Assume that a platinum crucible is to be weighed. 





* As most analytical balances will scarcely detect with certainty less 
than ; mgm., the weight is expressed only to the fourth decimal. If the 
fifth decimal place in a calculation amounts to six or more, the number in the 
fourth decimal place is increased one. 


i2 INTRODUCTION. 























: I. Point of Rest with Load — II. Point of Rest with Load 
Zero-point. of 12.052 gms. of 12.053 gms. 
Left. Right. Left. . Right. Left. Right. 
SB 17.6 5.8 18.7 3.5 15.8 
4.6 17:4 6.2 18.3 3.8 15.4 
5 et Ea pam De PNA os IS alae ABest a. ie 4.2 
Sum= 13.9 34.7 18.6 37.0 11.5 31.2 
Mean = 4.63 17.35 6.2 18.5 rs Bh Fae 15.60 
beste elation 8% YS eee cl 6.2 so eh ages ties ae 3.83 
Sum of both means= 21.98]........... pee Se ia beele oes | yee ; 19.43 
Mean we 1 OOO eos, aes SBR 4 rates oes 9.71 














Sensitiveness= 12.35 —9.71= 2.64 scale divisions. 
12.35 —10.99= 1.36 scale divisions. 

1.36 : 2.64=0.5 mgm. 

Weight of crucible= 12.052 + 0.0005 = 12.0525 gms. 


The sensitiveness of a balance varies slightly with the load. 
It is simplest to determine once for all the sensitiveness for 50 
gms., 20 gms., 10 gms., 5 gms., and 2 gms., place a card in the 
balance with the results obtained and use the numbers as required. 
In this way the sensitiveness of a balance 


with a load of was found to equal 
50 gms. 2.23 scale divisions 
205005 y Bla 2s 
10 “cc ; 2.64 cc i iT 3 
5 “cc 2.66 éc (73 
2 6c 2.66 6c“ 'f3 


The determination of the zero-point, however, must be made 
with every weighing. If a number of weights are to be made 
one after another it suffices to determine the zero-point at the 
beginning and at the end and to use the mean of the two deter- 
minations. ‘ In case of very heavy loads, however, the zero-point 
should be determined before and after each YOR and the 
mean value used. 


REDUCTION OF WEIGHING TO VACUO, 13 


Reduction of Weighing to Vacuo. 


Since most of our weighings are made in the air with brass 
weights, we are constantly introducing an error due to the dis- 
placement of air. This error is so small that it can be disregarded 
in ordinary analyses; in the case of the most accurate work, 
however, as in atomic weight determinations, calibrations of 
measuring vessels, etc., it should never be neglected. In such 
cases the apparent weight must be reduced to vacuo as follows: 
1 c.c. of air at 15° C. and 760 mm. pressure weighs 0.0012 gm. = A. 

The specific gravity of brass is 8.0=s’.* 

The specific gravity of the substance weight =s. 

The body that weighs po gms. in vacuo will be balanced by 
p gms. in the air. 


(es 


The loss in weight of the substance is io A gms. / 


The loss in weight of the brass weights i is 7 Pa ems, at 
(pot 


The total loss therefore, = (= 1-5 i). The weight of the 


sok 
$ 


substance in vacuo is: 


pod _pa 


Po=pt Pacueie™ and po= 


A 


On account of the small values of the fractions s and a 


sufficient accuracy is obtained by simplifying the expression so 


that it becomes 
R82 
poo[isd-d] 


Instead of making the computation, the following table of 
Kohlrausch may be used: 





_ * Brass has a density of 8.4, but the density of most analytical weights is 
nearer 8.0. 


14 


INTRODUCTION. 


REDUCTION OF A WEIGHING MADE WITH BRASS WEIGHTS TO VACUO. 


METHOD OF F. KOHLRAUSCH. 
































8 k s k s k 
0.7 | +1.56 2.0 | +0.45 gs | —0.00 
0.8 1.35 2.5 » 0.33 0.017 
0.9 1.18 3.0 0.25 10 0.030 
1.0 1.05 3.5 0.19 11 0.041 
tae 0.94 4.0 0.15 12 0.050 
1.2 0.85 4.5 0.12 13 0.058 
1.3 0.77 5.0 0.09 14, 0.064 
1.4 0.71 5.5 0.07 15 0.070 
1.5 0.65 6.0 0.05 16 0.075 
1.6 0.60 6.5 0.03 17 0.079 
1.7 0.56 7.0 0.02 18 0.083 
1.8 0.52 7.6 0.01 19 0.087 
1.9 0.48 8.0 + 0.00 20 0.090 
2.0 + 0.45 21 — 0.093 
b= 1.20( _ <5) mgm. If a substance of specific gravity s 


weighs g grams in the air, then g-k mgms. are to be added to the 
weight in air in order to obtain the weight in vacuo, 


TESTING OF WEIGHTS. 15 


Testing of Weights. 


Although it is now possible to buy nearly perfect weights, yet 
their accuracy should always be tested. 

For almost all analytical purposes it matters not whether the 
50 em. weight weighs exactly 50 gms., but it is essential that the 
individual weights represent the corresponding relations to one 
another. 

: In most sets of weights the following are found: 50 gm., 20 gm., 

10 gm., 10’ gm., 5 gm., 2 gm., 1 gm., 1’ gm., 1” gm., 0.5 gm., 0.2 gm., 
0.1 gm., 0.1’ gm., 0.05 gm., 0.02 gm., 0.01 gm., 0.01’ gm.; a rider 
(weighing either 10 or 12 mgms. according as to whether the bal- 
ance-arm is divided into 10 or 12 equal parts between the fulerum 
and the point of suspension of the right-hand balance-pan); and 
usually smaller weights weighing 5, 2, 1, 1, 1 mgms. 

The weights may be tested in the following manner: * 

The assumption is first made that the sum of the larger weights 
is equal to 100 gms. =100,000 mgms., and the weights of the single 
pieces obtained by the method of double weighing are compared 
with one another, e.g.: 


50 gm. wt. against 20+ 10+ 10’+5+2+1+41’+1” 


and it is found: 


Right 
(1) 50 gm. ey 31 mgm. = (20+ 10-4 10’+...) 
Left Right 


(2) (20+ 10+10’+...)=50 gm.+0.61 mgm. 
from which it follows: 


0.31 mgm.+ 0.61 mgm. 


5 =50 gm.+ 0.46 mgm, 


= (20+ 10+10’+...) 





50 gm.+ 


or 


50 gm. = (20+ 10+ 10’+...)—0.46 mgm. 
* Kohlrausch, Leitfaden der prakt. Phys., V. Auflage, p. 34. See also 
T. W. Richards, Journ. Am. Chem. Soc. (1900) XXII, 144, aoe the method 
used at Harvard is described. 





fh | INTRODUCTION. 


The other weights are compared in the same may, obtaining, for 
example, 


50 gm. = (20+ 10+ 10’+...) m,-+A mgm. 


20 “ = 10+10' +B « 
10°“ = 10 AR yt 
(5+2+1+...)= 10 +p * 


in which A, B, C, and D may be either positive or negative. 

The sum of the weights (50+20+10+10’+...) was assumed 
to equal 100 gms., and with the help of the preceding equations 
each weight is expressed in terms of the 10 gm. weight; then 

10x10+A+2B+4C+2D= (50+20+10+. . .)=100 gms. and 
the 10 gm. weight itself: 





A+2B+4C+2D 


10= 10— 10 


If we let Sac sth et then we obtain 





10=10 gm.—S 
10’=10 gm.—S+C 
54+24+141'+1’=10 gm.—S+D 
20=20 gm.—28+B+C 
50=50 gm.—5S+A+B+2C+D 
=50 gm.+4A 


The sum 4A+ B+2C+ D—5S should equal 0, which serves as 
a test for the accuracy of the observations. 

The 5 gm. weight is now compared with the 2+ 14 1’+1” in 
exactly the same way, with the result that 


§=2+1+4+1'+1"+a 


2=1+1' +b 
1 ce] +e 
he | +d. 





* It is well to mark the weights of the same denomination so that they 
may be distinguished from one another. 


TESTING OF WEIGHTS. 17 


According to the preceding work 
54+2+14+1/+1"=10,000—S+D 








consequently © 
10X 1-++-a+2b+4c+2d=10,000—S+D 
and : 
1=1000— a+ 2b+ 4c+ 2d+S—D 

10 

If welet s= eae, we obtain 
| 1=1000—s 
1’=1000—s+c 


1” =1000—s+d 
2=2000—2s+b+e 
5=5000—5s+ a+ b+ 2c+d. 


In the same way the smaller weights are tested until finally 
the following correction table is obtained. 


TABLE FOR CORRECTION OF WEIGHTS. 




















).5+0.2+0.1+0.1/ 

50 =50 g.+4A §4+24+14+1/4+17= +4* = 
20 =20 g.—2S+B+C 10 1 0.1 
10’=10 g.—S+C =10¢.—S+D =lg.—S+a 4=0.1 g.—s’+e 

§ =5g.—5st+tat+b+ |0.5 =1g.—5s’+ 8+ |0.05=0.5g.—5s’+1+ 
5 +2c+d +r+2d6+6¢ +m+2Zn+o 
2 2=2g.—2s+b+c 0.2 =0.2 g.—2s’+7+ |0.02=0.02 g. — 2s’’+ 
1 $}=10¢.—S+D +é +m+n 
r 1=lg.-—s 0.1 =0.1 g.— <8’ 0.01=0.01 g.—s” 
1 1’ =1g.—ste 0.1‘=0.1 g.—s’+6 0.01’=0.01 g.—s”’ +n 

1”“=1 g.—s+d =0.1 g.’—s’+o |Rider=0.01g.—s’’+0 
Sum =100 g. 10¢g.—S+D 1 g.—st+ea 0.1 g.—s’+¢ 





* 4=0.05+0.02+0.01+0.01’+Rider. (Rider=0.01 g.) 


The milligram weights may be standardized in exactly the 
same manner. It is much more convenient, however, and the 
accuracy attained is almost exactly the same, if instead of using 
these very small weights the rider is hung upon the whole 
divisions of the balance-arm in order to obtain the weight in . 
milligrams; for the estimation of the fractions of the milligram it 
is better to calculate them from the sensitiveness of the balance. 


18: INTRODUCTION. 


The weights should never be touched with the fingers, but should 
always be lijted by means of the pincers provided with each sel, and 
nothing should be placed on or removed from the balance-pans with- 
out arresting the balance, 7.e., raising the mechanical supports so 
that the beam no longer rests upon its knife-edges. 


Filtration and Washing of Precipitates. 


How large should the filter begand how many times should the 
precipitate be washed? 

With regard to the latter question it is evident that the pre- 
cipitate should be washed until the soluble matter is completely 
removed. Itis clear, however, that this point will never be reached 
because a part of the solution always remains on the filter, but it 
is not difficult to make the amount of the dissolved substance 
remaining so small as to be negligible. When the amount of 
dissolved substance remaining on the filter is so small that it could 
not be detected by our balance, we consider the precipitate to be 
completely washed. 

The aim should be not only to remove all of the soluble matter, 
but to accomplish this with as little wash water as possible. 

No precipitate is absolutely insoluble, so that it is clear that 
_ every unnecessary excess of wash water causes harm by removing 
a fraction of the precipitate, and the greater the excess of the wash 
water the greater the amount of the precipitate dissolved. 

The amount of wash water to be used depends largely upon 
the nature of the precipitate itself. Amorphous, gelatinous pre- 
cipitates always require more washing than crystalline, granular 
ones.* As a rule, it may be said that the process of washing 
must be continued until the substance which is being washed 
out can be no longer detected in the last filtrate. In case the 
filtrate must be used for another determination, it is obvious that 
the filtrate should not be tested too soon. When should the 


filtrate be tested? 
Let us assume the filter to hold 10 ¢.c., the solution to drain 





* The reason why some precipitates require more washing than others 
is due to the fact that the degree of adsorption varies. (Cf. Ostwald, Die 
wissenschaftl. Grundl. der analyt. Chem., p. 19.) , 


FILTRATION AND WASHING OF PRECIPITATES. 19 


to the last drop from the paper, the amount of the solution held 
back by the precipitate and filter to be 1 c.c. and to contain 
0.1 gm. of the solid substance which is to be removed by wash- 
ing. 
The filter is filled to the upper edge with wash water and 
allowed to drain to the last drop times, until not more than 
5/100 mgm. of the substance to. be removed by washing remains. 

According to our assumption. 9 c.c. drain off and 1 ¢c.c. remains 
behind; we have consequently: 


Removed by the There remains after the 
1st washing, 0.1-9/10 gm. Ist washing 0.1-1/10 gm. 
‘2d $3 0.1-9/10-1/10 gm. 2d ne 0.1-1/10-1/10 gm. 


3d “ 0.1-9/10- (1/10)? gm. 3d 4 0.1-1/10- (1/10)? gm. 


mth “ 0.1-9/10-(1/10)"-1gm. nth “ 0.1-1/10-(1/10)"—-1 gm. 


After washing ” times, therefore, the amount removed by 
washing is the sum of the decreasing geometric series of which the 
first term is 0.1-9/10 and the constant factor is 1/10. 

In case n =4, the sum of the series is 





=-(0.09999 gm. 


After washing the precipitate four times, therefore, 0.09999 gm. 
of the impurity has been removed. According to the assumption 
that there was originally 0.1 gm. of this substance, there remains 
in the precipitate only 0.00001 gm., or in other words a negligible 
amount. 

Consequently, the filtrate should be tested qualitatively for 
the substance to be removed only after the precipitate has’ been 
washed four times. 

Often the washing will be found to have been complete after the 
fourth washing, but as a rule this will not be the case, and in many 
cases it will be found necessary to repeat the operation for from 


290 INTRODUCTION. 


ten to twenty times. In the processes which are described 
it will usually be stated how far to carry the washing. 

Now with regard to the second point, how should a precipitate 
be washed with the least possible amount of wash water? Accord- 
ing to the above consideration it is necessary to wash every pre- 
cipitate at least four times, in which case the filter should be 
entirely filled each time, and it is evident that the size of the 
filter-paper will influence the amount of wash water used. 

The filter, therefore, should be made as small as possible, 
irrespective as to whether there is little or much liquid to filter. 
The size of the filter used should be regulated entirely by the amount 
of the precipitate and not at all by the amount of the liquid to be fil- 
tered. The mistake should not be made, however, of using too 
small a filter. The precipitate should never reach the upper 
edge of the paper; about 5 mm. should remain free, and even in 
this case the filter should not be so completely filled as in Fig. 4, a. 





























Fia. 4. 


It is better to have the filter filled about as much as is shown in 
Fig. 4, b, in order that sufficient room is left for the wash water. 

The use of too large filters is one of the inexcusable ana- 
lytical errors. 


THE DRYING AND BURNING OF PRECIPITATES. 21 


The Drying and Igniting of Precipitates. 


Before a precipitate can be weighed it must be absolutely dry. 
Those precipitates which do not undergo a change of weight on 
ignition are treated as follows: 


(a) THE PRECIPITATE IS IGNITED DRY. 


This method, in which the precipitate is separated from the 
filter, the filter burnt by itself, the ash added to the main part 
of the precipitate and the mixture then ignited to constant 
weight, is used in those cases when the ignited substance will be 
reduced by the burning paper, e.g., in the case of precipitates of 
silver chloride, lead sulphate, bismuth oxide, etc. 

In order to perform this operation it is first necessary that the 
filter and precipitate should be completely dried at 100°C. For 
this purpose the funnel containing the filter is carefully covered 
with a piece of filter-paper,* placed in a drying-closet (preferably 
one that is heated by steam) and dried at 100°C. When per-. 
fectly dry, a weighed crucible is placed upon a piece of glazed 
paper of about 20 sq. cm. (Fig. 6, left) and the dry precipitate is 
carefully shaken into the crucible, removing it from the paper as 
completely as possible by gentle rubbing with a platinum spatula. 
Any small particles of the precipitate which may have fallen 
upon the glazed paper are brushed into the crucible with the 
aid of a feather (Fig. 6). Small particles of the precipitate will 
still always adhere to the paper and these must be weighed. 
In order to accomplish this, the filter is burnt and the ash obtained 
is either weighed by itself or mixed with the main part of the pre- 
cipitate and weighed with it. 





* Wet a common cut filter, stretch it over the ground top of the funnel, 
and then gently tear off the superfluous paper. The cover thus formed 
continues to adhere after drying. Fresenius, Quant. Chem. Analysis.. 

+ By using filter-paper which has been carefully washed with hydro- 
chloric and hydrofluoric acids, it is permissible to neglect the weight of 
the ash from the filter itself. With an unknown paper it is necessary to 
determine the weight of the ash by a separate rs MEE and then correct 
the weight of the precipitate obtained. 


Ce INTRODUCTION. 


The combustion of the filter, to which small particles of the 
precipitate still adhere, is best accomplished by the method pro- 
posed by Bunsen as follows: The filter is folded together so that 
the precipitate occupies the position indicated in the shaded part 
of Fig. 5, a, and then it is further folded as indicated by ? and 7 of 
Fig. 5 to a narrow strip. The paper is then rolled between the 














Fiai. 5. 


fingers as indicated by 0, beginning at b, so that the portion of 
the filter which is free from the precipitate is on the outside. The 
roll is now enveloped with.a previously ignited heavy platinum 
wire, the wire is supported (as indicated in Fig. 6) by means of a 





Fig. 6. 


cork in the opening of a porcelain plate and the filter is ignited by 
means of the gas-flame. The flame is at once taken away and- 
the paper allowed to burn quietly. If carbonized particles still 
remain, the gas-flame is applied repeatedly until it is no longer 
possible to make the particles glow any more. (Too strong ignition 
should be avoided.) The ash is then added to the contents of the 


THE DRYING AND BURNING OF PRECIPITATES. 23 


crucible by gentle shaking and the final use of the feather. The 
cover is placed on the crucible, which is heated at first with a smal) 
flame, the temperature being 
gradually increased until the 
prescribed temperature of - = — 
ignition for the given precip- é : : 
itate is reached. The flame i 2 

is finally removed, the cruci- ff 
ble allowed to cool some- 
what, and while still warm, 
but not glowing, is placed 
in a desiccator (Fig. 7). 

After cooling (at least 7 
three quarters of an hour for Fig, 7. 
porcelain crucibles and 20 minutes for platinum ones) the crucible 
and its contents are weighed. 

Many precipitates (silver chloride, lead sulphate, etc.) are some- 
what reduced to metal by the above treatment. As, however, 
these metals are difficultly volatile, there will be no loss of the 
metal, only of the anion (chlorine in the case of silver chloride 
and SQ, in the case of lead sulphate). This loss may be readily 
replaced. The metal in the crucible is moistened with a few drops 
of nitric acid to dissolve it, a few drops of hydrochloric acid (in 
the case of a silver chloride precipitate), or of sulphuric acid (in 
the case of lead sulphate) are added, and after evaporating off the 
excess of the acid the crucible is weighed. The only danger in this 
method is that in burning the filter the ash is heated too hot, so 
that some of the reduced metal melts and alloys with the platinum 
wire. If, however, the filter-paper is rolled up as was directed, 
there is always some paper free from precipitate between the precipi- 
tate and the platinum wire, yielding an ash which, although its 
weight is inappreciable, is still sufficient to protect the wire and 
prevent the reduced metal from coming in contact with it, provided 
it is not heated strongly enough to melt the metal. 

Many precipitates (Mg(NH,)AsO,, K,PtCl,, etc.) are iaiped 
so much by this treatment that it would be impossible to obtain 
correct results. In such eases the filter cannot be burnt, but 
it is previously dried at a definite temperature and weighed; 





24 INTRODUCTION. 


afterwards the precipitate and filter are again dried at the same 
temperature and weighed again. 

In order to dry the filter, it is placed in a drying-closet * 
(Fig. 8a) upon a watch-glass and near an open weighing beaker, 
the temperature is brought to the desired point and kept there, 
with the help of the thermo-regulator 7, for 4 to 1 hour. By 
means of tongs the filter is quickly placed in the weighing beaker, 
and the latter in a desiccator filled with calcium chloride (Fig. 7), 
where it is kept for exactly 1 hour It is then covered, removed 
from the desiccator, allowed to stand in the air near the balance for 
20 minutes and then weighed. The heating and weighing is re- 
peated once more in exactly the same way until two consecutive 
weighings do not differ by more than 0.0002—3 gm. 

The precipitate is now collected upon the filter and after drying 
the filter in the funnel at 100°C. the filter and its conterts are 
removed, from the funnel and dried in exactly the same way as 
before. 

The same result is much more simply and accurately accom- 
plished by the use of the Gooch Crucible. 

This consists (as is shown in Fig. 9, page 25) of a@ cru- 
c.ble with a perforated bottom. The crucible is provided 
with an asbestos filter, weighed after drying at the pre- 
scribed temperature, then the precipitate is filtered off 
into the crucible, which is again dried and weighed. The 





* The drying-closet shown in Fig. 8a is fitted with six removable porce- 
lain plates which prevent any oxide falling from the metallic closet walls 
upon the substance to be dried, rendering it impure. The upper plate has 
two holes bored in it through which thermometer and thermo-regulator are 
placed. This upper plate is fastened to the top of the closet as follows: 
A glass rod provided with a broad rim rr and bulging out at aa is pushed up 
through the opening P of the porcelain plate (Fig. 8b) and K of the upper 
closet wall, and this is fastened by placing an asbestos ring A between aa 
and K. 

The bottom plate rests upona heavy iron wire so that it does not 
come directly in contact with the bottom of the closet. 

As the plates can be easily taken out, it is possible to clean them without 
difficulty. The only part of the apparatus that wears out is the bottom, 
so that it is best to have the closet so that it may be renewed from time 
to time without taking the apparatus to pieces. ; 

Several forms of electric ovens are also in use. These require little attens 
tion and can be regulated to almost any desired temperature. 


THE DRYING AND BURNING ,OF PRECIPITATES. 25 















Sas NT 


oa) 


























Po ee =~ 1svestoe 


Fia. 9, 


26 INTRODUCTION. 


use of these crucibles permits such accurate and rapid work that it 
is worth while to describe the method of using them more in detail. 


Preparation of Asbestos Filters. 


Some long-fibred, soft asbestos is cut into pieces $ em. long, 
and digested with concentrated hydrochloric acid upon the 
water bath for an hour. A good sample of asbestos will then 
be separated into very small fibres. 
The mass is collected in a funnel with 
a platinum cone, or upon a filter-plate, 
and washed with water. After dry- 
ing, the asbestos may be ignited, but 
for most purposes this is not only un- 
necessary but disadvantageous. 

For the preparation of a Gooch 
filter, a small flock of the material 
is shaken with water in a flask, so 
that a thin emulsion is formed. A 
piece of thin rubber tubing * (Fig. 10) 
is stretched over a funnel and the 
crucible 7 is placed in the opening. 
The funnel should be large enough so 
that the crucible is suspended by the 
rubber without touching the sides of 
the funnel. Enough of the emulsion is 
poured through the crucible to produce 
a layer of 1 to 2 mm. thickness, a 
small filter-plate (Fig. 9, P) is placed 
upon this layer and some more of the 
emulsion is poured into the crucible. 
Water must now be passed through 
the crucible until no asbestos fibres run through, and in order 
to see them the liquid is poured into a small beaker. Usually 
such a filter is prepared and used with a gentle suction,f 


aii 


\_ 
hy AE 








*In place of rubber tubing Bailey’s Gooch crucible holder or Walter’s 
Gooch crucible holder may be used to advantage. 

+t Too great a suction should not be employed during the filtration, for 
in that case the precipitate or even the asbestos itself will be so compremaat 
that the filtration will be prolonged and the washing made more difficult. 


THE DRYING AND BURNING OF PRECIPITATES. 27 


but in many cases it filters more rapidly than paper with- 
out it. 

The crucible is now dried at the proper temperature and after- 
wards weighed. The drying and weighing is repeated until a con- 
stant weight is obtained, when about half a liter of water is once 
more passed through the crucible (in order tobe sure that no asbestos 
fibres run through) and the crucible is again dried and weighed, 
after which, if the weight is constant, the crucible is ready for the 
filtration. 

The same crucible can be used for a large number of determina- 
tions. When the amount of the precipitate in the crucible be- 
comes too large, the upper part can be carefully removed and the 
crucible again used. 

If it is desired to ignite a precipitate contained in a Gooch 
crucible, it is placed (as shown in Fig. 11) within a larger porce- 
lain crucible and heated at first 
gently and finally more strongly, 
and when necessary it can even 
be heated over the blast-lamp. 

For many purposes it is prefer- 
able to use instead of the Gooch hae 
crucible a glass tube with an 
asbestos filter. This is particu- 
larly desirable when it is neces- 
sary to heat the precipitate in a Fie. 11. 
gas-stream. 

The so-called Munroe crucible,* in which the filtering medium 
consists of a porous felt of spongy platinum, is a modification of 
the Gooch crucible which permits rapid and accurate work. The 
felt is prepared by igniting a carefully-dried layer of ammonium 
chloroplatinate, which has been poured over the bottom of a 
platinum Gooch crucible in the form of an alcoholic sludge while 


r 


















By having the crucible suspended free by the rubber, the possibility of em- 
ploying too much suction is avoided, for, as soon’as_this has reached a certain 
tension the air is forced between the rubber and the ‘sides of thé crucible, so 
that we have the effect of a safety-valve to a certain extent. 

* C. E. Munroe, J. Anal. Chem., 2, 241; Chem. News, 58, 101. See also 
W. O. Snelling, J. Am. Chem. Soc., 31, 456, and O. D. Swett, 7572, 31, 928. 
The last reference gives a table of suitable solvents for removing ignited pre- 
cipitates from the Munroe crucible. 


28 INTRODUCTION. 


the crucible is held against several layers of filter paper. The 
felt can be shaped to the crucible during the ignition and subse-— 
quently burnished lightly with a glass rod of suitable form. In 
case imperfections develop, the felt should be saturated again 
with chloroplatinic acid, the crucible slowly lowered into a moder- 
ately concentrated solution of ammonium chloride, washed with 
alcohol, dried and ignited. 

The use of an electric furnace, Fig. 12, is very convenient for 
igniting the crucible and its contents, especially in the case of 








14 











Fra. 12. 


those precipitates which are likely to undergo change on coming 
in contact with a reducing flame. 


(b) THE PRECIPITATE IS IGNITED WET. 


Those precipitates which do not suffer any permanent change 
by the action of the products of combustion of the filter may be 
ignited wet. The precipitate is allowed to drain as much as 
possible and while still moist the filter and precipitate are placed 
in a platinum crucible, the paper being pressed down against the 


THE DRYING AND BURNING OF PRECIPITATES. 29 


sides of the crucible. The crucible is placed in an inclined position 
upon a triangle (Fig. 13),* with the cover inclined against the 
upper edge of the crucible and resting on the triangle. The flame 
of the burner is directed against the cover, which quickly dries 





Fig. 13. 


the filter, then scorches, carbonizes, and finally burns it. The 
flame is then slowly moved backwards under the crucible until 
finally the crucible is subjected to the whole heat of the burner, 
after which it can be heated over the blast-lamp if necessary. 





* In Fig. 13, the inner triangle is platinum wire, the outer triangle is heavy 
iron wire. Triangles of fused silica or of nickel-chromium alloy are suitable, 
but platinum alloys with iron, so that a hot crucible should never be placed 
in contact with iron wire. 


30 INTRODUCTION. 


THE EVAPORATION OFLIQUIDS. 


Liquids are usually evaporated upon the water-bath. In 
order to prevent anything from falling into the evaporating-dish 


it is well.to cover it with an evaporation-funnel, as shown in 
Fig. 14. 























Fia. 15, 


The funnel is suspended above the dish by: means of a porce- 
lain fork fastened to the iron rod (covered with hard rubber) which 
is attached to the water-bath. 

In case the laboratory is provided with a glass-covered hood 
‘with a good draft the use of the funnel is unnecessary. ‘“ 

If, however, the hood is directly connected with the chimney 
it often happens that on a windy day a considerable amount of 
dust falls into the hood. 


THE EVAPORATION OF LIQUIDS. 35 


In order to prevent this, the author has made use of the fol- 
lowing contrivance which has worked very satisfactorily for some 
years. The hood is provided with a glass roof, aa, Fig. 15, and about 
15 cm. below there is a second glass plate bb which does not quite 
touch the inner wall of the hood but is about 3 em. away from it 
throughout its whole length. Between the two plates there pro- 
jects a clay pipe R, about 15 cm. in diameter and about 5 em. 
above the inner edge of the lower glass plate, leading directly 
into the chimney K, in which there is a small gas-flame (not shown 
in the illustration). Any dust, sand, etc., from the chimney falls 
upon the plate bb; none can get into the hood. 

In the evaporation of liquids on the water-bath in weighed 
platinum crucibles or dishes, the platinum should not come in 
contact with copper or glass rings. Asa rule, porcelain rings should 
be used. In case the crucible is smaller than the ring, use is made 





Fia. 16. Fig. 16¢.—Water-bath with porcelain ring, 
platinum-brass cone, and crucible. 


of a truncated brass cone turned back at the base (Fig. 16), and 
lined with thin platinum foil. This is suspended in the ring and, 
the crucible placed within the cone (Fig. 16a). 

During evaporation many substances have the property 
of “creeping” over the edge of the crucible or dish, often causing 
a slight loss of the substance; furthermore there is often “‘ bump- 


32 INTRODUCTICN. 


ing,” so that in some cases the entire contents are thrown out of 
the crucible (cf. the determination of boric acid according to the © 
method of Gooch). Both of these phenomena can be readily 
prevented as follows: 

The crucible, at the most not more than two-thirds filled with 
liquid, is placed in the cylindrical tin or brass spiral kk (Fig. 17). 





Fie. 17.. 


The first two windings of the metallic spiral come into close con- 

tact with the sides of the crucible above the liquid, while the re- 

maining windings should not touch the crucible. When steam is 

passed through the spiral the 

upper part of the crucible is 

warmed first, so that there is no N= 

spattering, and furthermore Dy ~ Gseeeepeeesmemeeeneeres 

keeping the upper edge hot during 3 Fia. 18. 

the whole of the evaporation all a=asbestos ring. 
“creeping” of the substance is oe pe 

avoided. In this way it is possible e evaporate off alcohol rapidly 

without boiling the liquid. 

In case it is desired to evaporate high-boiling liquids, such as 
sulphuric acid, amyl alcohol, etc., the crucible is either heated 
cautiously over the free flame (continually moving it back and 
forth) or else the crucible is placed in an air-bath, which can be 
prepared in some such way as is represented by Fig. 18. 








DRYING SUBSTANCES IN CURRENTS OF GASES. 33 


Drying Substances in Currents of Gases. 


Substances may be dried at a high temperature in a current of 
air or of carbonic acid in a number of different ways. An oil-bath 
provided with a number of copper tubes (Fig. 19) may be used. 
The substance contained in a small “boat” is placed in a glass 
tube and the latter in one cf the copper tubes. The gas is now 
passed through one or more of the empty tubes (so as to warm it), 
and then through the tube containing the substance. 








Fia. 19. 
R=tube with thermometer for measuring the temperature of the gas-stream. 


In order to heat a crucible in a current of carbon dioxide, 
use can be made of Paul’s drying oven (Fig. 20). The crucible 
is placed in the glass pipe R and the pipe and copper cylinder K 
are covered with watch-glasses. Dry carbon doxide is conducted 
through the stem of the pipe, and the oven can be heated to any 
desired temperature. | 

In case it is desired to evaporate off a liquid in a flask and to 


34 INTRODUCTION. 


ignite the residue at a given temperature it is necessary to proceed 
somewhat as follows: 





LiL 


Lh. 






ddd idildiiddddsdipts 








Fie. 20. 





a 
a 
i 











UAHA A CAE ta 
1 


Fic. 21a. Fie. 216. 


The solution is placed in the open Erlenmeyer flask K and 
evaporated as far es possible over the free flame. The flask is 


PREPARATION OF THE SUBSTANCE FOR ANALYSIS. 35 


then placed in a metal beaker suspended in an oil-bath (Fig. 21a), 
and dry air is sucked through the spiral copper tube kk as shown in 
the illustration. Fig.21b shows the separate parts of the apparatus. 


PREPARATION OF THE SUBSTANCE FOR ANALYSIS, 


It is very difficult to give general rules for the preparation of 
substances for analysis, for it is necessary to proceed differently 
in different cases. Tor a scientific analysis (i.e., one in which it 
is desired to determine the atomic composition of a substance) 
it is necessary to choose pure material for the analysis. Although 
this sounds so simple it is often one of the most difficult conditions 
to fulfil. Many substances are hygroscopic and absorb moisture 
from the air, which can be removed by heating the substance or 
by simply allowing it to stand in a desiccator over calcium chlo- 
ride, provided the substance itself undergoes no change by this 
treatment. Many substances containing water of crystallization 
cannot even be dried in a desiccator, but must be analyzed air-dry. 
In all cases it is necessary to determine whether the substance to 
be analyzed possesses a constant weight. 

For technical analyses, the purpose being to determine the cost 
or selling price of an article or to control its manufacture, the sub- 
stance must be analyzed as 2t7s. In such a case the sample should 
represent as far as possible the average composition of the product. 

For our work we are concerned chiefly with scientific analyses 
and the first substances to be analyzed are easily crystallized 
from water. 

Many commercial salts are prepared extremely pure and could 
be analyzed directly; in most cases, however, we obtain them after 
they have stood for some time in the air and after they have been 
handled somewhat, so that they are not so pure as when freshly 
prepared. Consequently in case it is desired to test the accuracy 
of an analytical process, the purity of a commercial sample should 
never be taken for granted. The substance should be purified by 


Recrystallization. 


Ten or fifteen grams of the commercial salt are dissolved in 
the least possible amount of hot water (it is best to use not quite 


30. INTRODUCTION. 


enough water to completely dissolve the substance) and the hot 
solution is rapidly poured through a plaited filter contained in a 
funnel the stem of which has been broken off (Fig. 22). This 
serves to remove al! dust or other insoluble impurity. The filtrate 
is received with constant stirring in an evaporating-dish and is 
rapidly cooled by placing the dish in a larger one containing cold 
water. 





Fic. 22. 


By means of the rapid cooling and constant stirring, the 
salt is obtained in the form of a crystalline powder,* which is 
filtered off by pouring through a funnel provided with a perforated 
platinum cone. The mother-liquor is removed as much as possible 
by means of suction. The purity of the substance is then tested 
qualitatively by means of some suitable reaction. In case it is 
still not quite pure, the same process of recrystallization must be 
repeated until the presence of no impurity can be detected. 

The pure but still moist substance is placed upon a layer of 
several thicknesses of clean filter-paper, covered with another sheet 
of the same and allowed to stand for twelve hours at the ordinary 
temperature. One or two grams of the substance are then weighed 
upon a tared watch-glass, placed upon a dry glass plate, covered 
with another watch-glass and allowed to stand for several hours 
more. If the substance shows no change in weight it is ready for 





* Large crystals would be obtained by allowing the solution to cool 
slowly, but they are not desirable, as they usually contain more enclosed 
mother-liquor than do the smaller crystals. 


PREPARATION OF THE SUBSTANCE FOR ANALYSIS. 37 


analysis. Otherwise it must be dried in the air until it no longer 
shows a change in weight. It is not permissible to dry the sub- 
stance in a desiccator except in those cases in which the substance 
will not lose water of crystallization. Deliquescent substances 
of course should not be allowed to remain exposed to the air for 
very long. Such substances must be quickly dried upon a porous 
plate and transferred as soon as possible to a flask provided with a 
closely fitting ground-glass stopper. Further rules for the prepara- - 
tion of the substance for analysis will be given under the special 
cases. 


Pah. 
GRAVIMETRIC ANALYSIS. 


A. GRAVIMETRIC DETERMINATION OF THE METALS 
(CATIONS). 


METALS OF GROUP V. 
POTASSIUM, SODIUM, LITHIUM, AMMONIUM, AND MAGNESIUM. 
POTASSIUM, K. At. Wt. 39.10. 
Forms: * KCl, K,SO,, K,PtCl,, and KCIO,. 
1. The Determination as Chloride. 


THIs compound is chosen for the determination of potassium 
when it is already present as such, or in case the salt to be analyzed 
may be changed to the chloride by evaporation with hydrochloric 
acid. If the potassium is present in the form of its sulphate it 
may be transformed to the chloride by precipitation with barium 
chloride (see silicate analysis); if it is present as the phosphate, 
the phosphoric acid may be precipitated as basic ferric phosphate 
(Vol. I, p. 328, Ed. ID; or, finally, if it is present as chromate the 
CrO, ions may be reduced to chromic ions by evaporation with 
hydrochloric acid and alcohol and then precipitated by ammonia 
and filtered off. 

In almost all of these cases it is a question of separating the 
potassium chloride from the aqueous solution and in most cases 
of separating it from ammonium chloride as well. 

First of all the solution is evaporated to dryness on the water- 
bath in a platinum dish (or if necessary a thin porcelain dish may 





_* Under this heading will be given in every case the symbols of the com- 
pounds suitable for the determination of the element in question. 


DETERMINATION OF PO? ASSIUM AS CHLORIDE. 39 


be substituted), taking the precaution of stirring the liquid fre- 
quently with a heavy platinum wire, as soon as the salt begins 
to separate out, in order to hasten the evaporation of the 
enclosed water. In spite of long-continued heating and con- 
tinual stirring, however, it is not possible to completely expel all 
of the water enclosed within the crystals; this is effected by cover- 
ing the dish with a watch-glass and drying in the hot closet for 
an hour or two at 130°-150° C. The covered dish is then placed 
upon a platinum triangle and cautiously heated over a free 
_ flame, holding the burner in the hand and imparting to it a fanning 
motion. The dish is kept covered as long as a decrepitating sound 
can be heard. The cover is then taken off, any ammonium chloride 
on it is removed by careful heating, and it is then placed upon 
another clean watch-glass. The dish is then heated again over the 
constantly moving flame until the vapors of ammonium chloride 
cease to be given off, care being taken not to heat the potassium 
chloride too strongly on account of its volatility. Any potassium 
chloride remaining on the cover is then washed into the dish by 
means of a little water, the salt in the dish is brought into solution. 
by rotating this water in the dish and the almost ever-present 
carbon particles (from the carbonization of pyridine bases usually 
present to a slight extent in the ammonia and ammonium chloride) 
are filtered off through a small filter into a weighed platinum ecruci- 
ble. A few drops of HCl are added, the contents of the crucible 
evaporated to dryness on the water-bath, again covered and allowed 
to remain in the drying-closet for one to two hours at 130°-150° C. 
and once more heated over the free flame until all decrepitation 
has ceased, when the crucible is allowed to cool in a desiccator and is. 
weighed. After this the crucible is again heated for a few moments 
over the free flame so that the bottom of the crucible becomes a 
dark red (the cover of the crucible must not be lifted during this 
operation) ; it is allowed to cool, and is again weighed. The proc- 
ess is repeated until a constant weight is obtained. 

In the case of every analytical operation the heating and weigh- 
ing must always be repeated until two consecutive weights are 
the same. Therefore, whenever the terms ‘‘heated”’ (or ‘“‘ignited’’) 
and ‘‘ weighed” are used in this book, it is to be always understood 
that a constant weight is to be obtained. 


40 GRAVIMETRIC ANALYSIS. 


This method is capable of yielding exact results. 

Example: Determination of Potassium in Potassium Bichromate. 
—Commercial potassium bichromate usually contains potassium 
sulphate as impurity. The salt is therefore purified, as described 
on p. 35, by recrystallizing three times from water, placing the 
moist crystals in an evaporating-dish, heating on the water- 
bath with constant stirring and finally drying to constant weight 
in an oil-bath at 130° C. (ef. p. 33) in a current of dry air. 

The dry substance is then weighed upon a tared watch-glass, 
placed in a 300-c.c porcelain evaporating-dish, treated with 10 c.c. 
of concentrated HCl and 5 c.c. of alcohol, covered with a watch- 
glass and warmed upon the water-bath until the solution becomes 
a pure emerald-green. Any solution which may have spattered up 
on the cover-glass is washed into the dish by means of a stream 
of water from the wash-bottle and the solution is then evaporated 
to dryness. About 2 c.c. of concentrated HCl and -200 c.c. of water — 
are now added, the liquid is heated to boiling and precipitated 
with the least possible excess of ammonia, filtered and washed 
with hot water until 1 ¢.c. of the filtrate evaporated upon the cover 
of a platinum crucible leaves no residue. If, however, on making 
this test a residue remains, it must be redissolved in water and 
added to the rest of the filtrate. After the washing is found to 
be complete, the filtrate is evaporated to dryness as previously 
described,* the ammonium chioride expelled, and the residue of 
potassium chloride is weighed. 

If a is the amount of potassium bichromate taken, and p the 
weight of potassium chloride obtained, the amount of potassium 
present in the potassium bichromate may be calculated as follows: 


KCI:K =p:s 
74.56:39.10=p:s 





ous aa a3 P= weight of potassium in a gm. of Pnsonieys and ip 
percentage 
= ae Sone =per cent. K. 





* Frequently a little Cr(OH), separates out during the evaporation; it 
must be filtered off and washed free from the solution. 


DETERMINATIGN OF POTASSIUM AS POTASSIUM SULPHATE. 41 


It is customary to carry out the analysis in duplicate and to be 
satisfied only when two closely agreeing results are obtained, of 
which the mean is taken as the true value. According to the above 
method results are cbtained which are slightly lower than the theo- 
retical value, but this should not amount to more than 0.15 per 
cent. and the two “check” determinations should not differ by 
more than 0.1 per cent. from one another. 


2. Determination of Potassium as Potassium Sulphate. 


This method is chosen when the potassium is already present 
in solution as the sulphate, or when it is in such a form that it can 
be readily changed to sulphate by evaporation with sulphuric 
acid; it is most frequently used for determining the amount of 
potassium in combination with organic acids. 

Since the sulphate of potassium is much less volatile than the 
chloride, it is advisible to choose this method in case no other metal 
is present. On the other hand, when it is necessary to separate 
potassium from sodium, it is preferable to have the potassium in 
the form of the chloride. 

Example: Determination of Potassium in Potassium Bichromate. 
—About 0.5 gm. of the purified and dried salt is weighed, as described 
under 1, into a 300-c.c. porcelain evaporating-dish, treated with 20 
c.c. of a freshly prepared, saturated, aqueous solution of sulphur 
dioxide* and 5 .c. of double-normal sulphuric acid. The dish is 
covered with a watch-glass and warmed on the water-bath until 
there is no further evolution of gas perceptible, when the cover-glass 
is rinsed off, removed and the solution evaporated almost to dryness. 
About 200 c.c. of water are now added and the chromium is precipi- 
tated from the boiling solution by means of the slightest possible 
excess of ammonia. The precipitate is filtered off and washed 





* The solution of sulphur dioxide may be prepared as follows: Into a . 
300-c.c. Erlenmeyer flask about 150 c.c. of a saturated sodium bisulphite 
solution are placed, and concentrated sulphuric acid is slowly added from 
a drop-funnel, causing a lively evolution of SO, gas. This gas is passed first 
into a small wash-bottle containing water and then into another flask of 
distilled water, which is kept cool by placing it in a larger vessel filled with 
cold water. When the evolution of the SO, begins to slacken, it can be 
accelerated by gentle warming. 


42 GRAVIMETRIC ANALYSIS. 


until it can be shown by the test applied under 1 that it is com- 
pletely free from the solution. The filtrate, containing both potas- 
sium and ammonium salts, is evaporated in a platinum dish to dry- 
ness, the ammonium sulphaté is removed by gentle ignition (the 
salt melts and gases are evolved), the residue is dissolved in as little | 
water as possible and transferred to a weighed platinum crucible. 
After being evaporated on the water-bath to dryness the bottom 
of the crucible is heated by means of a free flame to dull redness 
until SOs vapors cease to come off. # The crucible is allowed to cool 
in a desiccator and then weighed. A piece of ammonium carbonate 
the size of a pea is placed in the crucible (see below), which is 
again heated and weighed, the process being repeated until a con- 
stant weight is obtained. 

If a is the weight of substance taken and p the weight of the 
K,SO, obtained, then the percentage of potassium in . the potassium 
bichromate may be calculated as follows: 





KeS04: Ko =p:s 
174.27 :78.20=p:s 
9690 
~ 174,27" 
tos 20 
vA VT. Ws eghteohede 
_ 100X738. 20 p 
and 174.97 a =per cent. K. 


In order to determine the amount of potassium in organic salts, 
a weighed sample is placed in a large platinum crucible, moistened 
with a little concentrated sulphuric acid, and heated over the 
free flame exactly as in the case of igniting a moist precipitate 
(p. 29), placing the crucible in an inclined position and directing 
the flame against the cover of the crucible. Thick, white fumes of 
sulphuric acid are soon evolved; as soon as these begin to diminish 
in quantity the flame is gradually brought toward the base of 
the crucible, finally heating it to a dull red until no more vapors 
are given off. The mass remaining in the crucible now consists 
of K,SO, and K,S,0,. The latter compound can be converted 
by stronger ignition into K,SO, with loss of SO,, but as this proced- 
ure involves a slight loss of potassium it is preferable to add a little 
solid ammonium carbonate, by means of which the excess of sul- 


SEPARATION OF POTASSIUM FROM SODIUM. 43 


phuric acid is converted into ammonium sulphate, which is readily 
volatile and can be driven off at a much lower temperature. 


3- Determination of Potassium as K,PtCl, and as KCIO,. 


These determinations are only employed when it is necessary 
to effect a separation of potassium from sodium. We will, there- 
fore, first consider the determination of sodium itself and after- 
wards the separation of the two metals. 


SODIUM, Na. At. Wt. 23.00. 


Sodium, like potassium, is determined in the form of its chloride 
and of its sulphate, and the same precautions which were discussed 
under potassium hold in the case of sodium. It may be men- 
tioned, however, that NaCl and Na,SO, are more difficultly fusible 
and much less volatile than the corresponding potassium com- 
pounds. , 


Separation of Potassium from Sodium. 


The solution should contain salts of no other metals with the 
exception of ammonium salts. In order to separate the sodium 
and potassium they should both be present as chlorides, the com- 
bined weight of which being first ascertained. The mixture is 
then dissolved and the potassium precipitated out either as 
chloroplatinate or as perchlorate. From the weight of the pre- 
cipitate, the corresponding amount of potassium chloride can be 
calculated, which value is deducted from the weight of the com- 
bined chlorides; this gives the weight of sodium chloride origin- 
ally present. The sodium, therefore, is determined by differ- 
ence. 


A. Separation of the Potassium as K,PtCl,. 


Principle. —K,PtCl, is practically insoluble in absolute alcohol, 
whereas the corresponding sodium salt is soluble. On the other 
hand, sodium chloride is insoluble in absolute alcohol, so that it 
is absolutely necessary to convert both the potassium and the 
sodium to the form of their chloroplatinates, as otherwise the 
K,PtCl, obtained will be contaminated with sodium chloride and 
too high a value will be found for the amount of potassium present. 

Procedure: 1. Transformation of the Chlorides into Chloroplati- 


44 GRAVIMETRIC ANALYSIS. 


nates —The assumption made is that the weight of the two chlo- 
rides p consisted entirely of sodium chloride, and from this the 
amount of chloroplatinic acid necessary to convert the chloride 
into chloroplatinate can be calculated: 


2NaCl: Pt=pix 


Pt 


= NaCl -p=weight of Pt in H,PtCl, required. 


Since our reagent (Vol. I, p. 236) contains 10 per cent. Pt, we have 


Pt 
10 gm. Pt:100 ¢.c.= = Na SNacl?: 2x 
2NacT -p=c.c. H,PtCl, required. 


The solution of the two chlorides in water loco in a 
platinum or porcelain evaporating-dish) is treated with a few 
tenths more than the calculated number of cubic centimeters of 
H2PtCle and is then evaporated almost to dryness on the water-bath 
at as low a temperature as possible (the water should not boil). 
After cooling, the residue is treated with a few c.c. of absolute 
alcohol (best methyl] alcohol *), after which the solid mass is broken 
up into a fine powder by means of a stirring-rod or a platinum 
spatula. The liquid is then decanted through a filter moistened 
with alcohol, and the treatment of the residue with alcohol together 
with the breaking up into powder, etc., is repeated until the alcohol 
runs through the filter completely colorless and the salt remaining 
assumes a pure, gold-yellow color without any orange-colored parti- 
cles being present (NagPtCle+6H,O). The precipitate is then 
carefully transferred to the filter, the alcohol is allowed to com. 
pletely drain off, and the precipitate is dried in the hot closet at 
80°-90° C. The greater part of the precipitate is then placed 
upon a clean watch-glass, the filter is replaced in the funnel, and 
the precipitate which still adheres to it (and likewise any pre- 
cipitate adhering to the dish in which the original precipitation 
took place) is dissolved off by means of a little hot water into a 
weighed platinum dish or crucible. The precipitate is evaporated 





* Dupré, Inaugural Dissertation, Halle, 1893. 


SODIUM : MODIFICATION OF THE CHLOROPLATINATE METHOD. 45 


to dryness on the water-bath at as low a temperature as possible, 
and to it is now added the precipitate from the watch-glass. It is 
dried at 160° C. and weighed. The calculation of the amount of 
potassium chloride corresponding to the weight of the precipitate 
is performed as follows: 

The weight p of the potassium chloroplatinate is multiplied by 
0.3056 and this gives at once the weight of the potassium chloride. 

Remark.—The coefficient 0.3056 is used instead of the true 
factor (0.3068), because the potassium chloroplatinate precipitate 
does not exactly correspond to the formula K,PtCl,.* It con- 
tains, in fact, a little more chlorine, besides oxygen and hydrogen, 
which are not given off as water at a temperature of 160°C. We 
must assume that the chloroplatinic acid is decomposed slightly 
on evaporation, perhaps according to the following equation: 

H,PtCl, + H,O@HC1+ H,PtCl,OH. 

By this hydrolysis a mixture of the potassium salts (K,PtCl, 
+KHPtCl,OH) is obtained, but fortunately if the work is always 
done in the same way these compounds are always formed in the 
same relative amounts. Innumerable determinations have shown 
that correct results are obtained if the factor 0.3056 is used in the 
calculations. 


Modification of Chloroplatinate Method. 


Instead of weighing the K,PtCl,, the dry precipitate may be 
heated in a stream of hydrogen, when HCl and H,O will be given 
off and a mixture of platinum and potassium chloride will remain 
behind. 

1. If the amount of hydrochloric acid evolved is determined 
(p gm.) and from this the calculation of the potassium chloride 
made according to the following equation, 


K,PtCl,+ 4H =4HCl+ Pt+2KCl 
4HC]: 2KCl=p:x 

_ KC 

e SHC "Pp, 

' * Cf. Fresenius, Z. anal. Chem., 1882, p. 234. Also F. Dupré, “Die 
Bestimmung der Kaliums als Kaliumplatinchlorid,’’ Inaugural Dissert., Halle, 
1893. Also W. Dittmar and McArthur, J. Soc. Chem., Ind. 6, 799, and Ber., 
1888, Ref. 412. 


x 





46 GRAVIMETRIC ANALYSIS. 


the result will be too low because less HCl is evolved than corre 
sponds to the above equation. . 

2. If the mixture of platinum and potassium chloride remaining 
in the dish is weighed (p gm.) and the amount of potassium 
chloride is calculated according to the equation 


(Pt-+2KCl):2KCl=p:% 
ROP: 
~Pt+ 2K ? 


too low a result will be obtained. 

3. Finally, if the mixture of platinum and potassium chloride 
is treated with water and, on the one hand, the weight of the plati- 
num remaining undissolved and, on the other hand, the weight of 
the potassium chloride which goes into solution (by evaporating 
the solution and weighing the residue) is determined, then the 
amount of potassium chloride calculated from the weight p of the 
platinum 


Pt: 2KCl=p:2 


7 2KCl. 
Beas ay 


again gives a result which is too low; while the amount of potas- 
sium chloride found in the aqueous solution corresponds to the 
amount of potassium chloride originally present. 

Inasmuch as the precipitate of potassium chloroplatinate 
possesses a constant composition it is possible to determine experi- 
mentally by working with pure materials the exact ratio which 
exists between (a) the amount of hydrochloric acid evolvea, (6) 
the mixture of potassium chloride and of platinum remaining 
after the ignition, (c) the weight of platinum remaining undissolved 
after treatment of the residue with water and the amount of 
potassium chloride originally present. According to Dupré, if 
the amount of platinum determined according to 3 is multiplied 
by the factor 0.76142 the true amount of potassium chloride 
will be obtained. 


MODIFICATION OF THE CHLOROPLATINATE METHOD. 47 


As an example of the modified chloroplatinate method we 
have the 


Neubauer-Finkener Method. * 


This method does not require that the sodium and potassium 
shall be present as chlorides. It depends upon the precipitation 
of potassium chloroplatinate in the presence of ether-alcohol, 
igniting the precipitate (K,PtCl,, Na,SO, etc.) in hydrogen, 
washing out the soluble salts, and weighing the residual plati- 
num. 

Procedure—The solution containing about 0.5 gm. of sub- 
stance is poured into a large porcelain casserole and treated 
with a few drops of hydrochloric acid. Somewhat more than 
enough chloroplatinic acid to precipitate the potassium is added, 
and the solution evaporated on the water-bath until its volume 
does not appear to diminish perceptibly; an unnecessarily long 
heating is to be avoided. After cooling the mass is moistened 
with about 1 c.c. of water and carefully crushed with the end 
of a flattened stirring-rod; then at least 30 c.c. of alcohol (93-96 
per cent. by volume) are added in portions of 10 e¢.c., each 
time crushing the mass with the rod. If considerable sodium 
or potassium is present, the mass toward the end assumes a soft, 
cheesy consistency but eventually becomes hard and crystalline. 
The covered casserole is next allowed to stand for half an hour, 
rubbing the precipitate from time to time. Then the super- 
natant liquid is poured through a platinum Gooch crucible and 
the precipitate washed by decantation with alcohol. After 
each addition of alcohol the crystals are forcibly crushed with 
the rod. As soon as the filtrate passes colorless through the 
filter the precipitate is transferred to the crucible with alcohol, 
the alcohol is removed by washing six times with ether, and 
the latter by sucking air rapidly through the crucible. The 
crucible is covered, and through an opening in the cover hydro- 
gen (or illuminating-gas)t is passed and the crucible is heated, 
at first very gently, to avoid losses by decreptitation. After five 





* Z. anal. Chem., 1900, 485. 
+ As in the determination of copper as cuprous sulphide, cf. p. 161. 


48 GRAVIMETRIC ANALYSIS. 


minutes the flame of the burner is turned a little higher so that 
the bottom of the crucible just shows a faint redness in the 
center,* and this temperature is maintained for at least twenty 
minutes. It is then allowed to cool. The contents are next 
moistened with cold water, and hot water is sucked through 
the crucible fifteen times to remove the -soluble salts com- 
pletely. To remoye calcium sulphate, or other difficultly soluble 
salts, the crucible is filled with 5 per cent. nitric acid (not HCl), 
which is allowed to act for about half an hour, from time to time 
replacing with a little fresh acid. Then wash with hot water, 
dry and weigh the platinum from which the corresponding 
amount of KCl is obtained by multiplying by 0.7612,7 or of K2,O 
by using the factor 0.4811. 

If hydrochloric acid were used instead of nitric acid in the 
above treatment, the platinum would subsequently run through 
the filter in a colloidal condition. 





*The crucible should be placed upon a piece of platinum foil so that 
the flame does not come directly in contact with the perforation in the bottom 
of the Gooch crucible. 

+t According to Neubauer the coefficient 0.7612 must be used in the 
presence of sulphates, and Dupré found that the factor 0.7614 holds in the 
case of chlorides. Similarly, Dittmar and McArthur (Z. anal. Chem., 28, 
767) state that the factor 0.7611 applies in the presence of much magne- 
sium. 


i Kling and Engels, Z. anal, Chem., 45, 315 (1906). 


DETERMINATION OF SMALL AMOUNTS OF POTASSIUM. 49 


Determination of Small Amounts of Potassium in the Pres- 
ence of Considerable Sodium. 


The solution may contain sodium, potassium, calcium,and mag- 
nesium in the form of their chlorides or sulphates, ete. Hydro- 
chloric acid gas is conducted into the <~ ACL 
solution, which has been concentrated as 
much as possible, until it has become sat- 
urated with the gas (the lower end of the 
delivery-tube should be enlarged, as indi- 
cated in Fig. 23, and should not dip into 
the liquid). ‘To every 100 c.c. of the solu- 
tion 2 c.c. of water are now added, the 
precipitated sodium chloride is allowed to 
settle and the solution poured through a 
funnel provided with a platinum filter- 
cone. The precipitated salt is washed three times by decantation 
with 95 per cent alcohol, transferred to the funnel, dried by suction 
and then washed three times more with alcohol. 

In the solution there remains all of the potassium, some sodium, 
and possibly calcium, magnesium, and sulphuric acid, 

The solution is evaporated to dryness on the water-bath if possi- 
ble (or if sulphuric acid is present the last traces of the free acid 
are removed by means of the free flame), the residue is weighed, 
for every decigram of the salt mixture 3 c.c. of double-normal 
hydrochloric acid are added with more than enough chloroplatinic 
acid to precipitate all of the potassium, and the liquid is evaporated 
to a paste. It is then treated with 20 c.c. of absolute alcohol, 
and well stirred. After standing five minutes 5 c.c. of ether are 
added, the mixture is allowed to stand half an hour under a 
bell-jar, and then filtered. As the residue often contains small 
amounts of other chloroplatinates, it should be purified as 
follows: The precipitate is allowed to dry in the air, it is dissolved 
in a little hot water, a few drops of chloroplatinic acid are added, 
and the above operation is repeated. The precipitate thus 
obtained contains all of the potassium in the presence of some 
sodium chloride and possibly sodium sulphate. It is washed 
with a mixture of ether and alcohol until the liquid runs through 











52 GRAVIMETRIC ANALYSIS. 


the filter completely colorless, after which the precipitate is dried, 
moistened with hot water, and digested on the water-bath with a 
few drops of chemically pure mercury,* constantly stirring with a 
glass rod, until the liquid appears perfectly colorless. 

By means of this treatment the potassium chloroplatinate is 
completely decomposed with separation of platinum: 


K,PtCl, + 4He=2KCl+ 2He,Cl,+ Pt 
or et 
K,PtCl, + 2Hg=2KCl+ 2H¢Cl,+ Pt. 


The mixture is thoroughly dried on the water-bath and gently 
ignited until the mercury is all volatilized; the platinum is changed 
at the same time to a denser form, which can be readily washed by 
decantation. After cooling, the mass is treated with water, the 
aqueous solution is decanted through a filter, and the residual 
metal is washed with hot water, dried, and cautiously ignited. The 
filter is ignited in a platinum spiral, its ash is added to the main 
portion of the platinum in the crucible which is now ignited, and 
weighed. The weight obtained p multiplied by 0.7612, gives the 
amount of potassium chloride, or multiplied by 0.3994, gives the — 
corresponding amount of potassium. 

The above method serves excellently for the estimation of 
small amounts of potassium in mineral waters. 


Separation of Potassium from Sodium by the Perchlorate 
Method. Schléssing-Wense.{ 


Principle-—This separation depends upon the insolubility of 
potassium perchlorate and the solubility of sodium perchlorate in 
97 per cent alcohol. 

The procedure is as follows: 

The chlorides of the two metals (sulphates must not be present 
on account of the insolubility of sodium sulphate in alcohol) are 
disso'ved, after weighing, in 20 c.c. of hot water, treated with 14 





* Sonnstidt, Z. f. anal. Chem., 36, 501. 
+ Z. angew. Chem., 1891, 691, and 1892, 233. 


SEPARATION OF POTASSIUM FROM SODIUM. St: 


- times as much of the perchlorate solution and evaporated with 
stirring to a syrupy consistency. A little hot water is added and 
the liquid evaporated with constant stirring until all of the hydro- 
chloric acid is expelled and heavy fumes of perchloric acid are given 
off, when a little more water is poured over the residue and the solu- 
tion is again evaporated with stirring. The perchloric acid lost by 
volatilization is replaced from time to time. After cooling, the 
mass is treated with about 20 c.c.of 97 per cent. aleohol to which 0.2 
per cent. by weight of perchloric acid has been added and the mix- 
ture is vigorously stirred.* It is important, however, not to break 
up the crystals of potassium perchlorate into too fine a powder as the 
latter would readily pass through the asbestos filter. After allow- 
ing the precipitate to settle the alcohol is decanted off through a 
Gooch crucible, the residue is washed again with the wash-alcohol, 
and the excess of the latter is removed by gentle warming The 
residue is dissolved in 10 c.c. of hot water, to which a little per- 
chloric acid has been added, and the solution is evaporated with 
stirring until fumes of perchloric acid are given off. One c.c. of 
the wash-alcohol is added; and, in order to avoid the employment 
of an excess of the latter, the precipitate is transferred by means of 
a rubber ‘‘policeman”’ to a Gooch crucible, washed with 50-75 c.c. 
of 97 per cent. alcohol, dried at 130° C., and weighed.f 

In case sulphuric acid was originally present, it is removed 
by precipitation with barium chloride. It is not necessary to 
~ remove phosphoric acid, but, in case this acid is present, it is advisa- 
ble to allow the potassium perchlorate to stand for some time 
with an excess of perchloric acid before treating it with alcohol. 

Preparation of Perchloric Acid According to Kreider.t—From 
100-300 gms. of commercial sodium chlorate (NaClO3) are placed 
in a round-bottomed flask and gradually heated until oxygen begins 
to be evolved slowly. This temperature is maintained until the 





* 100 e.c. of this alcohol dissolve about 5 mg. KClO, whereas 100 c.c. 
97.2 per cent. alcohol containing no free perchloric acid will dissolve 0.0158 g- 
KCIO, (W. Wense). 

+ Using this method, R. Fitzenkam obtained in three experiments, 
100.11, 100.04, 100.24, mean 100.13 per cent. of the potassium chloride taken. 

{ Z. anorg. Chem. IX, 342. 


52 GRAVIMETRIC ANALYSIS. 


mass becomes solid (requiring 14—2 hours), whereby the chlorate is 
almost completely changed to perchlorate and chloride. 

After cooling, the melt is dissolved in water, sufficient hydro- 
chloric acid is added to decompose any chlorate remaining, and the 
solution is evaporated to dryness (the liquid being constantly 
stirred from the time crystals begin to separate out.) 

The dry mass is broken up with a stirring-rod and then treated 
in a tall beaker with an excess of concentrated hydrochloric acid, 
by means of which sodium chloride geparates out after a few min- 
utes. The solution (it now contains perchloric and hydrochloric 
acids in tae presence of small amounts of sodium chloride) is poured 
through a Gooch crucible and the residue is washed once or twice 
bv decantation with concentrated hydrochloric acid. The filtrate 
is evaporated on the water-bath until the hydrochloric acid is 
completely expelled and heavy white fumes of perchloric acid are 
evolved. 3 ; 

Inasmuch as commercial sodium chlorate is often impure,it is 
necessary to test the perchloric acid, which has been prepared, for 
potassium. For this purpose a small amount of the solution is 
evaporated on the water-bath to dryness and the residue is treated 
with 97 per cent. alcohol, which will dissolve it readily in the absence 
of potassium perchlorate. If potassium is found to be present, 
the melt obtained by heating the sodium chlorate as above de- 
scribed is treated with HCl and evaporated to dryness in order to 
decompose any sodium chlorate remaining. The residue is finely 
powdered and treated with 97 per cent. alcohol (1 c.c. dissolves 0.2 
gm. NaClOs3) and filtered, and the process repeated until a little of 
the alcoholic solution when evaporated to dryness leaves abso- 
lutely no residue. 

The alcoholic solution, which is now free from potassium, is 
distilled from a spacious flask until the perchlorate begins to crys- 
tallize out, when it is poured rapidly into an evaporating-dish, 
evaporated to dryness, and treated as previously described, with 
hydrochloric acid, ete. 

One c.c. of a potassium perchlorate solution prepared accord- 
ing to the above directions gave a residue of 0.0369 gm. which 
was completely soluble in 97 per cent. alcohol, as it should be. 

In order to ascertain the approximate amount of perchloric 


DETERMINATION OF LITHIUM, POTASSIUM, AND SODIUM: 53 


acid contained in the solution, 1 ¢.c. should be treated with an 
excess of KCl, evaporated to dryness, treated with an excess of 
97 per cent. alcohol, filtered through a Gooch crucible, and washed 
until the filtrate shows no turbidity on being treated with silver 
nitrate solution. The precipitate is then dried and weighed. 


LITHIUM, Li. At. Wt. 6.94- 
Forms: Li,SO, and LiCl. 


The determination of lithium in the form of the above salts is 
carried out in practically the same way as in the case of potassium. 
It should be mentioned, however, that on evaporating a lithium 
salt with concentrated sulphuric acid the acid salt, LiHSO,, is 
formed, which on gentle ignition (even without the addition of 
ammonium carbonate) is changed to difficultly volatile Li,SO,. 

Since lithium chloride is a very hygroscopic salt, it is necessary 
to weigh it out of contact with moist air. To accomplish this, 
the platinum crucible, after being gently ignited. is placed in a 
desiccator which is provided with a calcium-chloride tube, and 
beside the crucible is placed a weighing beaker with ground glass 
stopper. After both crucible and beaker have assumed the tem- 
perature of the room, the former is quickly placed. within the 
latter which is then stoppered. It is allowed to stand for 20 min- 
utes in the balance case and then weighed. The salt is then placed 
in the crucible and the above process repeated. 


Determination of Lithium, Potassium, and Sodium in the 
Presence of One Another. 


After determining the weight of the combined chlorides, the 
potassium is determined in one portion as K,PtCl,, and in a second 
portion the lithium is determined according to one of the following 
methods: 


(a) Gooch’s Method.* 


- Principle—Anhydrous LiCl is soluble in anhydrous amv* 
alcoho] (15 parts of amyl alcohol dissolve in the cold 1 part ot 
LiCl, or 10 c.c. dissolve 0.66 gm. LiCl) while KCl and NaCl are 


* Proceedings of the Am. Acad. of Arts and Sciences. 22 [N. S. 14], 177. 





54 GRAVIMETRIC ANALYSIS. 


difficultly soluble in this liquid (solubility of NaCi=1:30,000 of 
KCl=1: 24,000). 

_ Procedure. —The solution, after having been concentrated as 
far as possible, and which should not contain more than 0.2 gm. 
LiCl, is placed in a 50 c.c. Erlenmeyer flask, 5-6 c.c. of amyl alcohol 
(boiling point 132°C.) are added and the flask is placed upon an 
asbestos plate and cautiously heated. The aqueous solution at 
the bottom of the beaker soon begins to boil and the water vapor 
escapes through the upper layer*of amyl alcohol.* As soon as 
all the water has been boiled off. the chlorides of sodium and 
potassium separate out, while the greater part of the lithium 
chloride is to be found in the alcoholic solution. During the 
evaporation of the aqueous LiCl solution, however, some LiOH is 
formed by hydrolysis, and the latter compound is insoluble in 
amyl alcohol. In order to bring this completely into solution, 
the clear amyl alcohol solution is treated with 2-3 drops of concen- 
trated hydrochloric acid, boiled two or three minutes and filtered 
while still warm through a small asbestos filter. The crust which 
remains is composed of sodium and potassium chlorides and is 
washed with hot amyl alcohol, which has been boiled. The fil- 
trate is evaporated to dryness, and the residue is dissolved in a 
little water after the addition of some dilute sulphuric acid. The 
solution is filtered from the carbonaceous residue into a weighed 
platinum crucible, evaporated as far as possible on the water- 
bath, the excess of sulphuric acid is removed by gentle heating 
over a flame (the crucible being held in an inclined position) and 
it is then weighed. The lithium sulphate thus obtained always 
contains small amounts of potassium and sodium sulphates in 
case these metals were present, so that from the weight obtained, 
0.00041 gm. should be deducted for every 10 ¢.c. of the filtrate 
(exclusive of the alcohol used in washing the residue) in case only 
sodium chloride is present, or 0.00051 if only potassium chloride is 
present, and 0.00092 if both sodium and potassium chlorides are 
present 





* To prevent loss by bumping at this point, the flask should be fitted 
with a cork stopper through which two tubes pass. If air is drawn through 
the liquid during the boiling, the water evaporates more quickly and without 
bumping. 


DETERMINATION OF LITHIUM, POTASSIUM, AND SODIUM. 55 


If 10-20 mgm. of lithium chloride were present in the original 
salt mixture, then the residue obtained after filtering and wash- 
ing with amyl! alcohol is dissolved in a little water and the above 
treatment is repeated, the lithium being determined in the com- 
bined filtrates. 

This method is very accurate, and, in the author’s opinion it 
is to be preferred to all other methods for the determination of 
lithium. 


(b) Rammelsberg’s Method. 


Principle.—Anhydrous lithium chloride is soluble in a mixture 
of equal parts alcohol and ether which has been saturated with 
hydrochloric acid gas, whereas the chlorides of sodium and potas- 
sium are practically insoluble therein. 

Procedure.—The solution of the chlorides is evaporated to dry- 
ness in a small flask made of Jena glass and provided with a 
ground-glass, two-way stopper (p. 34, Fig. 21a). During the evapo- 
ration a current of dry air is passed into the flask through the long 
tube @ and out through the short tube 6. As soon as the resi- 
due has become dry the flask is placed in an oil-bath and heated 
for half an hour at 140-150° C., during which time dry hydro- 
chloric acid gas is passed through the flask. The flask and its con- 
tents are allowed to cool with the hydrochloric acid still passing 
through the flask, after which the residue is treated with a few 
cubic centimeters of absolute alcohol, which has been saturated with 
hydrochloric acid gas and thereupon diluted with an equal volume of 
absolute ether. The flask is tightly stoppered and allowed to stand 
with frequent shaking for 12 hours. The solution is then poured 
through a filter, wet with the ether-alcohol mixture. and the residue 
is washed three times by decantation with ether-alcohol. A few 
more cubic centimeters of ether-aleohol are added to the contents 
of the flask and it is again allowed to stand for 12 hours; the liquid 
is then poured off and the residue is washed with ether-alcohol 
until a trace of the residue tested in the spectroscope shows the 
complete absence of lithium. The ether-alcohol extract is care- 
fully evaporated to dryness in a water-bath containing lukewarm 
water, the residue is dissolved (after the addition of a little dilute 
sulphuric acid) in as little water as possible, transferred to a weighed 


56 GRAVIMETRIC ANALYSIS. 


platinum crucible and treated with sufficient sulphuric acid to 
transform the lithium chloride present completely into sulphate.* 
The solution is evaporated as far as possible on the water-bath, . 
then cautiously over the free flame, after which it is gently ignited 
and the residue of lithium sulphate is weighed. 

Remark.—In the presence of considerable sodium and potas- 
sium salts it is advisable to remove the greater part of these by 
precipitation with hydrochloric acid gas (ef. p. 49), filtering 
through asbestos and washing th precipitate with concentrated 
hydrochloric acid until the residue no longer gives the lithium 
spectrum. The results obtained by this method are satisfactory. 

Besides the above methods for the separation of lithium from 
sodium and potassium there are two other methods to be men- 
tioned; that of W. Mayer + and that of A. Carnot.{t According 
to Mayer the lithiuin is precipitated in the presence of NaOH as 
Li,PO,, which, after being washed with ammonia water, is ignited — 
and weighed. Rammelsberg, however, claims that the Li,PO, 
always contains some sodium, so that the method is inaccurate. 
A great many experiments tried in the author’s laboratory have 
led to the same conclusion. 

According to Carnot the lithium is separated as the fluoride and 
then transformed to the sulphate. Walter§ claims that this 
method is accurate but tedious. 

Example for practice: Lepidolite analysis.- (See Index.) 


Indirect Determination of Lithium and Sodium. 


The mixture of the two chlorides is weighed and the chlorine 
determined either gravimetrically or volumetrically. (See p. 3.) 


Indirect Determination of Lithium and Potassium. 
The method is the same. 


* The above-described method has been modified by the author. Ram- 
melsberg evaporates the chlorides in the water-bath, heats the residue till 
it melts and then after cooling extracts with ether-alcohol. By the evapora- 
tion and fusion of the lithium chloride there is formed some lithium hy-— 
droxide which is changed by the carbonic acid of the air to carbonate. 
Lithium carbonate is insoluble in ether-aleohol so that the extraction with 
ether-alcohol is not complete. 

7 Ann. Chem. Pharm., 98, 193, and Merling, Z. anal. Chem., 18, 563. 

t Z. anal. Chem., 29, 332. § The Analyst, 16, 209. 





AMMONIUM. 57 


AMMONIUM NH,. Mol. Wt. 18.04. 
Forms: NH,, NH,Cl, (NH,),PtCl,, Pt, N. 


We have two cases to distinguish: 

1. The ammonium is present as chloride in aqueous solution. 

2. The ammonium is present in solution, together with other 
cations and anions. 

1. The solution contains i NH, and Clions. In this case the 
solution may be evaporated to dryness and the residue of ammo- 
nium chloride weighed; or the ammonium can be precipitated as 
(NH,),PtCl, and the precipitate weighed; or the ammonium 
chloroplatinate can be ignited and the residue of platinum weighed. 


(a) Determination as NH,Cl. 


The aqueous solution is treated with concentrated HCl and 
evaporated to a small volume on the water-bath at as low a temper- 
ature as possible, the solution is transferred to a platinum crucible 
(or one of porcelain), evaporated on the water-bath to dryness, and 
the covered crucible is dried to constant weight in a drying-oven. 
Good results are obtained, but they are always too low. On 
evaporating the aqueous solution some NH,Cl is driven off, and 
the amount lost increases in proportion to the amount of water 
used and the temperature at which the evaporation takes place, 
on account of the NH,Cl being ae 3 decomposed according to the 
equation 


NH,CI2NH,+ HCl 


into NH, and HCl, both of which are volatile.* 

If, on the other hand, a little hydrochloric acid is added to the 
solution the dissociation is for the most part prevented so that 
the loss is reduced to a minimum. The ammonium chloride must 
be dried in a covered crucible as otherwise a small amount of the 





* In cold, aqueous solution the NH,Cl undergoes ‘Slectrolytic dissociation 
simply: 


NH,Cl= NH, +C?. 


—~58 GRAVIMETRIC ANALYSIS. 


salt will be lost, but this amount is small in comparison with the 
amount which it is possible to lose during the evaporation. 


(6) Determination as (NH,),PtCl,. 


On heating (NH,),PtCl, to 130°C. the salt is unchanged; it 
suffers no dissociation, therefore, up to 130°C. In aqueous solu- 
tion it undergoes only electrolytic dissociation so that the above 
salt loses no (NH,) during the evapogation of its aqueous solution. 

The aqueous solution of ammonium chloride, therefore, is 
treated with an excess of chloroplatinie acid and a little hydro- 
chloric acid and evaporated at as low a temperature as possible 
to dryness. The residue is taken up in absolute alcohol and 
filtered through a Gooch crucible, dried at 130° C., and weighed. 
From this weight, the amount of ammonium chloride originally 
present can be correctly calculated by using the old atomic weight 
of platinum: Pt=196.9. If the new value for the atomic weight 
of platinum (Pt= 195.2) is used, too high a value will be obtained 
for the amount of ammonium present, as was explained in the 
case of potassium. . 

If the weight of the (NH,),PtCl,=p, then 


pX0.2400 =NH,Cl 
p X 0.08095 = NH, 
p.X 0.07643 = NH3. 


(c) Determination as Platinum. 


Instead of weighing the (NH,),PtCl, as such, it can be decoms 
posed by ignition* and the weight of the platinum remaining 
determined. If the old value for the atomic weight of platinum 
(196.9) is used in this determination the results obtained will be 





* As ammonium chloroplatinate decrepitates strongly on being heated, 
the ignition must take place in a large porcelain crucible which is provided 
with a close-fitting cover. The precipitate must be heated gradually at 
first to prevent loss. It is best ignited according to the directions of Rose. 
The precipitate and filter are placed in the crucible with the filter-paper on 
top, the crucible is covered and heated over a very small flame until the 
paper is completely charred without allowing the vapor to escape visibly 
from the crucible. The crucible is then strongly ignited with free access 
of air until the charred filter is completely consumed. 


AMMONIUM PRESENT WITH OTHER CATIONS. 59 


about 0.4 per cent too low, while if the new value (195.2)* is 
used, the results will be about 0.8 per cent too high. 

Correct results can be obtained by multiplying the weight of 
platinum (p) by the following factors: 


px 0.54527 = NH,Cl; 
p xX 0.18391 =NH,; 
p< 0.17364 = N Hs. 


2. The Ammonium is Present Together with Other Cations 
and Anions in Solution or in Solid Form. 


(a) The solution is distilled after the addition of a strong base 
(NaO H—Ca(OH),t), the ammonia evolved is absorbed in hydro- 
chloric acid and the resulting solution is analyzed according to 1. 





Fig. 24. 


Procedure.—About 1 gm. of the substance to be analyzed is placed 
in the 400-500 c.c. Erlenmeyer flask K, it is dissolved in 200 c.c. 
of water, a few drops of litmus solution are added, and in case the 


— 





*In analyzing chloroplatinates of organic bases (by weighing the plat- 
inum) correct results may be obtained by using for the atomic weight of 
platinum the value 196.9. 

+ MgO is frequently recommended for expelling the ammonia. Accord- 
ing to a private communication from Herrn Bormann, of Neunkirch, this 
base is absolutely unsuited for this purpose. 


60 GRAVIMETRIC ANALYSIS. 


solution reacts acid, sodium hydroxide solution (which has been 
previously boiled to expel traces of ammonia) is slowly added at TJ’ 
with constant shaking until the solution changes to blue, after 
which ten c.c. more of the caustic soda solution are added.* The 
liquid is then heated to boiling and 100 c.c. of it is carefully distilled 
into the receiver V, which already contains 20 cc. of 2 N. hydro- 
chloric acid. In order to make sure that no ammonia escapes 
from the receiver it is well to connect it with a small Peligot tube 
containing 5 ¢.c. of 2 N. hydrochloric acid and some distilled 
water. 

After 100 c.c. of the liquid have distilled over, all the ammo- 
nia will be found in the receiver and can be determined according 
to 1 (a) or 1 (6); preferably the latter. The determination can 
be carried out much more quickly, however, if the receiver contains 
a measured amount of standardized acid and the excess is deter- 
mined afterthe distillation by titrating with alkali (cf. Alkalimetry). 

It is also possible to make an accurate determination of the 
amount of ammonia present by measuring the volume of the gas.} 


Colorimetric Determination of Ammonium. 


For the determination of such small amounts of ammonia as 
occur in drinking-water, the above methods are not suited. In this 
case the procedure is the same as was described in Vol. I, p. 46. 
(In the case of mineral waters it is necessary to add more than one 
drop of the soda solution; the amount necessary is determined 
by adding litmus to a definite volume of the water and then adding 
the soda solution until the litmus changes to blue.) The distillate 
is received in 50 c.c. graduated Nessler tubes (in the fourth 
one there is usually no ammonia to be detected) and these are 
Nesslerized. The 50 c.c. of distillate is mixed with 2 c.c. of the 
Nessler sclution and the yellow color produced is compared with 
the colors produced in the same way from a series of tubes contain- 
ing known amounts of ammonia. When a standard is found of 





* The separatory funnel 7 should be roughly calibrated before setting 
up the apparatus, by pouring water into it, one cubic centimeter at a time, 
and marking with a pencil the level of the liquid on the glass, 

+ Cf. Part III, Gas Analysis. 


COLORIMETRIC DETERMINATION OF AMMONIUM. 61 


the same shade as the solution tested, then the two solutions contain 
the same amount of ammonia. 

The ammonium chloride solution necessary for preparing the 
standards is prepared as follows: 

3.141 * gms. of ammonium chloride which has been dried at 
100° C. are dissolved in 1 liter of water free from ammonia (cf. foot- 
note, Vol. I, p. 47). The solution now contains 
1 mgm. of ammonia (NH,) per cubic centimeter; 
this, however. is too strong for most purposes, so 
that 10 cc. cf it are taken and diluted to 1 
liter. Of this solution 1 ¢.c. contains 0.01 mgm. 
NH,. If the water to be analyzed contains 
considerable ammonia, a smaller portion should 
be taken for the analysis than in ordinary cases 
(500 ¢c.c.) as otherwise the first distillate (50 c.c.) 
would give too intense a color with the Nessler 
solution. In such a case only 50 c.c. of the 
water should be taken for the analysis and this 
should be diluted to 500 c.c. with water free 
from ammonia and then distilled. 

In order to ascertain how much of the water 
to take for the analysis, the following experiment should be made: 

About 100 c.c. of the water to be tested are placed in a narrow 
cylinder (which is provided with a ground-glass stopper), 2 c.c. of 
a strongly alkaline sodium carbonate solution ¢ are added to pre- 
cipitate the calcium which may be present, the mixture is vio- 
lently shaken and allowed to settle. From the clear supernatant 
liquid 50 c.c. are pipetted off into a Nessler tube, treated with 
2 c.c. of Nessler solution and mixed.{ If a strong yellow color, or 





Fic. 25. 





* NH,Cl+ NH, = 53.50 + 17.03 =3.141. 

+50 gms. NaOH and 50 gms. Na,CO, (calcined) are dissolved in 600 c.e. 
of pure distilled water and the solution boiled until the volume is only 500 c.c. 

t In the case of mineral waters rich in magnesium sulphate, the addition 
of “he 10 c.c. of sodium carbonate solution often fails to prevent a turbidity 
on adding the Nessler reagent, which would render a colorimetric determina- 
tion impossible. In this case 10 c.c. of a boiled BaCl, solution (120 gms. 
BaCl,+2H,O in 500 c.c. H,O) should be added before treating the water 
with the sodium carbonate solution. 


62 GRAVIMETRIC ANALYSIS. 


even. a precipitate, is obtained, then only 50¢.c. of the water should 
be taken for analysis. If, on the other hand, there is not more than 
a faint coloration apparent, then 500 c.c. must be taken for the 
determination. . 

For the Nesslerization, the three cylinders each containing 
50 c.c. of the distillate are placed over a sheet of white paper, 
treated with 2 c.c. of the Nessler reagent, and mixed. Beside 
them are placed a series of similar cylinders containing respectively 
0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 ¢.c? of the standard ammonium 
chloride solution diluted to 50 ¢.c. These are also treated with 
2 c.c. of the Nessler reagent and by matching the colors obtained 
in the test with those obtained from known amounts of ammonia 
the amount present in the water can be easily estimated. 

The Nessler reagent should give a distinct coloration with 500 c.c. 
of water containing 0.005 mgm. NH,; if this is not the case, it must 
be made more sensitive by the addition of mercuric chloride solu- 
tion. | 
For mixing the liquid in the cylinders it is convenient to employ 
a stirrer such as is shown in Fig. 25, the diameter of the bulb on 
the end being only slightly less than that of the cylinder. By 
moving this stirrer up and down twice the liquid becomes thor- 
oughly mixed. 


Kjeldahl’s Method for Determining Nitrogen. 


The methods which have been described thus far are suitable 
only for the determination of nitrogen when it is in solution in 
the form ot NH, ions. It is, however, of great importance to be 
able to determine nitrogen when it is present other than as an 
ammonium compound (in protein, coal, etc.). Inasmuch ag it 
is possible to determine the amount of ammonia present very accu- 
“rately, and by the employment of volumetric methods, very quickly, 
methods were sought for the transformation into ammonia of the 
nitrogen oviginally present in some other form. This is readily 
brought about by the method of Kjeldahl and its modifications. 

By the oxidation of nitrogenous organic substances with cou- 
centrated sulphuric acid, potassium permanganate, mercuric oxide, 
etc., the organic matter is destroyed and the nitrogen is completely 
changed to ammonium and held as ammonium sulphate, from 
which the ammonia an be readily distilled off. 


KJELDAHL’S METHOD FOR DETERMINING NITROGEN 63 


Procedure for Kjeldahl’s Nitrogen Determination (Wilfarth’s 
Modification*).—From 1 to 2 gms. of the substance to be analyzed 
are placed in a 500-600-c.c. flask, made of difficultly fusible potash 
glass, and to it are added 20 c.c. of sulphuric acid (3 vo_umes of con- 
centrated acid mixed with 2 volumes of fuming sulphuric acid) an? 
afew drops of mercury.t The flask is then heated in an iron dish 
covered with asbestos until its contents gently boil. It is impor- 
tant, however, to be sure that the substance should -be thoroughly 
moistened by the sulphuric acid before the heating, especially in 
the case of mealy substances. In order to avoid a loss of 
nitrogenous matter, it is first heated for half a minute over a 
very small flame and then over a larger one, but in no case 
should the flame touch the flask above the part occupied by the 
liquid, 

The heating is continued until the solution becomes clear and 
completely colorless. In the presence of iron compounds, how- 
ever, the liquid is sometimes slightly yellow. The decomposition 
is usually complete in two or three hours. The liquid is then allowed 
to cool, the sides of the flask are washed down and the solution is 
diluted with about 250 c.c. of water. After it is thoroughly cool, 
80 c.c. of caustic soda solution, free from nitrate, are quickly added 
and sufficient potassium sulphide solution (40 gms. commercial 
potassium sulphide to the liter) to completely precipitate the mer- 
cury and cause the liquid to appear black (25 c.c. of the potassium 
sulphide solution are usually sufficient). A few grains of powdered 
zine are then added and the flask is quickly connected with the distil- 
ling apparatus. The distilling-tube dips into a 250-300-c.c. Erlen- 
meyer flask containing a known volume of normal sulphuric acid 
(10-20 ec.c.) and sufficient water to cover the lower end of the con- 
denser tube. As soon as a noticeable amount of water vapor 
begins to come over, it is no longer necessary to have the condenser 
tube dip into the sulphuric acid solution in the receiver. After 
100 c.c. of the liquid have distilled over, the receiver is removed 
and the excess of sulphuric acid is determined by titration with 





* Chem. Ztg., 9, 502. 


{ Gladding uses a mixture of H,SO, and KHSO,, which usually works 
very well. Then, as no mercury is added, the subsequent addition of Na,S 
is unnecessary. 


64 GRAVIMETRIC ANALYSIS. 


one-tenth normal sodium hydroxide solution, using methyl orange 
as an indicator. * 

From the amount of sulphuric acid used, the amount of 
nitrogen present’ may be calculated as follows: Let ¢ be the 
number of cubic centimeters of normal sulphuric acid neutralized 
by the ammonia evolved from a gms. of the substance, then this 
corresponds to 

tx 0.01401 gms. nitrogen, 
and the percentage of nitrogen in the*substance is 
a:t 0.01401=100:z, 
_1,401xt 


ot a pee cent. nitrogen. 


If the nitrogen is originally present to a considerable extent in 
the form of nitrates, oxides, or cyanides, the above modifization of | 
_ Kjeldahl’s method will not serve to change all of the nitrogen into 
ammonia. In such cases it is best to use the modification proposed 
by M. Jodlbauer :} 

From 0.2-0.5 gm. of potassium nitrate (or the corresponding 
amount of another nitrate) are treated with 20 ¢.c. of concentrated 
sulphuric acid and 2.5 c.c. of phenolsulphonice acid (50 gms. of 
phenol dissolved in enough concentzated sulphuric acid of 66° Bé. 
to make 100 ¢e.c. of solution) 2-3 gms. of zinc dust and 5 drops of 
chloroplatinic acid are added and the mixture heated. After 
heating the substance with this mixture for four hours, the liquid 
becomes colorless and is ready to be distilled with the caustic soda 
solution. 





* In the titration it is best to add. the alkali until the solution turns 
yellow, and then finish by adding enough acid to get a change to a pale pink. 
In standardizing the acid and alkali, the volume of the solution and the 
method of titrating must be the same as during the analysis Piaer- 

¢ Z. anal. Chem., 27, 92. 


DETERMINATION OF MAGNESIUM. 65 


MAGNESIUM, Mg. At. Wt. 24.32. 
_ Forms: MgSO,, MgO, Mg,P,0.,. 
(a) Determination as MgSO,. 

This method for the determination of magnesium can always 
be employed when the magnesium is combined with an acid which 
can be volatilized by heating with sulphuric acid, and when no 
other metal besides ammonium is present. A weighed amount 
of the substance is placed in a platinum crucible and treated with 
a viight excess of concentrated sulphuric acid,* the mixture is 
evaporated on the water-bath as far as possible, and the excess of 
free sulphuric acid is removed by cautiously heating the crucible, 
- held in an inclined position, over a free flame. JT inally the dry 
mass is heated just to redness in a covered crucible, and after cool- 
ing in a desiccator, is weighed as quickly as- possible, as the 
anhydrous magnesium sulphate is hygroscopic. 


(6) Determination as MgO. 


This method is seldom used in practice, and then only in case 
the magnesium is present in a form that can be readily changed to 
the oxide by ignition—i.e., as carbonate, nitrate or salt of an organic 
acid.| The procedure consists simply of at first carefully heating 
in a covered crucible, and finally with the full heat of the Teclu 
burner in a half-covered crucible. 


(c) Determination as Magnesium Pyrophosphate. 


This, the most important of all the methods for the determina- 
tion of magnesium, is always applicable and depends upon the 
following principles: If the solution of a magnesium salt is treated 
with an alkali orthophosphate solution in the presence of ammonium 
chloride and ammonia, the magnesium is completely precipitated 





* Substances which react violently with concentrated H,SO, should be 
first treated with water, and dilute sulphuric acid ‘added little by little. 

+ Magnesium chloride can be changed to the oxide by ignition with mer- 
cur-¢c oxide in a porcelain evaporating-dish. Mercurie chloride and the excess 
of mercuric oxide are volatilized. In this way magnesium is often separated 
from the alkalies. (Translator.) 


66 GRAVIMETRIC ANALYSIS. 


as magnesium amrnonium phosphate, which by ignition is changed 
to magnesium pyrophosphate: 


2MgNH,PO,—2NH,+H,0+ Mg,P,0,,. 


Formerly it was a common practice to precipitate magne- 
sium ammonium phosphate in the cold. Neubauer* showed, 
however, that this sometimes leads to high results while at 
other times the results are low. The latter is the case when the 
precipitation takes place in strongly ammoniacal solutions con- 
taining but little ammonium salts, particularly when the phos- 
phate solution is added slowly. Tribasic magnesium phosphate, 
Mg3(PO,4)2, contaminates the precipitate. On the other hand, 
the results are too high if the precipitation takes place in neutral 
or slightly ammoniacal solution in the presence of considerable 
ammonium salts. In this case more or less monomagnesium 
ammonium phosphate, Mg(NH4)4(PO,4)2, is formed. This com- 
pound is changed to magnesium metaphosphate on gentle 
ignition. 

2Mg(NH4)4(PO4)2 = 2Mg(PO3)2+8NH3+4H20, 


and the results are too high. When only a little of the meta- 
phosphate is present, the temperature of the blast-lamp will 
eventually lead to volatilization of some phosphorus pentoxide, 
so that nearly correct results are then obtained. 


2Mg(PO3)2=Mg2P207+ P2035. 


Neubauer recommends adding an excess of sodium phosphate to. 
the slightly acid solution of the magnesium salt, then stirring 
in one-third of the solution volume of 10 per cent. ammonia, 
filtering after four or five hours, washing with 2.5 per cent. 
ammonia, dissolving the precipitate in a little dilute hydro- 
chloric acid, adding a few drops of sodium phosphate solution, 
and precipitating by the addition of one-third volume of 10 per 
cent. ammonia. This method, however, gives too high results, 





-*Z, angew. Chem., 1896, 439. See also Gooch and Austin, Z. anorg. 
Chem., 20, 121. 


DETERMINATION OF MAGNESIUM. 67 


e.g., 9.97, 9.95, and 9.98 per cent. Mg in MgSOu, 7H20 instead 
of 9.88 per cent. Mg. 

Correct results can be obtained by the method of B. Schmitz,* 
or of W. Gibbs. 


Method of B. Schmitz. 


The acid solution, containing magnesium in the presence of 
ammonium salts,{ is heated to boiling and then treated with an ex- 
cess of sodium or ammonium phosphate. One-third the solution’s 
volume of 10 per cent. ammonia is at once added, the solution 
allowed to cool and filtered through a Munroe crucible after 
standing for several hours. The precipitate is washed with 2.5 
per cent. ammonia, dried and ignited very slowly, gradually 
increasing the heat until the precipitate is white. After cool- 
ing, the MgP207 is weighed: 


2MgNH4PO4 = 2NH3 + H.O + MgeP207. 


From the weight of the latter, p, the amount of magnesium can be 
calculated as follows: 


_2Mg 


Me,P.0; -p=weight of magnesium. 
2 


The precipitate can be filtered upon an ordinary filter, but 
in all cases the ignition must. be gradual or it is almost impossible 
to. obtain perfectly white pyrophosphate. 





Sean 


* Z. anal. Chem, 1906, 512; cf. Jorgensen, wbid. 1906, 278. 

+ Am. J. Sci. (8), 5, 114. 

t Ammonium salts do no harm when the precipitation takes place in hot 
solution, but, on the contrary, cause the formation of a coarsely crystalline 
precipitate which is easy to filter. 


68 GRAVIMETRIC ANALYSIS. 


Method of W. Gibbs. 


The neutral, not too concentrated, solution of magnesium 
salt containing ammonium salts is treated at the boiling tem- 
perature with a normal solution of microcosmic salt, NaHNH4PO, 
+H.2O, until no further precipitation takes place. Almost 
90 per cent. of the magnesium present is at once thrown down 
as amorphous magnesium hydrogem phosphate, MgHPOx: 


NaHNH,PO.4+ MeClo= NaCl +NH.Cl+MgHPO,. 


Then, while stirring the hot solution, about one-third volume of 
10 per cent. ammonia is added whereby the amorphous pre- 
cipitate is transformed into crystalline magnesium ammonium 
phosphate: | 


MgHPO,g+ NH3 = MgNH4P0x. 


At the same time the magnesium remaining in solution is thrown 
down almost completely. 

After standing two or three hours, the supernatant liquid 
is filtered off, and the precipitate washed three times by decan- 
tation with 2.5 per cent. ammonia, finally transferred to the 
filter, washed completely with 2.5 per cent. ammonia and dried 
in the hot closet. The dried precipitate is transferred as com- 
pletely as possible to a weighed platinum crucible, the filter- 
paper burned in a platinum wire spiral, and the ash added to the 
main portion of the precipitate. The covered crucible is heated 
very gently at first until the ammonia is all dried off, then more 
strongly until the mass is snow white. The crucible is cooled 
in a desiccator and weighed. 


Separation of Magnesium from the Alkalies. 


The methods of Gibbs and of Schmitz serve to separate mag- 
nesium from the alkalies in those cases where the dstermination 
of magnesium alone is desired. 

If it is desired to determine magnesium and the alkalies in one 


DE si aici TION OF MAGNESIUM. ' 69 


and the same sample, it is best to use the Gooch and Eddy * 
modifications of the 


Schaffgotsche Method} 


The method is based upon the fact that magnesium can be 
precipitated quantitatively, by means of an alcoholic solution of 
ammonium carbonate, as crystalline magnesium ammonium car- 
bonate, MgCO,- (NH,),CO,-6H,0. 

_ Preparation of the Precipitant—A mixture of 180 c.c. con- 
centrated ammonia, 800 c.c. water, and 900 c.c. absolute alcohol 
is saturated with commercial ammonium carbonate. The liquid 
is shaken with the powdered carbonate, and after several hours 
the excess of the latter is removed by filtration. 

Procedure-—The neutral solution containing only magnesium 
and the alkalies (lithium must not be present), preferably in the 
form of chlorides, is treated with an equal volume of absolute 
alcohol and then with an excess of the ammonium carbonate re- 
agent. The liquid is vigorously stirred for a few minutes and 
allowed to stand for at least half an hour, whereupon it is filtered 
through a Gooch or Munroe crucible. The precipitate is washed 
with the precipitant, dried, ignited and weighed as MgO. 

If considerable alkali is present the precipitate always con- 
tains a small quantity of it. In such cases the precipitate is dis- 
solved in hydrochloric acid, the solution evaporated to dryness, the 
residue taken up in a little water, and the precipitation repeated. 

The combined filtrates are evaporated to dryness and the 
alkali determined as described on page 43 et seq. 

If, however, it is desired to separate magnesium from the alka- 
lies in order that the latter may be determined, the magnesium 
may be precipitated as magnesium hydroxide, from a solution 
free from ammonium salts, by the addition of barium hydroxide 
solution.t The barium is then removed by ammonium carbonate 
and the alkalies determined in the filtrate. For the detailed 
description of this method see Silicate Analysis. Even in this 
case, however, the use of the Schaffgotsche method of separating 
magnesium from the alkalies is more satisfactory. 

* Z. anorg. Chem., 58, 427 (1908). + Pogg. Ann., 104, 482 (1858). 

t Cf. footnote to page 65. 





79. GRAVIMETRIC ANALYSIS. 


, METALS OF GROUP IV. 
CALCIUM, STRONTIUM, BARIUM. 
CALCIUM, Ca. At. Wt. 40.09. 
Forms: CaO, CaCOs, CaSO4. 
1. Determination as Calcipm Oxide (Lime), Cad. 


For the determination of calcium as CaO, it is best precipi- 
tated as the oxalate and converted to the oxide by strong ignition. 

Procedure.—The neutral or slightly ammoniacal solution, 
which besides magnesium and the alkalies should contain no other 
metal,* is treated with ammonium chloride, heated to boiling, and 
precipitated by the addition of a boiling solution of ammonium 
oxalate. After standing some time, the precipitate becomes 
coarsely crystalline and settles-to the bottom of the beaker, when 
a little more ammonium oxalate solution is added to make sure that 
the precipitation has been complete. It is allowed to stand four 
hours, when the clear liquid is poured through a filter, the pre- 
cipitate is covered with boiling water containing ammonium 
oxalate, allowed to settle, filtered, and the operation repeated 
three times. ‘ Finally the whole precipitate is transferred to the 
filter and washed with hot water containing ammonium oxalate 
until free from chloride. The precipitate is warmed in the hot 
closet until nearly dry, when it is placed together with the filter 
in a platinum crucible and ignited wet. It should be heated 
cautiously at first in order that the too rapid evolution of carbonic 


oxide will not cause loss. After the filter is burnt the crucible is © 


covered and strongly heated at first over the Teclu burner and 
finally over the blast-lamp for twenty minutes. 

The crucible while still quite warm is placed, in the desiccator 
shown in Fig. 7, near an open weighing-beaker and allowed to re- 
main there for one hour. During the cooling, the air enters the 


wee ome 


desiccator, through the U tube, whose outer half is filled with 
* If magnesium is present, the precipitation should be carried out as 





described on page 76. 

+T. W. Richards found that the precipitate was appreciably soluble in 
pure water but practically insoluble in a dilute solution of ammonium oxalate 
(Z. anorg. Chem., 28, 85 (1901)). 


DETERMINATION OF CALCIUM AS SULPHATE. 71 


soda-lime and whose inner half contains calcium chloride, in a dry 
condition and free from carbonic acid gas: The crucible is now 
placed in the weighing-beaker, quickly covered and allowed to 
_ stand for half an hour in the air near the balance, after which it 
is weighed. The crucible is again heated over the blast-lamp for 
ten minutes and is cooled in exactly the same way and weighed. 
Should the weight not be found constant, the process must be 
repeated. The above directions when carefully followed usually 
enable one to obtain a constant weight on the second ignition. 

Example.—Caleite: 0.5 gm. of the finely powdered material 
which has been dried at 100° is placed in a 300-c.c. beaker and 
moistened with 20 c.c. of water. The beaker is covered with a 
watch-glass, concentrated hydrochloric acid is added drop by drop, 
and the liquid is finally heated until all is dissolved. The solution 
is then boiled for fifteen minutes to expel all carbon dioxide, neutral- 
ized carefully with ammonia, diluted with 150-200 c.c. of hot water, 
and precipitated with ammonium oxalate as above described. 

Remark.—If both solutions are not boiling hot during the 
precipitation, the calcium oxalate forms very fine crystals; it 
then settles very slowly and passes readily through the filter. 

Calcium oxalate is almost insoluble in water and acetic acid in 
the presence of ammonium oxalate, but readily soluble in mineral 
acids. 

2. Determination of Calcium as Sulphate, CaSO,,. 


This method is chiefly used for the analysis of calcium salts 
of organic acids. For this purpose the ealcium salt is ignited in 
a weighed platinum crucible, after which the crucible is covered 
with a watch-glass, carefully treated with dilute sulphuric acid 
and warmed upon the water-bath until there is no longer any 
evolution of carbon dioxide. The under side of the watch-glass 
is carefully washed and the liquid evaporated as far as possible 
on the bath. The excess of sulphuric acid is then carefully driven 
off by inclining the crucible and heating over the free flame (or 
in an air-bath) (cf. Fig. 11, p. 27). The residue is genily ignited 
and weighed. By strong ignition, calcium sulphate loses SO,.* 





* 0.2052 gm. CaSO, remained unchanged in weight after heating for one 
hour to dark redness; but on heating with the full flame of a Teclu burner, 
it lost 0.0004 gm. in weight. On heating for one hour over the blast lamp 
it lost 0.0051 gm. (J. Weber.) 


72 GRAVIMETRIC ANALYSIS. 


Calcium may also be precipitated as calcium sulphate. The 
solution, which should contain as little free acid as possible, is 
treated with an excess of dilute sulphuric acid, four volumes of 
alcohol are added, and the mixture is allowed to stand 12 hours. 
It is then filtered off, washed with 70 per cent. alcohol, dried, 
separated from the filter as completely as possible, the filter burned 
in a platinum spiral, and the ash added to the main part of the 
precipitate in a platinum crucible, gently ignited and weighed. 

4 > 


3. Determination of Calcium as Carbonate, CaCO,. 


Only in rare cases is calcium precipitated as carbonate by 
ammonium carbonate in the presence of ammonia. ‘The filtered 
and washed precipitate is gently ignited and weighed as carbonate. 
After weighing it is necessary to moisten the residue with a 
little ammonium carbonate solution, evaporate to dryness on 
the water-bath, and again ignite gently. This is done in order to 
change small amounts of calcium oxide, which may have been 
formed during the burning of the filter-paper, back to carbonate. 

In the presence of considerable ammonium chloride the pre- 
cipitation of calcium by means of ammonium carbonate is not 
quite complete, whereas the precipitation with ammonium oxalate 
always is. Consequently it is advisable in all cases to precipitate 
the calcium as oxalate and weigh it as the oxide. 1f the oxalate 
is ignited gently, however, it can be weighed as the carbonate. 


STRONTIUM, Sr. At. Wt. 87.63. 
Forms: SrSO,, SrCO,, SrO. 


The determination as the sulphate is the most accurate, 


Determination of Strontium as Sulphate, SrSO,. 


Procedure-—To the neutral solution containing strontium, 2 
slight excess of dilute sulphuric acid is added and as much alcohol 
as there is volume of solution. After stirring well, the mixture 
is allowed to stand twelve hours, filtered and washed, at first 
with 50 per cent. alcohol, to which a little sulphuric acid has 
been added, and finally with pure alcohol until the wash water 
no longer gives the sulphuric acid reaction. The precipitate is — 


DETERMINATION OF STRONTIUM AS OXIDE OR CARBONATE. 73 


dried and ignited as described under the determination of cal 
cium as sulphate. 


Solubility of Strontium Sulphate according to Fresenius. 


6895 parts of water at the ordinary temperature (17-20°) dis- 
solve 1 part of SrSO,. 

9638 parts of bailing water dissolve 1 part $180,. 

The sulphate is less soluble in water containing sulphuric acid: 

12,000 parts of water containing sulphuric acid dissolves 1 part 
SrSO,4. The precipitate is soluble in concentrated sulphuric acid, 
so that the water should not contain very much sulphuric acid. 

In cold, dilute hydrochloric or nitric acid, strontium sulphate is 
considerably more soluble, and also in solutions containing acetic 
acid, magnesium chloride, or alkali chloride. 

If, therefore, considerable free acid is present, it should be 
removed by evaporating the solution to dryness and dissolving 


the residue in water. ‘The strontium is then precipitated as above 
described. | 


Determination of Strontium as Oxide, SrO, or as Carbonate, SrCO,. 


The strontium is precipitated as carbonate, or in some cases 
as oxalate, and changed Py ignition to the oxide as described 
under calcium. 

Strontium carbonate is decomposed by heat more difficultly 
than calcium carbonate and the determination as carbonate is 
very satisfactory. It is advisable to treat the precipitate as 
described under calcium, although it is usually unnecessary to 
heat with additional ammonium carbonate. 


Solubility of Strontium Carbonate in Water according to Fresenius. 


18,045 parts of water dissolve at ordinary temperatures 1 part 
of SrGO,. 

In water containing ammonium carbonate the salt is much 
less soluble; on the other hand, ammonium chloride and ammo- 
nium nitrate increase its solubility. 

In case calcium, strontium, magnesium and alkali salts are 
present together, as in minerals and in mineral waters, the calcium 


74 GRAVIMETRIC ANALYSIS. 


and strontium are both precipitated as oxalates and transformed 
by ignition into the oxides. Cf. pp. 78, 79. 


Solubility of Strontium Oxalate in Water. 


12,000 parts of water at ordinary temperatures dissolve 1 

part of SrC,0,+24H,0. | : 

The solubility is very much less in water containing ammonium 
oxalate. 


BARIUM, Ba. At. Wt. 137.37. 
Forms: BaSO,, BaCrO,, BaCo,. 


1. Determination as Barium Sulphate. 


The solution, slightly acid with hydrochloric acid, is heated to 
boiling and precipitated by the addition of an excess of hot, dilute 
sulphuric acid. It is allowed to stand on the water-bath until the 
precipitate has settled, the solution is then poured through a filter, 
and the precipitate is washed four times with 50 c.c. of water to 
which a few drops of sulphuric acid have been added. The pre- 
cipitate is transferred to the filter and washed with hot water 
until the wash water ceases to give the sulphuric acid reaction. 
It is then dried somewhat, ignited wet in a platinum crucible, and © 
weighed without previous heating over the blast-lamp. 

Remark.—By the combustion of the filter-paper there is usually 
a partial reduction of the barium sulphate to sulphide, but the 
Jatter, on being gently ignited in an inclined crucible, is readily 
changed back to sulphate, so that there is no loss to be feared. 

The procedure for the determination of barium as carbonate is 
the same as was described under calcium. 

Solubility of Barium Sulphate in Water.— 344,000 parts of water 
dissolve 1 part of BaSQ,. 


2. Determination of Barium as Chromate. 


The neutral solution of the barium salt is diluted to about 200 
e.c., treated with 4-6 drops of acetic acid (sp. gr. 1.065), heated to 
boiling, precipitated with a slight excess of ammonium chromate 
(prepared by adding ammonia to a solution of ammonium bichro- 
mate free from sulphate, until the color becomes yellow), and 


DETERMINATION OF BARIUM AS CHROMATE. 75 


allowed to cool. The precipitate is filtered off through a Gooch 
crucible and washed with hot water until 20 drops of the filtrate 
give scarcely any reddish-brown coloration with a neutral solution 
of silver nitrate. The precipitate is dried in the hot closet, after 
which the crucible is fastened to a larger porcelain crucible by 
means of an asbestos ring, so that there remains a space of about 
4cm. between the two crucibles (cf. p. 27), and the open crucible 
is ignited over the free flame until the precipitate becomes a bright 
yellow.* 


Solubility of Barium Chromate.t 


86,957 parts of water at ordinary temperatures dissolve 1 part BaCrO,, 

23,000 “ “ boiling water dissolve 1 part BaCrO,,. 

49,381] “ “a 0.75 per cent. ammonium acetate solution (at 15°) dissolve 
1 part BaCrO,. 

45,152 parts of a 0.5 per cent. ammonium nitrate solution (at 14°) dissolve 
1 part BaCrO,. 

23,555 parts of a 1.5 per cent. ammonium acetate solution (at 15°) dissolve 
1 part BaCrO,. | 

22,988 parts of 0.5 per cent. ammonium nitrate solution dissolve 1 part 


BaCrO,. 
3,670 parts of 1 per cent. acetic acid solution dissolve 1 part BaCrQ,. 
2618 “ck sé 5 “é Cag “ it3 cc “ 1 &é “ 
1.986 ‘ce ce 10 &e ée ce ce 6c “ 1 “ce “ce 
? 


1813 “ “10 “ “ chromic acid solution dissolve 1 part BaCrO,, 


The solubility of barium chromate, therefore, increases con- 
siderably with increasing concentrations of either acetic or chromie 
acids; the solubility is affected to a much less degree by solutions 
containing neutral ammonium salts. By the additions of small 
amounts of neutral ammonium chromate the solubility becomes 
lessened to nearly zero. 





* Oftentimes small amounts of the precipitate are reduced to chromic 
oxide by traces of organic matter, whereby it appears slightly greenish. 
By long-continued ignition in an open crucible, the chromic oxide is changed 
back to chromate, when the precipitate appears a homogeneous yellow 
throughout. 

+ P. Schweizer, Z. anal. Chem., 1890, p. 414, and R. Fresenius, tid., 1890, 
p. 418. 


75 GRAVIMETRIC ANALYSIS. 


SEPARATION OF THE ALKALINE EARTHS FROM MAGNESIUM AND 
FROM THE ALKALIES. 


i. Separation of Calcium from Magnesium (and Alkalies). 


The separation depends upon the different solubilities of ‘the 
two oxalates. Calcium oxalate: is practically insoluble in hot 
water, whereas magnesium oxalate 1 is fairly soluble; 1500 parts 
of cold water, or 1300 parts of boiling water, dissolve 1 part of 
MgC,0,-2H,O. In an excess of the precipitant, however, mag- 
iuin aeataes is much more soluble owing to the formation of 
complex salts. 

If calcium is precipitated as oxalate from a dilute solution in 
the presence of magnesium, some magnesium oxalate is occluded 
by the calcium oxalate precipitate even when the solution is by 
no means saturated with magnesium oxalate, and high results 
are obtained for calcium. In such cases the error is usually 
compensated according to Fresenius, by dissolving the pre- 
cipitate in hydrochloric acid and repeating the precipitation with 
ammonia and ammonium oxalate. 

The work of T. W. Richards * has shown, however, that the 
quantity of magnesium oxalate occluded is dependent upon the 
concentration of the undissociated magnesium oxalate in solution, 
and upon the time in which the calcium oxalate is in contact 
with the solution. Consequently anything that prevents the 
dissociation of magnesium oxalate will tend to increase the 
amount of occlusion and thereby increase the result of the calcium 
determination. Anything that will cause the ionization of the 
magnesium oxalate serves to reduce the amount of error. 

Concentrating the solution and increasing the amount of 
oxalate ions in solution by the addition of considerable ammo- 
nium oxalate, both tend to repress the dissociation of the mag- 
nesium oxalate. The dissociation of this compound is favored 
by the presence of hydrogen ions and by dilution. 

A considerable excess of ammonium oxalate is necessary in 
the quantitative precipitation of calcium and, moreover, mag- 





* Z. anorg. Chem., 28, 71 (1901). 


ALTERNATE METHOD. 77 


nesium oxalate forms readily soluble complex salts with undisso- 
ciated ammonium oxalate. It is desirable, therefore, to prevent 
the dissociation of the ammonium oxalate as much as possible, 
and this is accomplished by the addition of another ammonium 
salt, preferably the chloride. 

Procedure.—Dilute the solution with hot water so that the 
Magnesium is present in a concentration of not over fiftieth 
normal, and add a considerable quantity of ammonium chloride, 
if it is not already present. To precipitate the calcium, add 
a sufficient volume of boiling oxalic acid solution, containing 
three or four equivalents of hydrochloric acid to lessen the 
dissoziation of the oxalic acid. Introduce a little methyl orange 
into the boiling sclution, and add ammonia until a yellow colora-. 
tion is produced. The ammonia should not be added all at 
once, but in small quantities from time to time so that about 
half an hour is consumed in the operation. 

After the neutralization add a considerable excess of hot 
ammonium oxalate solution, allow to stand four hours but not 
longer on account of the fact that the occlusion increases with 
the length of time, filter, and wash with hot, one per cent. am- 
monium oxalate until the filtrate gives no test for chloride after 
acidifying a sample with nitric acid. » 

Although this precipitate contains a little magnesium (0.1- 
0.2 per cent.) it is also true that an almost equal amount of 
magnesium passes into the filtrate, so that it is not advisable to 
repeat the precipitation when made in this way. 


Alternate Method. 


W. ©. Blasdale * has studied the separation of calcium from 
different amounts of magnesium and succeeded in getting excellent 
results by a somewhat simpler method. The acid solution con- 
taining not more than 0.6 gm. of calcium and magnesium oxides is 
brought to a volume of 309 c.c., and heated to boiling; 3.5 gms. of 
ammonium chloride are aided and 1 gm. of oxalic avid, and the 
complete precipitation of the calcium is brought about by neutral- 





* J. Am. Chem. Soc., 31, 917 (1999). 


78 GRAVIMETRIC ANALYSIS. 


izing with 1 per cent. ammonia added during five minutes. The 
precipitate is allowed to stand for an hour before filtering. If 
considerably more magnesium than calcium is present, the 
precipitation is effected in two stages. First only enough oxalie 
acid is added to combine with all the calcium present. The 
solution is slowly neutralized as before and allowed to stand ten 
minutes. Then the balance of the oxalic acid is added, the solu- 
tion made alkaline and allowed to stand for an hour. With more 
than ten times as much magnesium 4s calcium, a double precita- 
tion is desirable. 

In the filtrate the magnesium can be veeakatad by the 
addition of ammonia and sodium phosphate. If, however, large 
amounts of ammonium salts are present it is preferable to 
evaporate to dryness in a platinum or porcelain dish, to dry 
the residue at 110-130° for an hour or longer, and to expel the 
ammonium salts by gentle ignition. The residue is taken up 
in a little warm hydrochloric acid, diluted with water, a little 
carbon residue filtered off and the magnesium determined as 
pyrophosphate. (Pages 66-69.) 


II. Separation of Strontium from Magnesium. 


This separation finds practical application in the analysis of 
almost all mineral waters and of minerals containing strontium. 
In all of these cases, however, strontium occurs in relatively small 
amounts in the presence of large amounts of calcium and varying 
amounts of magnesium, so that it is a question, first, of separating 
calcium and strontium from magnesium. This separation is 
effected by the precipitation of the calcium and strontium as 
oxalates as described on pp. 70 and 73. 

The filtrate containing magnesium may also contain traces of 
strontium; hence, after the removal of the ammonium salts by 
ignition, the residue is dissolved in hydrochloric acid, sulphurie 
acid and alcoho! are added, and the solution is allowed to stand 
for twelve hours. Any resulting precipitate, consisting of strontium 
or barium sulphate, is filtered off and weighed. From this filtrate 
the magnesium is precipitated as magnesium ammonium phosphate 
as described on p. 66, and weighed as the pyrophosphate. 


eee 


SEPARATION OF BARIUM FROM MAGNESIUM AND STRONTIUM. 79 


III. Separation of Barium from Magnesium. 
In case it is desired to separate only barium from magnesium, 
the solution (which must be free from nitric acid) is acidified 
with hydrochloric acid, heated to boiling, and the barium precipi- 


- tated by the addition of boiling, dilute sulphuric acid (cf. p. 74). 


The magnesium is precipitated from the filtrate as magnesium 
ammonium phosphate in the usual way. In most cases, however, 
a separation of barium, strontium, and calcium from the magnesium 
is involved. For this purpose the three alkaline earths are pre- 
cipitated as oxalates, and any barium or strontium remaining in 
the filtrate is precipitated as described under II. The magnesium 
is determined in the final filtrate. 


IV. Separation of the Alkaline Earths from One Another. 


Principle-—The mixture of the dry nitrates is treated with 
ether-alcohol, which dissolves calcium nitrate alone. The residue 
is taken up in water, the barium is precipitated as chromate, and 
the strontium is determined in the filtrate as sulphate. 


PROCEDURE. 


(a) Separation of Calcium from Strontium and Barium accord- 
ing to Rose-Stromeyer-Fresenius. 


The three metals are assumed to be present together in solution 
in the form of their nitrates. The solution is evaporated in a 
small Erlenmeyer fiask, as described under lithium, p. 55, in an 
oil-bath, meanwhile passing a stream of dry, warm air through the 
flask. When all the water is evaporated, the temperature of the 
bath is raised to 140°C. and maintained at this temperature for 
one-to two hours, still passing the current of warm air through 
the flask. After cooling, the dry residue is treated with ten times 
its weight of absolute alcohol, corked up, and allowed to stand 
with frequent shaking for one to two hours. An equal volume 
of ether is now added, the flask closed, shaken, and again allowed 
to stand twelve hours. It is then filtered through a filter moist- 
ened with ether-aleohol and washed with ether-alcohol until a few 
drops of the filtrate evaporated on platinum-foil leave no residue. 


80 GRAVIIAETRIC ANALYS-S. 


The filtrate is evaporated to dryness in a lukewarm water-bath, 
the calcium nitrate is dissolved in wate-, precipitated as the oxalate, 
and after ignition is weighed as the oxide. 

_ Remark.—In case only a small amount of calcium is present 
(not more than about 0.5 gm.) the above separation is complete. 
With large amounts of calcium, the residue of strontium and 
barium nitrates almost always contains some calcium. In this 
case the aqueous solution is again evaporated to dryness in the 
same way as before and the treatment with alcohol and ether re- 
peated. The calcium is then determined in the combined filtrates. 

This separation finds application in the analysis of most min- 

eral waters. 


(b) Separation of Barium from: Strontium according to 
Fresenius. * 


Requirements.—1. A solution of (NH,),CrO, (1 ¢.c. of the solu- 
tion should contain 0,1 gm. of the salt). The solution is 
prepared by adding ammonia to a solution of ammonium 
bichromate (free from sulphate) until the color of the solution be- 
comes yellow. The solution should be left acid rather than alkaline. 

2. A solution of ammonium acetate (1 c.c. containing 0.31 gm. 
of the salt). 

3. Acetic acid of sp. gr. 1.065. 

4. Nitric acid of sp. gr. 1.20. 

Procedure.—The residue, consisting of strontium and barium 
nitrates, is dissolved in a little water and for each gram of salt 
mixture the solution is diluted until the concentration corre- 
sponds to 300 c.c., heated to boiling, treated with six drops of 
acetic acid and about 10 c.c. of ammonium chromate solution 
(this should be an excess over the theoretical amount necessary) 
and allowed to stand one hour. The precipitate of barium chromate 
is washed by decantation with water containing ammonium chro- 
mate until the wash water ne longer gives a precipitate with ammo- 
nia and ammonium carbonate; it is then washed with pure hot 
water until the last washing gives only a slight reddish-brown 


Ses 





* 7, anal. Chem., 29, 427 (1890). 


SEPARATION OF BARIUM FROM STRONTIUM, — 81 


coloration with neutral silver nitrate solution. The precipitate 
cn the filter still contains a little strontium. It is carefully washed 
back: into the vessel in which the precipitation took place, while 
any precipitate remaining on the filter is dissolved in a little warm 
dilute nitric acid and washed into the dish, finally adding enough 
nitric acid to the precipitate so that it dissolves completely on 
warming (about 2 c.c. of nitric acid being usually necessary). The 
solution is then diluted to 200 c.c., heated to boiling, treated with 
5 ¢c.c. of ammonium acetate solution, added little by little, and 
finally with enough ammonium chromate to cause the disappear- 
ance of the odor of acetic acid from the solution (usually about 
10 e.c. are necessary). After standing one hour the liquid is 
poured through a Gooch crucible, the residue is treated in the dish 
with hot water, allowed to cool, then filtered and washed with cold 
water until the filtrate gives only a slight opalescence with neutral 
silver nitrate. The precipitate is dried, ignited gently in an air- 
bath (cf. p. 75), and weighed as BaCrO4. The filtrate is treated 
- with 1 ¢.c. nitric acid, concentrated somewhat, and the strontium 
precipitated as carbonate by the addit:on of ammonia and am- 
monium carbonate. The precipitate, which always contains some 
chromate, is washed once with hot water, dissolved in hydrochloric 
acid, and thestrontium determined as sulphate, according to p. 72. 
The results obtained according to this method are very satis- 
factory. Experiments performed in this laboratory * completely 
confirm the results obtained by Fresenius. 
_ Remark.—In the opinion of the author, all other methods for 
the separation of the alkaline earths give incorrect results; for 
that reason they will not be discussed in this book. 





* The results of seven experiments gave (a) for the percentage of barium 
chromate obtained: 99.9, 99.9, 100.3, 100.4, 100.7, 100.6; mean, 100.3 per 
cent.; (b) for strontium sulphate, 100.9, 99.73, 99 °6, 99.£4, 99.47, 9¢.77. 
29.6; mean, 99.75 per cent. (H. Schmidt.) 


82 GRAVIMETRIC ANALYSIS, 


METALS OF GROUP III. 


ALUMINIUM, CHROMIUM, TITANIUM, IRON, URANIUM, NICKEL, 
COBALT, ZINC, AND MANGANESE. 


A. DIVISION OF THE SESQUIOXIDES. 
ALUMINIUM, CHROMIUM, IRON, TITANIUM, AND URANIUM. 
ALUMINIUM, Al. At. Wt. 27.1. 

Form: A1,0,. 


In order to determine aluminium in this form, the metal is pre- 
cipitated as its hydroxide and converted to its oxide by ignition 
of the precipitate. 

It must not be forgotten, however, that aluminium hydroxide 
exists in a soluble form (hydrosol) and in an insoluble form (hydro- 
gel); and further that the hydrosol is not completely changed by. 
boiling into insoluble hydrogel. ‘To accomplish this the presence of 
salts in solution (preferably ammonium salts) is also necessary. 
Since, however, ammonium salts become acid on long boiling (due 
to the escape of ammonia) there is danger of the aluminium hy- 
droxide being redissolved. Furthermore, itis true that the hydrogel 
gradually changes over into hydrosol by standing in a solution 
containing only a small amount of dissolved salts, or by remaining 
in a hot solution containing only a small amount of dissolved 
salts. 

From these facts the following procedure is derived: 

The solution containing the aluminium (but no phosphoric 
acid, or anything but aluminium that is precipitated by ammonia) 
is treated with considerable ammonium chloride, or ammonium 
nitrate, heated to boiling in a platinum or porcelain vessel, and 
a slight excess of ammonia * is added. The precipitate is allowed 
to settle, after which the clear solution is poured through a filter 
which rests on a platinum cone, but without applying suction. 





* The ammonia should be freshly distilled. When it has been kept for . 
any length of time in glass bottles, the ammonia invariably contains silica, 
and this leads to high results in the case of all precipitates formed by adding 
ammonia to an acid solution. 


DETERMINATION OF ALUMINIUM, | 83 


The precipitate is washed three times by decantation with hot 
water to which a drop of ammonia and a little ammonium nitrate 
has been added, and finally transferred to the filter. The pre- 
cipitate is now washed as quickly as possible with the hot wash 
liquid (so that the precipitate is thoroughly churned up each 
time) until the filtrate ceases to give a test for chlorine. The pre- 
cipitate is then dried as completely as possible by the applica- 
tion of suction and ignited wet in a platinum crucible. After the 
precipitate and ash have become white, the covered crucible is 
heated over the blast-lamp for about ten minutes, cooled in a 
desiccator and weighed. The process is repeated until a constant 
weight is obtained. 


Determination of Aluminium by the Method of Chancel.* 


In the determination of aluminium in a sample of alum by 
precipitation with ammonia, there are a number of difficulties to 
overcome. In the first place the precipitate always contains more 
or less basic aluminium sulphate, and it requires long ignition to 
expel the last traces of sulphuric anhydride. It is, to be sure, 
possible to wash out all the sulphate by means of a solution of 
ammonia and ammonium nitrate; the operation, however, is 
very tedious and a large quantity of the wash liquid is required. 
Another disadvantage arises from the fact that the filtration takes 
place very slowly, even when the precipitate contains basic sul- 
phate, on account of the slimy nature of aluminium hydroxide. 

By the method of Chancel and in the two following methods 
these difficulties are overcome. 

The principle of this and the following methods consists in 
neutralizing, by salts of weak acids, the mineral acid that is set 
free in the hydrolysis of an aluminium salt; the weak acid is 
finally removed by boiling or neutralization. In the Chancel 
method the mineral acid is neutralized by sodium thiosulphate, 


-2AICl, + 6H,022Al (OH), + 6HCI, 
- GHC1-+3Na,8,0,—6NaCl +3H,0 +380, +38. | 
Procedure——The dilute, neutral solution (about 0.1 gm. Al 





_ * Compt. rend., 46, 987; .:Z. anal. Chem., 3, 391. 


84 GRAVIMETRIC ANALYSIS. 


in 200 c.c.) is treated with an excess of sodium thiosulphate and 
boiled until all traces of SO, are expelled. Enough ammonia * 
is then added so that the odor is barely perceptible after blowing 
away the vapors, and the boiling is continued a little longer. 
The precipitate of Al(OH), and § is filtered off, washed with hot 
water, and ignited in a porcelain crucible. Such a precipitate 
of Al(OH), is much denser than that produced by direct pre- 
cipitation with ammonia and is very easy to filter and wash. 

Remark.—This method is often employed for separating alu- 
minium from iron. Ferric iron is reduced to ferrous salt by the 
sodium thiosulphate and is not precipitated. In this case, how- 
ever, the final neutralization with ammonia, as prescribed in the 
above directions, should not be carried out or a little iron will 
come’ down. 


Determination of Aluminium by the Method of Alfred Stock.} 


The aqueous solution of the aluminium salt, which on account 
of hydrolysis always shows an acid reaction, 


AICI, + 3HOH@Al(OH),+3HCl. 


is treated in the cold with a mixture of potassium iodide and 
potassium iodate. This mixture in the presence of acid is decom- 
posed in accordance with the equation 


KIO3 +5KI+6HC1=3H,0 + 6KC1431b», 


whereby the equilibrium which existed in the hydrolysis of the 
aluminium salt is disturbed and the first reaction continues to 
take place until all the aluminum is precipitated. If now the 
iodine in the solution is made to react with sodium thiosulphate 
and the mixture is heated on the water bath for half an hour, the 
precipitate collects in such a condition that it can be quickly 
filtered and washed. 

Procedure.—The solution in which the aluminum is to be 
determined should be very slightly acid; if more acid is 
present sodium hydroxide is added until a slight permanent 
precipitate is obtained which is redissolved by means of a 





* If the addition of ammonia is omitted, the solution will retain traces of 
aluminium. 


T Ber., 1900, 548. 


DETERMINATION OF ALUMINIUM. 85 


few drops of hydrochloric acid. Thereupon equal volumes of a 
25 per cent. potassium iodide solution and a saturated potassium 
iodate solution (about 7 per cent. KIO3) are added. After about 
five minutes the solution is decolorized by the addition of a 20 
per cent. sodium thiosulphate solution and a little of the potassium 
iodide and iodate mixture is added in order to make sure that 
‘enough was added in the first place. One or two c.c. more of 
sodium thiosulphate are added and the solution heated half an 
hour on the water bath. The pure white precipitate settles out 
well, and can be filtered through a filter with relatively wide pores, 
washed with hot water, ignited and weighed as Al2O3. 

Remark.—The presence of calcium, magnesium and boric acid 
does not interfere with the above determination, but if phosphoric 
acid is contained in the solution, the phosphate of aluminum is 
precipitated. It is obvious that the method cannot be employed 
in the presence of organic substances such as tartaric acid, citric 
acid, sugar, etc., which prevent the precipitation of aluminium 
hydroxide. 


Determination of Aluminium by the Method of G. Wynkoop * 
and of E. Schirm.7 


_ Principle—If a neutral solution of aluminium (iron, chro- 
mium, or titanium) is boiled with an excess of sodium or ammo- 
nium nitrite until no more fumes are evolved, aluminium 
hydroxide is precipitated in a form that can be as easily filtered 
as that of the last methods. 


2A1Cl, + 6HOH@2Al(OH),+6HCl; 
6HCl+6NaNO,=6NaCl1+6HNO,; 
6HNO,=3H,0 +3NO0O+3N0,. 


Procedure.—lf the solution is acid, enough ammonia is added 
so that the precipitate first formed dissolves only slowly on 





* J. Am. Chem. Soc., 19, 434 (1897). 
t Chem, Ztg., 1909, 877. 


86 GRAVIMETRIC ANALYSIS. 


stirring. An excess of a 6 per cent. solution of pure ammonium 
nitrite* is then added, the solution diluted to 250 c.c. and 
boiled until no more fumes of nitrous oxides are evolved 
(about 20 minutes). The precipitate is filtered, washed with 
hot water, ignited wet in a platinum crucible, and weighed as 
ALO. 

Remark.—In the presence of more than 1 per cent. of 
ammonium salts, these are hydrolyzed enough so that the solution 
remains acid and the precipitation of the aluminium is incomplete 
even after long boiling. In such a case, after the vapors of 
nitrogen peroxide are no longer visible, ammonia is added drop 
by drop until the odor is barely perceptible. The precipitate is 
allowed to settle while the beaker is on the water-bath, and the 
analysis is finished as above. 

If the solution contains only aluminium in the form of its chlo- 
ride, nitrate, or sulphate, it can be determined by evaporating the 
solution in a platinum crucible on the water-bath with the addi- 
tion of a little sulphuric acid, the excess of the latter being finally 
removed by cautious heating over the free flame in an inclined 
crucible. The residue of aluminium sulphate is then changed to 
the oxide by strong ignition over the blast-lamp. 

In the case of organic’ salts of organic acids, the oxide is 
readily obtained by careful ignition of the salt in a platinum 
crucible. 





* Sometimes the reagent contains a little barium which should be pre- 
cipitated with ammonium sulphate before using it in an analysis. 


IRON. i 87 


Tron, Fe. At. Wt. 55.84. 


Forms: Ferric Oxide, Fe,0,, and Metallic Iron. 
Determination as Fe,0,. 
(a) By Precipitation with Ammonia. 

This is the form chiefly used for the gravimetric determination 
of iron. The solution containing the ferric salt in the presence of 
ammonium chloride is heated to about 70° C. » 
in a porcelain dish or Jena beaker * and pre- 
cipitated by means of a slight excess of am- 
monia. The precipitate is washed by decan- 
tation with hot water and finally with a 
strong stream of hot water from the wash- 
bottle.t It is ignited gradually in a plati- 
num crucible and finally in the half-covered 
crucible over the Bunsen burner.t The 
_ ferric oxide obtained varies in its appear- 
ance according to the temperature to which 
it has been heated. Gently ignited ferric 
oxide is reddish brown, whereas when 
strongly ignited it has almost the appearance 
of graphite. Both forms are difficultly 
soluble in dilute hydrochloric acid, but can 
be readily dissolved by digesting with con- 
centrated hydrochloric acid on the water-bath. 

(b) By Precipitation with Ammonium Nitrite. 

The precipitation of Fe(OH), from neutral solutions of ferric 

salts takes place as described for aluminium, p. 85. 





Fic. 26. 





* Too high results are invariably obtained in ordinary glass vessels. The 
ammonia should be freshly distilled or the results will be high. 

{ For this purpose a wash-bottle such as is shown in Fig. 26 is useful. 
By blowing through the long arm of the U-tube (which is provided with a 
Bunsen valve) and placing the thumb over the short arm a continuous 
stream of water is maintained which can be stopped at any time by removing 
the thumb. 

t 1t is not advisable to heat the covered crucible over the blast on account 
of te danger of forming some Fe,O,._ The magnetism of such a precipitate 
can be s own by placing a magnet outside the platinum crucible and moving 
it slowly up and down. 


88 GRAVIMETRIC ANALYSIS. 


If the iron is in solution either as the ferrous or ferric salt of a 
volatile acid, it can be readily converted into ferric oxide by evapo- 
ration with sulphuric acid and ignition of the residue. 


2. Determination as Metallic Iron. 


Iron may be determined by electrolysis, but this method offers 
no advantages over the gravimetric method just described or the 
following volumetric process, so that it will not be discussed in this 
book. 

In the case of the analysis of oxide iron ores or of mixtures of 
considerable iron oxide with comparatively little alumina, titanium 
dioxide, or silica, the following method is accurate and rapid. y 

The finely powdered and weighed + substance contained in a 
porcelain boat is introduced into a tube of difficultly fusible glass 
and heated to redness in a stream of dry hydrogen until no more 
drops of water condense on the cool front end of the tube and the 
contents of the boat appear gray and not black. By this means 
the ferric oxide is reduced to metallic iron: 


Fe203 + 3H». = 3H2O + 2Fe. 


After cooling in the stream of hydrogen, the boat and its contents 
are again weighed after remaining some time in a desiccator. The 
loss in weight p represents the amount of oxygen originally com- 
bined with the iron, from which the amount of iron can be calcu- 
lated: 


30:2Fe=p:z 
rare. 
30 


Remark.—In attempting to reduce ferric oxide to iron by 
means of hydrogen, it is very important to heat the oxide to bright 


* Rivot, Ann. Chem. Phys., 3. Serie, 30, 188 (1850); Liebig’s Ann., 78, 
211 (185). | 

{ Ferric oxide after having been powdered and ignited is so hygroscopic 
that the porcelain boat should be placed within a weighing-beaker with 
ground-glass top immediately after removing. it from the desiccator, and 
then weighed. 





VOLUMETRIC DETERMINATION OF IRON. 89 


redness. At a dull red heat, the oxide is to be sure reduced to 
metal, but in such cases black, pyrophoric iron is formed and the 
latter cannot be exposed to the air and weighed without becoming 
oxidized. By heating to a bright red heat, however, the iron 
becomes gray, is no longer pyrophoric, and can, if allowed to cool 
in the stream of hydrogen, which is subsequentiy replaced by 
earbon dioxide, be safely weighed in the air without fear of 
oxidation. 

Although this method is extremely simple, and the correspond- 
ing oxides of aluminium, chromium, titanium and zircon, etc., 
are not reduced under the same conditions, it should be used with 
caution and only when the ferric oxide greatly predominates in a 
mixture of oxides. Otherwise the reduction of the iron is incom. 
plete on account of some of the ferric oxide being enclosed within 
the particles of foreign oxide. This has been proved by the work 
of Daniel and Leberle * and by Treadwell and Wegelin.t 

It is still more accurate to dissolve the metallic iron produced 
in dilute sulphuric acid out of contact with the air and determining 
the amount present volumetrically by titrating with potassium 
permanganate solution. 


3. Volumetric Determination of Iron, according to Margueritte.{ 


Although the volumetric methods are discussed in the second 
part of this book, this determination is so important and is so 
often used to test the purity of the iron oxide produced by a gravi- 
metric analysis that it seems proper to discuss it at this place. 


Principle of the Method. 


Ferrous salts are oxidized by potassium permanganate in acid 
solution to ferric salts: 


2KMn0O,+10FeSO,+8H,SO,=K,SO,+ 2MnSO,+8H,0+5Fe,(SO,),. 


If, therefore, a potassium permanganate solution of known 
strength is slowly added to the solution of a ferrous salt, it will 





* Z. anorg. Chem., 34, 393 (1903). 
. +A table is given, showing the results of twelve experiments, in the 
German edition of this book. 
t Ann. de chim. et de phys. [3], 18 (1846), p. 244. 


go «GRAVIMETRIC. ANALYSIS. 


be decolorized as long as there remains ferrous salt to react with 
it. As soon as all of the ferrous salt has been oxidized, the next 
drop of the permanganate will impart a permanent pink color to 
the solution, whereby the end-point of the reaction is determined. 


Preparation and Standardization of the Permanganate Solution. 


In most cases @ 7/5 normal solution of potassium permanganate 
is suitable, i.e. one which contains,in one liter enough oxygen to 
oxidize ;!y of a gram-atom of hydrogen (1.008 gm. of hydrogen). 

_ Potassium permanganate in acid solution reacts according 
to the equation 


K,O, Mn,0,=K;0+2Mn0+50, 





2KMn0O, 


so that from two molecules of permanganate five atoms of oxygen 
(=10 atoms of hydrogen) are available, thus: 


2KMnO, KMn0O, _ 158.03 
TC ea 
oxygen =1 gm.-atom of hydrogen. 





=31.61 gm. KMnO,=3 gm.-atom of 


Consequently it is necessary to take ;1, of a gram-molecule of 
potassium permanganate (3.161 gms.) for a liter of 74; normal solu- 
tion. 

Although it is possible to purchase very pure potassium per- 
manganate, it is not advisable to take the trouble of weighing 
out just this amount of the substance and dissolving it in exactly 
the right amount of water, for although we might in this way obtain 
the correct strength of solution, yet on the following day its value 
would be different, for the distilled water in which the perman- 
ganate is dissolved almost always contains traces of organic matter 
oxidizable by the permanganate. Consequently we weigh out 
on a watch-glass approximately the right amount of permanganate 
3.1-3.2 gms.), dissolve it in a liter of water, and allow it to stand 
eight to fourteen days* before using it. After this time all of the 
oxidizable matter in the water will have been completely destroyed. 


The solution is filtered through an asbestos filter and then stand- 
ardized. 


* Cf. Morse, Hopkins, and Walker, Am. Chem. Jour., 18 (1896), p. 401. 





VOLUMETRIC DETERMINATION OF IRON. gr 


Standardization of the Potassium Permanganate Solution. 


It is possible to standardize the solution by a number of differ- 
ent methods, as will be discussed in detail under volumetric analysis. 
In this case we are concerned with the determination of iron only, 
so that the most natural way for us to standardize the solution will 
be by means of chemically pure iron. An accurately weighed por- 
tion of iron is dissolved in dilute sulphuric acid out of contact with 
the air and permanganate solution is added from a glass-stoppered 
- burette until the solution remains pink for one-half minute after 
thoroughly stirring or shaking. 
If for the oxidation of a grams of iron¢ cubic centimeters of the 
permanganate solution were necessary, then 


1 a ; 
¢.c. => gm. iron, 


The value - represents the titration value of the solution. 


Remarks.— The chief difficulty lies in the procuring of a 
suitable standard. It is difficult to prepare iron which is exactly 
100 per cent. pure. Moreover, the purity of a sample of iron wire, 
such as is ordinarily used for standardization, cannot be deter- 
mined satisfactorily by means of a gravimetric analysis because 
the analytical errors are greater than is usually supposed. The 
ferric hydroxide precipitate is bulky so that it is customary to take 
only about 0.2 gm. of wire for analysis which yields approximately. 
0.283 gm. of Fe2O3 so that an error of 0.0003 gm. in the final 
weight corresponds to 0.1 per cent. of iron in the sample. The 
precipitate of ferric hydroxide, moreover, often contains silica 
and alumina when the analysis is carried out in glass beakers, or 
when the reagents used have been in contact with glass for some 
time. This leads to high results and the error may amount to 
0.003 gm. Again, in igniting a precipitate of hydrated ferric 
oxide great care must be taken or some of it will be reduced to 
magnetite as can be shown by a magnet held outside the crucible.* 
oo ‘ 

* The carbon from the filter-paper causes the reduction when the pre- 


cipitate and filter are heated together and too much heat is used at the start. 
Heating the covered crucible over the blast lamp alsoconverts Fe,O, into Fe,Ox. 





Q2 GRAVIMETRIC ANALYSIS. 


When magnetite is thus once formed it is practically impossible 
to change it back to ferric oxide by further heating or by treating 
the precipitate with concentrated nitric acid. This leads to low 
results. When thrown down from hydrochloric acid solutions 
by the addition of ammonia, the precipitate often contains some 
chloride which it is hard to remove by washing, and on igniting 
such a precipitate there is danger of losing a little ferric chloride 
by volatilization. On account of these sources of error it is 
difficult to carry out a perfect gfavimetric estimation of iron 
with an accuracy sufficient for standardization purposes. The 
most satisfactory method is to determine the impurities present, 
but even although this may give the correct percentage of iron, 
if there is any carbon, phosphorus, silicon or sulphur present, as 
is usually the case, compounds may be left in solution which are 
oxidizable by potassium permanganate so that a sample of iron 
wire may show against permanganate an apparent iron value 
of from 100 to 101 per cent., in spite of the fact that it contains 
only 99.7 per cent. of pure iron. 

A. Classen * has proposed, therefore, to standardize pateneiaie 
permanganate against pure electrolytic iron and in the author’s 
laboratory this has been found to be very satisfactory. There is 
a possibility, however, of such electrolytic iron containing occluded 
hydrogen, carbon, etc., which may exert some effect upon the 
titration, although idea the electrolysis is properly carried out, 
these errors cannot be large. G. Lunge, in his report to the Sixth 
International Congress of Applied Chemists + (Rome, 1907), 
recommended that permanganate should be standardized against 
one of four substances: 

(1) Pure oxalic acid, the exact value of which has been deter- 
mined by titration against standard barium hydroxide solution. 
The barium hydroxide is standardized against hydrochloric acid, 
which is in turn titrated against sodium carbonate. (See 
Acidimetry.) 





* Mohr-Classen, Lehrbuch der chem.-analyt. Titriermethode (1896). 
¢ See also Z. Angew. Chem. 17, 195 (1904). 


PREPARATION OF ELECTROLYTIC IRON. 93 


(2) Pure iron wire, the exact value of which is known by a 
comparison with oxalic acid (standardized as above). 

(3) Sodium oxalate. (See Acidimetry.) 

(4) Hydrogen peroxide by the Nitrometer Method. 


Preparation of Electrolytic Iron. 


Some commercial ferrous chloride is dissolved in water and a 
little hydrochloric acid, the solution is saturated with hydrogen 
sulphide, and filtered. After boiling off the excess of hydrogen 
sulphide, the ferrous chloride is oxidized by means of potassium 





chlorate and hydrochloric acid, the excess of chlorine boiled off, 
and the solution neutralized by the careful addition of sodium 
hydroxide. The iron is then precipitated by the barium carbonate 
method (see p. 149). The well-washed precipitate is dissolved in 
hot, dilute hydrochloric acid and freed from barium by a double 
precipitation of the iron with ammonia. The hydrated ferric 
oxide thus obtained is dried, ignited, and reduced to metal in a 
stream of hydrogen as described on p. 88. The metallic iron is 
dissolved in the calculated amount of dilute sulphuric acid out of 
contact with the air (passing CO. through the solution*). The 

* For the generation of carbon dioxide, an apparatus similar to that shown 
in Fig. 30 is used, only the wash-bottle A is filled with permanganate solv- 
tion, and the tower C contains pumice soaked with copper sulphate solution, 


above which is a plug of cotton. 
The potassium permanganate and copper sulphate both serve to remove 


H,S from the CO,. 





94 _ GRAVIMETRIC ANALYSIS. 


solution is then diluted with water until 20 ¢.c. contain about 
0.35 gm. iron. 

In addition it is also necessary to provide a solution of am- 
monium oxalate, saturated at the room temperature. 

For the electrolysis, two electrodes K (Fig. 27) are prepared 
by taking two pieces of platinum-foil about 25 sq. cm. surface and 





Fie. 28. 


fastening a piece of fairly heavy platinum wire to each; they 
are bent so that they will conveniently pass through the neck 
of a liter-flask. The electrodes are cleaned by boiling in con- 
centrated hydrochloric acid and finally igniting them over the 
free flame. To accomplish the latter purpose, it is convenient 
to hang them upon a heavy platinum wire which is itself placed 


PREPARATION OF ELECTROLYTIC IRON. 95 


on an iron ring; they are then heated over the non-luminous 
flame of the Teclu burner (Fig. 28).* 

After the ignition the electrodes are allowed to cool in a desic- 
cator and weighed accurately by the method of swings (cf. p. 
10). About 350 c.c. of the ammonium oxalate solution are now 
placed in a 400-c.c. beaker and 20 c.c. of the iron solution 
(about 0.35 gm. Fe) are added. The beaker is covered with a 
glass plate containing three holes (Fig. 29). At the ends of 
the plate are fastened two corks which serve to support the 
two heavy platinum wires a and b. Through the two side 
holes are passed from below the bent platinum wires of the 





cathodes K, leaving them suspended from a; while through 
the middle hole the end of the spiral anode passes and is 
suspended from the cross-wire 6. The wire a is now con- 
nected with the negative, and wire b with the positive pole of a. 
battery, and the solution is electrolyzed for from one and one-half 
to two hours at about 60° with a current of 0.5-0.7 ampere. At 
the end of this time there will be firmly attached to each of the 
cathodes about 0.15-0.17 gm. of a bright, steel-gray deposit. 
The circuit is broken, one of the electrodes is removed, and 
the circuit again closed. The electrode which has been removed is 





* The electrodes must be above the inner flame mantle. 


96 GRAVIMETRIC ANALYSIS. 


at once plunged into a beaker of distilled water, taken out, the 
bottom edge touched with a piece of filter-paper to remove 
the greater part of the adhering water, and then washed with a 
liberal quantity of absolute alcohol that has been distilled over lime. 
The lower edge is again touched with filter-paper, then washed 
with ether which has been distilled over potash, after which it is 
dried in the hot closet until the ether has evaporated (this takes 
about half a minute). It is then placed in a desiccator. The sec- 
ond electrode is now removed from*the circuit and subjected to 
precisely the same treatment. After the electrodes have been in 
the desiccator for fifteen minutes they are weighed. 

While the solution is being electrolyzed the solvent for the iron 
should be prepared. In the liter-flask K (Fig. 30) are placed 
500 c.c. of water and 50 c.c. of chemically pure concentrated sul- 
phuric acid. The contents of the flask are heated to boiling, 
meanwhile passing a stream of carbon dioxide through the liquid. 
After the latter has boiled vigorously-for ten minutes the flask 
is closed at b, removed from the flame, placed in cold water, and 
allowed to cool in an atmosphere of carbon dioxide. 

In this manner a solution of sulphuric acid is obtained com- 
pletely free from air, so that there is no danger of its oxidizing any 
of the ferrous salt. 

One of the weighed electrodes, on which the iron has been de- 
posited, is thrown into the flask containing the sulphuric acid; the 
flask is immediately closed and gently heated on the water-bath, 
or better to boiling, still passing carbon dioxide through the 
apparatus. The iron dissolves very quickly, leaving no residue.* 
The flask is then closed at b, placed in cold water, and titrated 
with permanganate solution added from a glass-stoppered burette. 
After noting the burette reading, the permanganate is added drop 
by drop and the flask is constantly rotated to insure thorough mix- 
ing of the permanganate with the ferrous solution. When the solu- 

tion possesses a slight pink color, permanent for half a minute, the 





* Sometimes a minute trace of carbon remains undissolved, but it is so 
small in amount that it can safely be disregarded. 
+ See under Volumetric Analysis. 


PREPARATION OF ELECTROLYTIC IRON. 97 


end-point is reached, and after the burette has drained a second 
reading is taken. A blank test is made with another portion of 











Fie. 30. 


500 c.c. of water and 50 c.c. sulphuric acid solution (boiled free from 
air in the same way and allowed to cool in a stream of carbon 
dioxide), to see how much permanganate is necessary to impart 
this pink color in the absence of iron. This amount should be 
subtracted from the total number of cubic centimeters of the per- 
manganate solution used in titrating the iron. 

The results obtained by this procedure are excellent. 

After the strength of the permanganate solution has been 
accurately determined by the above method the apparent iron 
value of a sample of iron wire may be determined. When thisis 
known it is possible to determine accurately the strength of a new 
permanganate solution, or of the same solution at a future date, 
by titrating against a solution of the same wire. 





{ Thus Dr. Schudl obtained the following values by using three methods: 


1 c.c. KMnO,=0.005485 g. Fe with electrolytic iron; 
Te.c,. ~** °° =0,005470'g. ** + *‘* iodine; 
lec. ‘§ =0.005468 g. ff ‘* oxalic acid. 


98 GRAVIMETRIC ANALYSIS. 


Determination of the Apparent Iron Value of Iron Wire. 


The wire is cleaned by rubbing with a piece of emery paper until 
it is perfectly bright. It is then passed through filter-paper until 
it no longer leaves a gray mark on the paper. The wire is wound 
round a dry glass rod, making a spiral, and a portion of 0.15-0.2 gm. 
is weighed out. This is dissolved, as described on p. 601, in a 
flask which is fitted with a Bunsen valve * and contains 55 c.c. 
of dilute sulphuric acid (50 ¢.c. water+5 e.c. cone. H2SO4); the 
solution is boiled + a few minutes after the iron has all dissolved. 
It is allowed to cool, and is then titrated with permanganate 
which has been standardized against electrolytic iron or sodium 
oxalate. The apparent iron value of the wire is then calculated. 

Every time a new supply of iron wire is obtained its apparent 
iron value should be determined. 

The following results of determinations made with great care 
by W. A. K. Christie in the author’s laboratory illustrate the 
process: 

The permanganate solution was standardized against iron 
wire and it was found that 1 c.c.=0.005600 gm. iron, The purity 
of the sample of commercial iron wire was found in three titra- 
tions to be 99.93, 100.0 and 99.92 per cent., the average of 
which is 99.94 per cent. The actual purity of the wire was deter- 
mined to be 99.7 per cent. The apparent purity, therefore, is 
greater than the actual purity, and if the latter were to be used 
in the computation the permanganate solution would be found a 
little too strong. | 

The author recommends the standardization against elec- 
trolytic iron and compares the value obtained with that found 
with iron wire, the work all being done on the same day. Then 
the apparent iron value of the wire will be known for future — 





* Still better is the use of the glass valve as recommended by Contat 
(Chem. Ztg., 1898, 298) and improved by Géckel (Z. Angew. Chem., 1899, 
620). When the boiling is stopped, sodium bicarbonate is sucked back 
into the solution, and there is no overpressure 01 the outside. Cf. p. 601. 

+ The boiling of the solution is necessary, as otherwise hydrocarbons or 
other reducing substances remain in solution so that too much perman- 
ganate is used 


ANALYSIS OF FERRIC COMPOUNDS. 99 


standardizations and it will be necessary to standardize against 
electrolytic iron only when a new supply of iron is purchased. 
On the other hand, it must be admitted that the standardization 
against sodium oxalate is full as accurate and is easier to carry 
out. See Volumetric Analysis, p. 597. 3 


Analysis of Ferric Compounds according to the Method of 
| Margueritte. 


From what has already been said, it is evident that in order to 
determine the amount of iron present in a solution by titration 
with potassium permanganate, it is necessary for the iron to be 
present entirely in the ferrous condition. In order, therefore, 
to apply this method to the analysis of ferric compounds, it is 
first necessary to reduce them completely. 

To effect the reduction of a ferric sulphate solution we can 
proceed as follows: The solution is placed in a 200-c.c. flask, acidified 
with one-tenth its volume of pure, concentrated sulphuric acid, 
the flask is closed with a stopper provided with two tubes through 
which gas can enter and leave the flask, the contents of the flask 
are heated to boiling and hydrogen sulphide is passed through the 
solution until it is perfectly colorless. The boiling is continued 
and carbon dioxide is now passed through the solution until the 
excess of hydrogen sulphide is completely removed. The solu- 
tion is then allowed to cool in an atmosphere of carbon dioxide 
and titrated exactly as in the standardization of the solution of 
permanganate. 

If tc.c. of permanganate were necessary to completely oxidize 
the solution and 1 c.c. of the permanganate corresponds to a gm. 
of iron, then the titrated solution evidently contains a.t gm. of 
iron. | 

Besides hydrogen sulphide, a great many other substances 
can be used to. reduce the ferric salt, e.g., zinc, sulphurous acid, 

.stannous chloride. The use of these substances will be discussed 
in the portion of this book devoted to Volumetric Analysis. 

Remark.—The titration of a solution by means of potassium 
permanganate takes place preferably in a sulphuric acid solution; 

in the case of hydrochloric acid too high results will be obtained 


100 GRAVIMETRIC ANALYSIS. 


(due to the fact that: the permanganate oxidizes some of the acid), 
unless the oxidation takes place in a dilute solutior in the presence 
of a large excess of manganous sulphate. See Volumetric Analysis. 


TITANIUM, Ti. At. Wt. 48.1. 


Titanium, when present in large amounts, is determined as its 
dioxide, TiO,; but if only small amounts are to be determined, 
as in the case of many rocks and iron ores, the colorimetric 
method is preferable. 


(a) Determination as Titanium Dioxide. 


The titanium is precipitated from solution either by means of 
ammonia, or by boiling a solution strongly acid with acetic acid 
and containing considerable ammonium acetate; or, finally, by 
boiling the slightly acid solution of the sulphate. In all these 
cases it is precipitated as titanic acid, from which it is changed by 
ignition into TiO,. 

The two former methods are preferable to ig third. See 
separation of titanium from aluminium. 


(b) Determination of Titanium Colorimetrically ; Method of 
A. Weller.* 


(Suitable for small amounts of titanium.) 


This determination depends upon the fact that acid solutions 
of titanium sulphate are colored intensely yellow when treated 
with hydrogen peroxide; the yellow color increases with the 
amount of titanium present and is not altered by an excess of 
hydrogen peroxide. On the other hand, inaccurate results are 
obtained in the presence of hydrofluoric acid (Hillebrand); con- 
sequently it is not permissible to use hydrogen peroxide for this 
determination which has been prepared from barium peroxide by 
means of hydrofluosilicie acid. Furthermore, chromic, vanadic, 
and molybdie acids must not be present, since they also give 
colorations with hydrogen peroxide. The presence of small amounts 
of iron docs not affect the reaction, but large amounts of iron cause 
trouble on account of the color of the iron solution. If, however, 
phosphoric acid is added to the colored ferric solution it becomes 





* Berichte, 15, p. 2593. 


DETERMINATION OF TITANIUM. 101 


decolorized, and from sucha solution the determination of titanium 
offers no difficulty. The solution in which the titanium is to be 
determined must contain at least 5 per cent. of sulphuric acid; 
an excess does not influence the reaction. The reaction is so 
delicate that 0.00005 gm. of TiO, present as sulphate in 50 c.c. 
of solution give a distinctly visible yellow coloration. 

For this determination a standard solution of titanium sulphate 
is required. This can be prepared by taking 0.6000 gm. of po- 
tassium titanic fluoride which has been several times recrystal- 
lized and gently ignited (corresponding to 0.2 gm. of TiO,). This is 
treated in a platinum crucible several times with a little water 
and concentrated sulphuric acid, expelling the excess. of acid by 
gentle ignition, finally dissolving in a little concentrated sulphuric 
acid and diluting with 5 per cent. sulphuric acid to 100 c.c. One 
cubic centimeter of this solution corresponds to 0,002 gm. TiO,. © 

The determination proper is carried out in the same way as 
described on p. 60, under the colorimetric determination of am- 
-monium. | , 

50 c.c. of the solution which has been brought to a definite 
and accurately measured volume is placed in a Nessler tube be- 
side a series of other tubes, each containing a known amount 
of the standard titantium solution, filled up to the mark with 
water and each treated with 2 c.c. of 3 per cent. hydrogen per- 
oxide * (free from hydrofluoric acid). The color of the solution 
in question is compared with the standards. This method is 
only suitable for the estimation of small amounts of titanium, as 
the shades of strongly colored .solutions cannot be compared 

accurately. 

According to J. H. Walton, Jr.,f titanium in the presence of 
iron may be determined after fusing the finely powdered sub- 
stance with two or three times as much sodium peroxide. On 
extracting with water, sodium pertitanate goes into solution and 
ferric oxide is left behind. The filtered solution is acidified with 
sulphuric acid, which is added until 5 per cent. of free acid is 





*The hydrogen peroxide solution is prepared shortly before using by 
dissolving commercial potassium percarbonate in dilute sulphuric acid. 
7 J. Am. Chem. Soc., 29, 481 (1907). 


102 GRAVIMETRIC ANALYSIS. 


present. The color of the solution is then compared with that 
obtained by fusing a known weight of TiO, with Na,O,, etc. 


CHROMIUM, Cr. At. Wt. 52.1. 
Forms: Chromic Oxide, Cr,0,; Barium Chromate, BaCrO,. 


(a) Chromic Compounds. 
Determination as Chromic Oxide. 
1. By Precipitation with Ammonia or Ammonium Sulphide. 


If the chromium is present in solution as chromic compound 
it can be precipitated exactly as described under aluminium, by 
means of a slightest possible excess of ammonia* in the presence 
_ of considerable ammonium salts (or better still, by the addition of 
freshly prepared ammonium sulphide solution to the boiling solu- 
tion). The precipitated Cr(OH)3 is washed with dilute ammonium 
nitrate solution and ignited wet in a platinum crucible, being there- 
by changed to the oxide, Crg03. The results obtained are always 
a few tenths of a per cent. too high on account of the formation 
of small amounts of alkali chromate even though the entire opera- 
tion takes place in platinum vessels. The alkali comes from the 
reagents. It can be shown that the ignited product contains 
a little chromate, as the aqueous extraction always possesses a 
slight yellow color and gives with silver nitrate a red precipitate 
of silver chromate. 

If phosphoric acid is present, it will be found in the precipitate. 
In this case the dried precipitate is fused in a platinum crucible 
with sodium carbonate and potassium nitrate, whereby sodium 
chromate and sodium phosphate are obtained. The melt is dis- 
solved in water, acidified with nitric acid, and the phosphoric 
acid precipitated by means of ammonia and magnesia mixture, 
as described under Phosphoric Acid. From the filtrate the 
chromium is determined as barium chromate in acetic acid solu- 
tion as described below. 





* An excess of ammonia prevents the complete precipitation of the 
chromium hydroxide, the filtrate is then colored pink. Im such cases the 
filtrate must be boiled until the excess of ammonia is expelled, and the 
chromium is all precipitated. 


CHROMIUM. 103 


2. By Precipitation with Potassium Iodide—Iodate Solution. 
Method of Stock and Massaciu.* 


The determination is carried out as in the case of aluminium 
(cf. p. 83). The slightly acid f solution, contained in a porcelain 
dish, is treated with a mixture of potassium iodide and iodate, 
decolorized after a few minutes by means of sodium thiosulphate 
solution, treated with a little more iodide and iodate and then again 
with a few c.c. of sodium thiosulphate, and heated half an hour on 
the water-bath. The flocculent precipitate of chromic hydrox- 
ide settles quickly, and is filtered preferably through a hot water 
filter under slight suction. The precipitate is ignited wet in a 
platinum crucible. 


3. By Precipitation with Ammonium Nitrite.t 


If the solution of the chromic salt is acid, it is neutralized 
with ammonia until a slight permanent precipitate is obtained. 
This precipitate is dissolved by the addition of a few drops of 
hydrochloric acid and then an excess of 6 per cent. ammonium 
nitrite solution is added and the liquid boiled until all nitrous 
fumes have been expelled. By this means practically all of the 
chromium will have been precipitated, but in order to throw down 
the last traces, ammonia is added drop by drop until the odor 
of free ammonia barely persists in the solution. The precipitate 
is allowed to settle while the beaker remains on the water-bath, 
and is finally filtered off, washed with hot water, ignited wet in 
a platinum crucible, and weighed as Cr,O,. 


(6) Chromates. 


If the chromium is present in solution in the form of an alkali 
chromate, free from chloride and large amounts of sulphuric acid, 





* Ber., 1901, 467. 

+ If the solution is strongly acid, it is neutralized by the addition of pure 
KOH solution drop by drop, until a faint permanent turbidity is obtained. 

t E. Schirm, Chem. Ztg., 1909, 877. Cf. p. 111. According to Scholler 
and Schrauth (ibid., 1909, 1287) iron, chromium, aluminium, and zine can be 
precipitated by means of aniline. 


104 GRAVIMETRIC ANALYSIS. 


it may be determined very accurately by precipitation with mer- 
curous nitrate solution as mercurous chromate; on ee the 
latter is changed to Cr,Q,. 

Procedure. —The neutral or weakly acid solution is treated 
with a solution of pure mercurous nitrate whereby brown, basie 
mercurous chromate, (4Hg,O,3CrO,), is formed. On heating to 
boiling, the precipitate becomes a beautiful, fiery red, being con- 
verted into the neutral salt Hg,CrO,. This red salt settles very 
quickly, and if the precipitation is cOmplete the solution above the 
precipitate will be colorless. After cooling, the precipitate is filtered 
off, washed thoroughly with water containing a little mercurous 
nitrate, dried and separated from the filter as completely as possible. 
The filter is burned in a platinum spiral and ignited with the main 
portion of the precipitate, gently at first and finally strongly, in 
a platinum crucible under a hood witha good draft, afterwards 
weighing the residue as Cr,O,. | 

The purity of the mercurous nitrate must be tested before using 
it. 5 gms. of the salt should leave no residue after being ignited. 

This excellent method for the determination of chromium 
unfortunately permits only a very limited application. If 
the solution contains any considerable amount of chloride, 
mercurous chloride will be precipitated with the mercurous chro- 
mate, which, although volatile on ignition, renders the precipitate 
too bulky and the method inaccurate. 

If, therefore, it is necessary to determine chromium present 
-as chromate in a solution: containing chloride, two other methods 
are at our disposal. The chromate may be reduced by boiling 
with sulphurous acid (or by evaporating with concentrated hydro- 
chloric acid and alcohol) and analyzed according to (a), or it 
may just as accurately, and much more conveniently, be deter- 
mined by precipitating as 


Barium Chromate, 


which is weighed after gentle ignition. 

Procedure.—The neutral solution, or one weakly acid with acetic 
acid, is treated at the boiling temperature with a solution of barium 
acetate added drop by drop,* and after standing for some time, 





* If the barium acetate solution is added too quickly some of it will be 


BARIUM CHROMATE. 105 


is filtered through a Gooch crucible (without using very strong 
suction, as otherwise the filter will soon get stopped up and the 
solution will filter extremely slowly). The precipitate is washed 
with dilute alcoho] and dried in the hot closet. The crucible is 
suspended in a larger one of porcelain by means of an asbestos 
ring (cf. page 27) and heated, at first gently, and finally over the 
full flame of a good Bunsen burner. After five minutes the cover 
is removed and the heating is continued until the precipitate 
appears a uniform yellow throughout, when it is cooled in a desic- 
cator and weighed. 

Sometimes the precipitate appears green on the sides of the 
-erucible owing to a slight reduction (by means of dust, traces 
of alcohol, etc.) of chromic acid to chromic oxide. The latter 
gradually takes on oxygen from the air during the long-continued 
heating of the open crucible, so that the green color gradually 
disappears. 

If a grams of chromate were taken for analysis, and the 
barium chromate precipitate weighed p grams, then the amount 
of chromium present may be calculated as follows: 


BaCrO, : Cr=p:s 


oe Beate 
~ Bacro, /” 


and 
> Cr filets > 
100 Cr p_ 
Bary). a ews per cent. Cr. 


Example for practice: Potassium bichromate, K,CreQv7, 
purified and dried as described on pages 33 and 35. 

Chromium originally present as chromate, or obtained as 
such after suitable oxidizing treatment, may be determined 
- accurately by volumetric methods described in Part IT. 





carried down with the barium chromate, so that too high results will be 
obtained. 


106 GRAVIMETRIC ANALYSIS. 


URANIUM, U. At. Wt., 238.5. 
Forms: U,0, and UO,, 


(a) Determination as U,0,. 


Uranium is almost always precipitated by means of ammonia 
as ammonium uranate and changed to U,O, by gentle ignition 
in a platinum crucible with free gccess of air. According to 
Zimmerman * this transformation is only complete when the 
precipitate is ignited in a stream of oxygen; the error is, however, 
so small that for ordinary purposes it can be neglected. 

According to the temperature of ignition, the U,O, appears 
dirty green or black, and is difficultly soluble in dilute hydro- 
chloric or sulphuric acids; in nitric acid it dissolves gradually. 
By heating with dilute sulphuric acid (1 vol. cone. H,SO,+6 vol. 
H,O) in a closed tube at 159°-175° C. for a long time (W. F. 
Hillebrand),+ the U,O, is completely dissolved with the formation 
of uranous and uranyl sulphate: 


U,0,+ 4H,SO,=2U0,(SO,)+U (SOs) 2+ 4H,0. 


U,O, is also readily soluble in dilute sulphuric acid in the 
presence of potassium bichromate. These two last facts are taken 
advantage of in the volumetric determination of uranium (which 
see). 


(6) Determination as UO,. 


The ignited precipitate, obtained in exactly the same way 
as before, is heated over a good Teclu burner, or over the blast- 
lamp, in a current of hydrogen, untila constant weight is obtained 
whereby it is quantitatively changed to UO,. This is the most 
accurate method for the determination of uranium. 

The UO, thus obtained is a brown powder, insoluble in dilute 
hydrochloric and sulphuric acids, but soluble in concentrated 
sulphuric acid after long (Mea best in a closed tube. ‘This 
oxide is also soluble in nitric acid. 


se * Ann. d. Ch. und Ph., 232 (1886), p. 287. 
f Bull. U.S. Geol. Survey, 78, p. 90. 





SEPARATION OF IRON FROM ALUMINIUM. 107 -" 


Separation of Iron, Aluminium, Chromium, Titanium, and 
Uranium from Calcium, Strontium, Barium, and Magne- 
sium. 3 

The solution containing the above substances in the presence 
of considerable ammonium chloride is placed in an Erlenmeyer 
flask and treated with a slight excess of freshly prepared ammo- 
nium sulphide free from sulphate and carbonate. After stand- 
ing overnight the precipitate is filtered off and washed with 
water containing ammonium sulphide. It contains the iron and 
uranium as sulphides, the aluminium, chromium, and titanium 
as hydroxides. In case large amounts of magnesium are present, 
some of it is almost always present in the precipitate, so that it 
is then necessary to dissolve the precipitate, after filtration, in 
hydrochloric acid and to reprecipitate with ammonium sulphide. 

Instead of using ammonium sulphide, the separation can pe 
accomplished satisfactorily with ammonia; the iron must then 
be in the ferric condition. 


Separation of Iron from Aluminium. 


(1) The solution is treated in a porcelain dish with pure 
potassium hydroxide solution until strongly alkaline, boiled, 
diluted with hot water, and filtered. The precipitate contains 
the iron as hydroxide, while the solution contains the aluminium 
as aluminate.* For the iron determination the precipitate is 
dissolved in hydrochloric acid, reprecipitated with ammonia,t 
dried, and weighed as FeO, (see page 83). The aluminium is 
precipitated as hydroxide from the filtrate by acidifying with 
nitric acid and then adding ammonia. . 
_ (2) The acid solution is treated with tartaric acid (three parts 
of tartaric acid for each part of the mixed oxides (Fe,0,+ Al,0,)), 
hydrogen sulphide is passed into the solution until it is saturated, 
. as slight an excess as possible of ammonia is added, and the sulphide 
of iron is allowed to settle in a closed Erlenmeyer flask. It is then 





* If the precipitate is large, it should be dissolved in hydrochloric acid 
and again precipitated with KOH. 

+ It is very hard to wash the KOH precipitate free from alkali so that the 
first precipitate should not be weighed. 


108 GRAVIMETRIC ANALYSIS. 


filtered, washed with water containing ammonium sulphide, dis- 
solved in hydrochloric acid oxidized with a little potassium chlorate 
or nitric acid, and precipitated as ferric hydroxide by the addition 
of ammonia. The aluminium is determined in the filtrate by 
evaporating to dryness with the addition of a little sodium carbo- 
nate and potassium nitrate. The residue is gently ignited in a 
platinum dish in order to destroy the tartaric acid, after which 
it is dissolved in dilute nitric acid, the carbon filtered off, and the 
aluminium precipitated from the Solution by the addition of 
ammonia. ; 

(3) The neutral solution of the chlorides or sulphates (not 
the nitrates) is treated with sodium carbonate until a slight 
permanent precipitate is formed, which is dissolved by the addi- 
tion of a few drops of hydrochloric acid. The solution is diluted 
to about 250 c.c. for each 0.1 or 0.2 gm. of the metals present, an 
excess of sodium thiosulphate solution is added, and the solution 
boiled until every trace of SO, has disappeared. By this opera- 
tion the ferric salt is reduced to ferrous salt: 


2Na,8,0, +2FeCl, = 2NaCl+ Na,S,0,+2FeCl,, 
and the aluminium is precipitated as the hydroxide: 
2AI1Cl, +3H,O +3Na,8,0, = 6NaGl+3S0, +35 + 2Al(OH);. 


The precipitate of aluminium hydroxide and sulphur is filtered 
off, washed with hot water, dried, transferred as completely as 
possible to a porcelain crucible, the filter burned in a platinum 
spiral and the ash added to the crucible, which is ignited gently 
until all the sulphur has been expelled and then more strongly 
over the blast or a Teclu burner until the weight is constant. 

To determine the iron, the filtrate may be acidified with hydro- 
chloric acid, the SO, boiled off, the sulphur filtered off, the solu- 
tion oxidized with nitric acid and precipitated by ammonia as 
described on page 87. It is still better to precipitate the 
iron with ammonium sulphide, filter, dissolve in hydrochloric 
acid, oxidize with nitric acid, and then precipitate with am- 
monia. 


SEPARATION OF IRON FROM ALUMINIUM. 109 


(4) Both of the metals are precipitated with ammonia, filtered, 
washed, dried, ignited in a platinum crucible, and the weight of the 
combined oxides determined. The mixture is then digested with 
concentrated hydrochloric acid to which a little water has been 
added (10HC1:1H,O) in a covered crucible until the iron is com- 
pletely dissolved. If ferric oxide predominates, as is frequently 
the case, the solution is effected in one or two hours. If, on the 
other hand, a relatively large amount of alumina is present (as is 
usually the case with silicates), and which can be detected by the © 
color of the precipitate produced by ammonia, the precipitate 
then dissolves very slowly and in many cases only incom- 
pletely. 

In the latter case the ignited oxides are brought into solution 
by fusing with 12-15 times as much potassium pyrosulphate, 
K,S,O7 (cf. Vol. I).* The decomposition of the oxides is usually 
complete in 2-4 hours. The crucible together with its cover 
is placed in a beaker, water and a little sulphuric acid are added, 
and the melt is dissolved by warming gently, and passing a current 
of air through the solution in order to keep the liquid in motion. 
A small amount of platinum is always dissolved by this treatment. 
After removing the crucible and its cover, the solution is heated 
to boiling and saturated with hydrogen sulphide. The solution 
is then filtered into a flask and carbon dioxide is passed through 
it until the excess of hydrogen sulphide is completely removed. 
The contents of the flask are then cooled by placing the flask in 
cold water, the carbon dioxide still passing through the flask. 
The iron is then titrated with potassium’ permanganate solution 
as described on page 99. The aluminium is determined by differ- 
ence, from the weight of the combined oxides. For the determina- 
tion of iron in silicates the above process is most suitable (Hille- 
brand). The reduction of the ferric salt to ferrous salt by means of 
hydrogen sulphide possesses great advantages over the reduction 
by means of zinc, for in the former case no foreign element is intro- 
duced, and furthermore zinc serves to reduce the titanic acid 





* E. Deussen finds that fusion with KF.HF works better. The platinum 
is not attached and the solution is effected more readily.—Z. angew. Chem., 
1905, 815. 


110 GRAVIMETRIC ANALYSIS. 


that is almost always present in rocks, and this will be again oxidized 
by the permanganate, so that too high an iron value will be ob- 
tained. : . 

If the iron is all dissolved by treating the oxides with hydro- 
chloric acid, the solution is evaporated to dryness and the residue 
is treated with a few cubic centimeters of dilute sulphuric acid, 
evaporated on the water-bath as far as possible, and then heated 
over the free flame until fumes of sulphuric acid are evolved. 
After cooling, the product is dissolved in«water and the ferric 
sulphate reduced to ferrous sulphate by introducing a piece of 
zine, free from iron, into the crucible and covering the latter with a 
watch-glass.* The reduction is complete in 20-30 minutes. The 
slight residue of platinum f is filtered off with the excess of zine 
into a flask already filled with carbon dioxide. The residue is 
washed with water that has been boiled, and the solution is titrated 
with potassium permanganate solution. | 

The latter method is to be recommended for the determination 
of small amounts of iron in the presence of still less aluminium, as 
is the case in the analysis of mineral waters. ) 

The following procedure leads to ibe same end, but the results’ 
are not quite so reliable: | 

The solution from which the iron and aluminium are to be 
determined is diluted to a definite volume (e.g., 250 ¢.c.) and two 
aliquot portions are taken by means of a pipette (usually 100 
c.C.). 

In one portion the weight of the combined oxides of iron and 
aluminium is determined by precipitation with ammonia and ignition 
of the precipitate, while in the other the iron is determined by 
titration. If the solution contains hydrochloric acid, as is usually 
the case, the iron is first precipitated with ammonia, filtered, 





* If titanium is present, the solution is reduced by means of hydrogen 
sulphide. 

t Platinum is perceptibly attacked by long digestion with ferric chloride’ 
solution: 


4FeCl, + Pt + 2HCl= H,PtCl, + 4FeCl.. 


The chloroplatinic acid is reduced to platinum by the action of zinc. 


SEPARATION OF IRON, ALUMINIUM, PHOSPHORIC ACID: 111 


washed, and dissolved in dilute sulphuric acid. The solution is 
then reduced and titrated as previously described.* 


Separation of Iron, Aluminium, and Phosphoric Acid. 


Although the determination of phosphoric acid has not. yet 
been considered, we will describe its determination in the presence 
of iron and aluminium because this highly important separation is 
necessary in the analysis of almost all minerals containing iron 
and aluminium as well as in the analysis of many mineral waters. 
Two cases are to be distinguished: 

1. The solution contains only a small amount (a few centigrams 
or less) of iron, aluminium, and phosphoric acid. 

2. The solution contains large amounts of these substances. 

1. In the first case the determination of all three constituents 
must be undertaken in the same portion, as otherwise errors would 
be introduced on account of the small amounts to be determined. 
The solution is first treated with ammonia whereby the iron, alu- 
minium and phosphoric acid are precipitated. 

The precipitate is ignited in a platinum crucible and weighed: 


Fe,0,+ Al,O,+ P,0,=A. 


The product is then fused with six times its weight of a mixture 
consisting of four parts anhydrous sodium carbonate and one part 
pure silica. The mixture is heated over the blast-lamp, the melt 
is extracted with water, to which a little ammonium carbonate 
has been added, and filtered. The filtrate contains all of the phos- 
phorie acid and a very little silicic acid, while the residue contains 
all of the iron and aluminium and considerable silica. 

For the determination of the phosphoric acid, the filtrate is 
evaporated with hydrochloric acid on the water-bath to dryness, 





* It is necessary to get rid of the hydrochloric acid on account of its action 
“upon potassium permanganate (cf. Vol. Anal., under Iron). 

+ The phosphoric acid is usually present in such small amounts that the 
iron and aluminium are more than sufficient to effect the precipitation of all 
the phosphoric acid, on the addition of ammonia, as phosphates of these 
metals. 


II2 GRAVIMETRIC ANALYSIS. 


in order to remove the silica, the residue is moistened with hydro-< 
chloric acid, taken up in a little water, filtered, and the phosphoric 
acid precipitated in the filtrate by the addition of ammonia and 
“magnesia mixture.” The precipitate of magnesium ammonium 
phosphate is changed to magnesium pyrophosphate by ignition and 
from its weight p the amount of phosphoric anhydride, P,O,, is 
calculated (=B): 


> 
Mg,P,0,:P30;=p:B, 


ae POs “?p 
Mg,P.0, ~~ 





B 


By subtracting B from A the combined weight of the iron and 
aluminium oxides is obtained, in which the iron is determined 
volumetrically and the aluminium by difference. For the deter- 
mination of the iron, the insoluble residue, obtained after treating 
the product of the fusion with water and ammonium carbonate, 
is digested with hydrochloric acid in a small porcelain crucible 
until the iron oxide is completely dissolved. The solution is treated 
with dilute sulphuric acid, evaporated on the water-bath as far 
as possible, and then over a free flame until fumes of sulphuric 
anhydride are evolved. After cooling, water is added and after 
digesting on the water-bath for a long time the silica is filtered 
off, the solution reduced by means of hydrogen sulphide (cf. p. 
109, sub. 4), and, after removing the excess of hydrogen sulphide, 
the iron is titrated with permanganate solution.* From the 
amount of permanganate used, the amount of ferric oxide (C) can 
be calculated, and by deducting this amount from the weight 
of the combined oxides, the weight of the Al,O, is ascertained: 

A—(B+C) =Al,0,. 

2. In case the solution contains large amounts of iron, alu- 
minium, and phosphoric acid, it is divided into three aliquot por- 
tions and in one the value of ‘‘ A ” is determined by precipitation — 
with ammonia; in the second the phosphoric acid is determined 
by the molybdate method; and in the third the iron is determined 
by titration. 


* Instead of reducing the iron, the ferric salt may be titrated directly 
with titanous chloride (cf. p. 699, or iodimetrica!ly (ef. p. 681). 





SEPARATION OF IRON FROM CHROMIUM. 113 


Separation of Iron from Chromium. 


1. The chromium is oxidized in alkaline solution by means of 
chlorine or bromine to a soluble chromate and the insoluble ferric 
hydroxide is filtered off. 

Procedure.—The solution of the chlorides, which should be 
placed in an Erlenmeyer flask of Jena glass provided with a 
ground-glass stopper and tubes by which gas may enter and leave 
the flask, is treated with potassium hydroxide solution until 
strongly alkaline, warmed on the water-bath and chlorine gas is 
conducted through the liquid, or bromine water is added, until 
it becomes distinctly yellow and the ferric hydroxide has as- 
sumed its characteristic reddish-brown color. When the oxidation 
is performed by chlorine gas, 0.5 gm. of the mixed oxides will be 
completely oxidized in fifteen to twenty minutes. The solution is 
diluted with water and filtered. The filtrate is carefully acidified 
with acetic acid, the chromium precipitated by the addition 
of barium acetate, and the precipitate of barium chromate is 
treated as described on p.104. The ferric hydroxide is dissolved 
in hydrochloric acid, reprecipitated with ammonia and weighed 
as ferric oxide. 

Remark.—lf the chromate is to be determined as barium chro- 
mate, the solution must contain no sulphuric acid. If the latter is 
present, the chromate is reduced by evaporating with hydrochloric 
acid and alcohol; the solution of chromic chloride thus obtained 
is precipitated with ammonia and the chromium determined as 
chromic oxide. 

In the case of a precipitate containing iron and chromic oxides, 
it is fused with sodium carbonate and a little potassium chlorate, 
the melt is extracted with water, and the chromium is determined in 
the solution by precipitating with barium acetate. The insoluble 
residue from the aqueous extraction of the fusion is dissolved in 
hydrochloric acid, precipitated with ammonia, and the iron deter- 
mined as ferric oxide. 

If it is desired to precipitate the chromium as mercurous 
chromate, the precipitate containing the iron and chromic oxidcs 
is fused with sodium carbonate and potassium nitrate, the melt 


114 3 GRAVIMETRIC ANALYSIS. 


extracted with water, the solution neutralized with nitric acid and 
precipitated with mercurous nitrate solution, as described on p. 104. 

2. It has been proposed to analyze the mixture of ferric and 
chromic oxides by strongly igniting them in a stream of hydrogen 
whereby the ferric oxide is reduced to metallic iron, while the 


chromic oxide is unchanged. The iron could then be determined ~~ 


by the loss of weight. This method, although theoretically very 
simple, seems from experiments. carried out in the author’s 
laboratory to be absolutely inadequate, for the ferric oxide is so» 
enveloped in chromic oxide that it is not even approximately re- 
duced even when heated over the blast-lamp. 

3. Iron may be separated from chromium by precipitating 
the former with ammonium sulphide from a solution containing 
sufficient ammonium tartrate to prevent the precipitation of the 
chromium. The separation is the same as was described under 
aluminium, p. 107, sub. 2. 


Separation of Aluminium from Chromium. 


If the chromium is present as chromic salt, it. is oxidized by 
means of chlorine or bromine in a solution made strongly alkaline 
with potassium hydroxide. The solution is then acidified with 
nitric acid, and the aluminium precipitated by ammonia as hydrox- 
ide, being weighed as the oxide. In the absence of sulphuric acid 
the chromium may be determined in the filtrate as barium chro- 
mate (cf. p. 104). If sulphuric acid is present, the chromate is 
reduced to chromic salt again by the action of concentrated hydro- 
chloric acid and alcohol, precipitated with ammonia, and weighed 
as the oxide 

If, however, the chromium is already present as chromate, the 
aluminium is at once precipitated with ammonia as hydroxide. 


Separation of Iron from Titanium. 


It is frequently necessary to determine both iron and titanium — 
in a precipitate produced by ammonia consisting of a mixture of 
these two oxides alone, but it is more often necessary to determine 


SEPARATION OF IRON FROM TITANIUM. II5 


titanium in the presence of iron, aluminium, and phosphoric acid, 
all of which are precipitated by ammonia in the analysis of 
rocks. 

For the separation of titanium from iron in the absence of 
alumina, the following methods are suitable: 

1. The precipitate produced by ammonia is ignited and then 
fused with 15-20 times as much of previously dehydrated potas- 
sium pyrosulphate over a small flame until completely attacked. 
After cooling, the melt is dissolved in cold water containing: sul- 
phuric acid, and the solution is hastened by keeping the liquid in 
motion by means of a current of air passed through it. — 

The solution thus obtained is diluted to a definite volume, and 
after being thoroughly mixed is divided into two portions, one being 
used for the determination of titanium and the other for the deter- 
mination of iron. For the iron determination, the acid solution is 
saturated with hydrogen sulphide in the cold, heated to boiling, and 
the precipitate of platinum sulphide, sulphur, and a little titanium 
is filtered off into a flask filled with carbon dioxide, and washed 
thoroughly with hot water. The filtrate is heated to boiling and 
carbon dioxide is passed through the solution until the excess of 
hydrogen sulphide is completely removed, when it is cooled in an. 
atmosphere of carbon dioxide and then titrated with perman- 
ganate. For the titanium determination, the other part of the 
solution is treated with sodium carbonate solution until a slight 
precipitate is formed; this is dissolved in as little sulphuric acid as 
possible, saturated with hydrogen sulphide in the cold, and 5 gms. 
of sodium acetate which has been neutralized with acetic acid* are 
added. Carbon dioxide is conducted through the solution, it is 
neated to boiling, filtered hot, washed with water containing hydro- 
gen sulphide, ignited wet in a platinum crucible, and weighed as 
TiO,. 

Remark.—lIf considerable iron is present, the titanic oxide thus 
obtained is likely to contain iron. It is brought into solution 
again by fusing with potassium pyrosulphate and the precipitation 
is repeated exactly as before. In this way a precipitate free from 
irou is obtained. 

2. The Chancel-Stromayer method is also satisfactory. The 
solution from the pyrosulphate fusion, in this case after being 





* Cf. footnote to paze 150. 


116 GRAVIMETRIC ANALYSIS. _! 


neutralized with sodium carbonate, is treated with an excess of 
sodium thiosulphate, diluted to about 400-500 c.c. and boiled for 
some time. In this way metatitanic acid and sulphur are precipi- 
tated, while iron remains in solution. During the filtration, how- 
ever, the finely divided sulphur passes through the filter, so that 
the first method is preferable. In the presence of considerable 
iron the metatitanic acid obtained by this method is also contam- 
inated with iron, so that the separation must be repeated. 


Separation of Aluminium from Titanium. 


It has been proposed to effect this separation by diluting to a 
considerable volume the slightly acid solution of the melt obtained 
by the potassium pyrosulphate fusion and boiling for some time, 
thereby. precipitating the titanium and leaving the aluminium in 
solution. This method, however, is useless, for alumina is pre- 
cipitated with the metatitanic acid unless the solution contains 
enough acid to prevent this hydrolysis, in which case a considerable 
amount of titanic acid remains in solution. 

The best separation is that of Gooch;* it consists of boiling a 
solution of the two elements containing considerable free acetic 
acid and alkali acetate; by this means all of the titanium and none 
of the aluminium is precipitated. If, however, the amount of 
aluminium present is large (as is usual in rock analysis), the pre- 
cipitate will contain some aluminium, so that the separation must 
be repeated. In no case is there danger of the precipitation of the 
titanium being incomplete. 

In practice it is almost always necessary to separate the titanium 
not from aluminium alone, but from iron and aluminium, so that 
the method of Gooch will be described for this more general case. 

The solution obtained by dissolving the pyrosulphate melt in 
cold water is treated with three times as much tartaric acid as 
the weight of the oxides, is saturated with hydrogen sulphide 
gas, and then made slightly ammoniacal. By this means all of the 
iron is precipitated as ferrous sulphide, while the aluminium and 
titanium remain in solution. The sulphide of iron is filtered off, 
the filtrate is acidified with sulphuric acid, heated to boiling, and 
the precipitate of sulphur and platinum sulphide (the latter from 
the platinum crucible in which the fusion with pyrosulphate was 


* Chemical News, 52, pp. 55 and 68. 





SEPARATION OF ALUMINIUM FROM TITANIUM. II7 


made) is filtered off. The filtrate is boiled to expel the last traces 
of hydrogen sulphide and the tartaric acid is destroyed by adding 
24 times as much potassium permanganate as the amount of tar- 
taric acid present. Sulphurous acid is then added until the precipi- 
tated manganese dioxide is redissolved, after which a slight excess 
of ammonia is added and then 7-10 c.c. of glacial acetic acid for 
each 100 c.c. of solution. The solution is boiled for one minute, 
the precipitate is allowed to settle, and the filtrate is decanted 
through a filter,* transferred to the filter, washed with 7 per cent. 
acetic acid and finally with hot water. The dried precipitate is 
ignited over a Bunsen burner for from fifteen to twenty minutes 
and then weighed. : 

The precipitate contains manganese and aluminium, so that it 
is fused with three times as much sodium carbonate. The melt 
(colored green by the manganese) is leached with cold water, leaving 
sodium metatitanate t and some alumina undissolved. The precipi- 
tate is filtered off by means of a small filter, is ignited in a platinum 
crucible, and fused again with a little sodium carbonate. After 
cooling, the contents of the crucible are dissolved in 1.9 c.c. of 
sulphuric acid (1 vol. conc. H,SO,:1 vol. H,O) diluted to about 
150-200 c.c. and treated with 5 gm. sodium acetate and one-tenth 
of its volume of giacial acetic acid. After boiling one minute and 
allowing to stand until settled, the precipitate is filtered off, washed 
with 7 per cent. acetic acid, then with water, dried, ignited, and 
weighed. This precipitate usually contains aluminium, so that 
it is again fused with sodium carbonate and the melt again treated 
with sulphuric acid, etc., exactly as described above. . This time 
the precipitate is usually free from aluminium, but the process 
should be repeated until a constant weight is obtained. 

This analysis does not require much time, for usually the 
amount of titanium present is so small that the precipitates filter 
and wash quickly. 

For the determination of very small amounts of titanium, it is 
advisable to use the colorimetric method proposed by Weller 
(cf. p. 100). Under the analysis of silicates will be discussed a 
practical example of this determination. 





* Schleicher & Schiill’s filter-paper No. 589 is satisfactory for this purpose. : 
+ The sodium metatitanate undergoes hydrolysis and forms a precipitate 
containing a much higher percentage of TiOs. 


118 GRAVIMETRIC ANALYSIS. 


Determination of Titanium in Rutile and Iron Ores. Method 
of O. L. Barnebey and R. M. Isham.* 


This method is based on the volatilization of the silica by hydro- 
fluoric acid in the presence of sulphuric acid, evaporation to | 
dryness and fusion with sodium carbonate and a little potassium 
nitrate (which converts the iron and titanium to insoluble ferric 
oxide and sodium acid titanate) extraction with hot water to 
remove the soluble phosphates, sulphates and aluminates, solu- 
tion of the ferric oxide and sodium titanate in hydrochloric acid, 
extraction of ferric chloride with ether, reduction of slight traces 
of iron with sulphurous acid, precipitation of the titanic acid by 
boiling in acetic acid solution, filtration and ignition to titanium ~ 
oxide (or the titanium may be determined colorimetrically? 
The method is accurate and not long. 

Procedure.—The sample is weighed into a platinum crucible, 
treated with a little water, 5 to 10 drops of sulphuric acid, 
and 1 c.c. of hydrofluoric acid, and the mixture heated care- 
fully until finally no more sulphuric acid fumes are evolved. 
Five or 10 grams of sodium carbonate and a little potassium 
nitrate are added and the mixture fused at least thirty minutes. 
The crucible and cover are cooled, placed in a beaker, covered 
with hot water, and heated until the melt is disintegrated. 
Ferric oxide and sodium titanate are left insoluble in hot water. 
The crucible is removed, washed, and any adhering particles of 
ferric oxide and sodium titanate are dissolved in hot hydrochloric 
acid (sp. gr. 1.1). This solution is saved. The residue in the 
beaker is filtered and washed with hot water. The filter is per- 
forated and the residue carefully washed into a clean beaker with 
hydrochloric acid (sp. gr. 1.1). (No water is to be added from 
this stage of the analysis until after the treatment with ether.) 
The hydrochloric acid washings from the platinum crucible are 
transferred to the beaker and the whole heated on the hot plate 
until solution is complete and the total volume reduced to 15 to 
20 c.c. The solution is then cooled, and transferred to a separa- 
tory funnel, the beaker being rinsed with hydrochloric acid 





* J. Am. Chem. Soc., 32, 957 (1910). 


DETERMINATION OF TITANIUM IN RUTILE AND .IRON ORES. ‘19 


(sp. gr. 1.1). An equal volume ot ether, which has been saturated 
with concentrated hydrochloric acid solution, is added to the 
solution in the funnel, a rubber stopper is inserted in the top, 
the funnel is inverted, the stop-cock opened, and the whole 
thoroughly shaken. The stop-cock is then closed, the funnel 
placed in an upright position and allowed to stand ten minutes, 
when the aqueous layer is drawn off into a second separatory 
funnel. The ether is rinsed twice by shaking well with 5 to 10 
c.c. portions of hydrochloric acid (sp. gr. 1.1) and the washings 
are added to the aqueous solution. The treatment with ether 
is repeated two or three times until the last portion of ether 
fails to show any greenish tinge due to the presence of dissolved 
ferric chloride. 

The aqueous solution containing all the titanium in the pres- 
ence of little, if any, iron and aluminium, is heated to expel the 
dissolved ether, 20 c.c. of sulphuric acid (1: 1) are added, and 
the solution evaporated until fumes of sulphuric anhydride are 
evolved. The cooled solution is diluted to about 100 c.c. and 
nearly neutralized with ammonia. One or two grams of ammo- 
nium bisulphite are added and the solution heated on the hot 
plate for half an hour. Ten to 15 grams of ammonium 
acetate are now added and the solution boiled for fifteen minutes. 
The precipitated titanic acid is filtered off, washed with 7 per 
cent. acetic acid, ignited and weighed as TiO,. 


Separation of Uranium from Iron and Aluminium. 


The slightly acid solution, containing considerable quantities 
of ammonium salts, is treated with an excess of ammonium car- 
bonate and then with ammonium sulphide, allowed to stand for 
some time in a closed flask, finally filtered and washed with water 
containing ammonium sulphide. 

The precipitate contains the iron as sulphide and the aluminium 
as hydroxide; in the filtrate is found all of the uranium as 
UO2(CO3)3(NH4)4. The precipitate is dissolved in hydrochloric 
acid, its solution freed from hydrogen sulphide by boiling, the 
Serrous salt oxidized to ferric salt by the addition of potassium 


120 . GRAVIMETRIC ANALYSIS. 


chlorate, and the iron and aluminium determined by one of the 
methods described on pages 107-109. 

The filtrate containing the uranium is evaporated almost to — 
dryness, acidified with hydrochloric acid, boiled, and the uranium 
precipitated, by the addition of ammonia, as ammonium uranate. 
The precipitate is filtered off, washed with 2 per cent. ammonium 
"nitrate solution to which a little ammonia has been added, dried, 
ignited, and weighed as U3QOs. 

The result obtained may be verified by heating the residue 
repeatedly in a current of hydrogen in a Rose crucible (see Copper 
Determination) until a constant weight is obtained; weighing 
as UO,. The purity of the precipitate may also be tested volu- 
metrically (see Volumetric Analysis). 


B. DIVISION OF THE MONOXIDES. 
MANGANESE, NICKEL, COBALT, ZINC. 


MANGANESE, Mn. At. Wt. 54.93. 
Forms: MnS0O,, MnS, Mn30,, Mn>P.0-. 
1. Determination as Manganous Sulphate, MnSO,. 


This method, first proposed by Volhard,* has recently been 
tested by Gooch and Austin, and has been found strictly accurate. 
Experiments performed by Schudel in the author’s laboratory 
completely confirm Gooch’s results. 

Procedure-—The oxide obtained by the ignition of the car- 
bonate, sulphide, or of manganous manganite, is dissolved in as 
slight an excess of sulphuric acid{ as possible in a porcelain 
erucible, evaporated as far as possible on the water-bath, after 
which the excess of acid is removed by heating in an air-bath. 
‘A porcelain crucible provided with an asbestos ring (see Fig. 11, 
p. 27) serves for the air-bath. The walls of the smaller crucible 
should be separated from those of the larger one by about | cm. 
After the sulphuric acid has been removed, the two crucibles 





* Ann. d. Chem. u. Pharm., 198, p. 328. 

Tt Zeit. f. anorg. Chem. (1898), 17, p. 264. 

t The manganous manganite (Mn,O,) requires the Pigencig of reducing 
agent (best SO,) or of pure hydrogen peroxide. 


SEPARATION OF MANGANESE AS CARBONATE. 121 


are covered and heated to redness over a good Bunsen burner, 
allowed to cool in a desiccator and weighed. From the weight 
of the manganous sulphate, the amount of manganese present 
may be calculated as follows: 


MnSO,:Mn=p:2 
Hege 3 
‘ ane MnSO, a 


(a) Separation of Manganese as Carbonate. 


This method for the separation of the manganese permits only 
a limited application, because no other metal that is precipi- 
tated by alkali carbonates can be simultaneously present. The 
method, therefore, is only suitable for the determination of man- 
ganese in solutions of pure manganese salts containing nething 
else except alkali and ammonium salts. 

According to H. Tamm, * the precipitation is best accomplished 
by means of ammonium carbonate. For this purpose the neutral 
solution (which may contain other ammonium salts) is treated 
with a slight excess of ammonium carbonate, warmed gently, and 
the beaker containing the solution is allowed to remain in a luke- 
warm water-bath until the precipitate has settled and the upper 
liquid has become clear. 

The precipitate is filtered off, washed with hot water, dried, 
ignited, and weighed either as sGiphats according to 1 or as Mn,O, 
according to 2. 

Remark.—If either sodium or potassium carbonate is used to 
precipitate. the manganese, the precipitate will always contain 
alkali carbonate that cannot be removed by washing. After the 
precipitate has been ignited, however, the alkali carbonate can be 
easily extracted by water. Furthermore, the precipitation is not 
quite quantitative; the filtrate always contains small amounts of 
manganese. In order to remove this, it is necessary to evapo- 
_ rate the aqueous solution to dryness, whereby the manganous car- 
bonate is completely decomposed hydrolytically into carbonic acid 
and manganous hydroxide, and the latter in contact with the air 
‘ehanges to brown manganic oxide, Mn,0,.--The-residue obtained 
after the evaporation is treated with water, the small amount 





+ Ch. News, 26 (1872), p. 37, and Zeit. f. anal. Chem., 11 (1872), p. 425. 


122 GRAVIMETRIC ANALYSIS. 


of. brown manganese compound filtered. off, ignited, and added te 
the main part of the precipitate. 


(b) Separation of Manganese as Sulphide. 


[his method is employed when it is necessary to separate man- 
yanese from calcium, strontium, barium, and magnesium. 

We will distinguish between two different cases: 

(a) The solution contains, besides manganese, large amounts 
of the alkaline earths or magnesium. 

(@) The solution contains only small amounts of the alkaline 
earths or magnesium. 

(a) In case large amounts of the alkaline earths or magnesium 
are present, the manganese sulphide must be precipitated in the — 
cold in the presence of considerable ammonium salts. _ 

- The solution is placed in an Erlenmeyer flask of Jena glass and 
about 5 gm. of ammonium chloride or ammonium nitrate are added. - 
In case the solution reacts acid, ammonia is added until it is slightly 
alkaline, and a slight excess of freshly-prepared, colorless ammo- 
nium sulphide solution is added. The flask is now nearly filled 
with cold distilled water that has been boiled, corked, and allowed 
to stand twenty-four hours, or, better, still longer. After this time 
the flesh-colored precipitate will have settled. The clear upper liquid 
is carefully decanted through a filter,* taking pains not to disturb 
the precipitate and to keep the filter filled with liquid all the time. 
If the precipitate is at all bulky, it is washed three times by decan- | 
tation with a 5 per cent. solution of ammonium nitrate to which has 
been added 1 ¢.c..of ammonium sulphide. The precipitate is then 
transferred to the filter and washed with dilute ammonium sul- 
phide water until 20 drops of the filtrate evaporated to dryness on 
a crucible-cover leave no residue. Now for the first time the filter 
is allowed to drain completely and is dried. As much of the pre- 
cipitate as possible is transferred to a small thin-walled porcelain 
crucible, the filter-paper is burned in a platinum spiral, and the ash 
added to the main portion of the precipitate in the crucible. The 
uncovered crucible is heated over a small flame until the greater 
part of the sulphur has been burned off, when the flame is increased — 





* Schleicher & Schiill’s filter-paper No. 590 can be used to advantage. 


SEPARATION OF MANGANESE AS MANGANESE DIOXIDE. 123 


and the crucible is finally heated over the flame of a Teclu burner, 
cooled, and weighed as Mn,O, (cf. p. 125, sub. 3). The heating is 
repeated until a constant weight is obtained. Manganous sulphide 
is readily changed to Mn,O, if the amount of sulphide is compara- 
tively small. In case more than 0.2 gm. is present there is danger 
of getting a too high result on account of some manganous sulphate 
not being decomposed. In this case it is advisable to dissolve the 
washed precipitate of manganous sulphide in dilute hydrochloric 
acid, to evaporate the solution to dryness in order to remove all 
hydrogen sulphide, to dissolve the residue in a little water and to 
precipitate the manganese as carbonate according to 1; or the 
manganous sulphide can be weighed as such. (See p. 125.) 

(3) In case only small amounts of alkaline earths are present, 
the following procedure can be used: The neutral solution is heated 
to boiling, an excess of ammonia and some ammonium sulphide 
is added and the boiling is continued until the manganous sulphide 
has become a dirty green. The precipitate is allowed to settle for 
some minutes and is then filtered and washed with water contain- 
ing a little ammonium sulphide. From this point the procedure 
is the same as described under (a). 


(c) Separation of Manganese as Manganese Dioxide. 


If a dilute solution of a manganous salt is treated with 
bromine water and boiled, the reaction 


MnCl, -+ Br, + 2H,O@MnO, + 2HCl+2HBr 


does not take place unless the halogen acids are neutralized as 
fast as they are formed. This neutralization can be accom-_ 
plished by means of the salt of a weak acid, such as sodium 
acetate, even when the solution contains free acetic acid, which 
is scarcely ionized at all in the presence of its alkali salt. Thus 
in a solution such as is obtained after the removal of iron and 
aluminium by a basic acetate separation (cf. p. 152), the man- 
ganese can be precipitated quantitatively by boiling with an 
excess of bromine water. The oxide does not correspond exactly 
to MnO,, although most of the manganese is in the quadrivalent 


124 GRAVIMETRIC ANALYSIS. 


condition.* When the precipitate has collected together in large 
flocks, the boiling is discontinued and the precipitate allowed to 
settle; it is filtered and washed with hot water. Some chemists 
ignite this precipitate and weigh as Mn,O, but it is more accurate 
to dissolve the precipitate in a mixture of HCl and H,SO, and to 
precipitate the manganese finally as eaneenere ammonium 
phosphate. (See 4, p. 126.) 

Chlorine, aL peroxide, hypochlorites, etc., may be used 
instead of bromine, but these reagents have no peeled advan- 
tages. 

When the solution of the manganous salt contains ammonium 
salts, the precipitation of the manganese does not take place by 
the above procedure, because the sodium acetate serves rather 
to neutralize the acid set free by the following reaction: : 


2NH,Cl+3Br,=N, +2HC1+6HBr. 


Upon the audition of ammonia, however, the precipitation of the 
manganese can be effected. In this case, it seems fair to assume 
that the reaction goes through the following stages: 


MnCl, +2NH,OH@Mn(OH),+2NH,Cl, 
Mn(OH), + Br+NH,OH = MnO(OH), +NH,Br. 


The precipitation with bromine and ammonia is not so satis- 
factory as with bromine alone in the presence of acetic acid and 
sodium acetate and in the absence of ammonia or ammonium 
salt, because when ammonia is present much of the bromine is 
used up in oxidizing the ammonia or ammonium salt. In that case 
there is considerable solution of nitrogen, and, moreover, when 
an excess of bromine is added the solution may become acid 
enough to dissolve the precipitated manganese: 


2NH, +3Br,=6HBr +N). 





* The MnO, acts as the anhydride of metamanganous acid, H,MnO,, and 
some manganous manganite, MnMnO, or Mn,0O,, is contained in the pre- 
cipitate. 


DETERMINATION OF MANGANESE. 125 


It is necessary, therefore, when ammonium salts are present 
to make sure that the solution is ammoniacal at the end of the 
operation, 

This method of precipitating manganese from solutions pos- 
sesses disadvantages which make it useless in many cases. If, 
besides manganese, the solution contains calcium, zinc, etc., man- 
ganites of these metals are precipitated with the manganese. In 
this case the precipitate must be dissolved in hydrochloric acid 
and the precipitation repeated several times, but even then it is 
not possible to obtain a precipitate altogether free from these 
metals. If the other metals are present only in small amounts, the 
results obtained by this method are sufficiently accurate. The 
precipitation of manganese as sulphide in the presence of other 
metals is always satisfactory and should be used in almost all cases. 


2. Determination of Manganese as Sulphide. 


If the manganese has been precipitated, as described on p. 
122 as sulphide, the precipitate is separated from the filter as 
completely as possible, placed in a Rose crucible (of unglazed 
porcelain), the filter is burned in a platinum spiral, and the ash 
added to the main portion of the precipitate. Some pure sulphur 
which has been crystallized from CS, is added, after which the 
crucible and its contents are heated in a current of hydrogen by 
means of a Bunsen burner exactly as described under the Deter- 
mination of Copper as Sulphide. After the excess of sulphur has 
distilled off and been burned, the crucible is cooled in a stream 
of hydrogen and the precipitate is weighed as MnS. 


3- Determination of Manganese as Mn,Q,. 


Inasmuch as all the oxides of manganese, as well as those 
compounds which are converted into oxide on ignition (manga- 
nous salts of volatile organic and inorganic acids, with the ex- 
ception of the halogen salts), are converted into Mn,O,* on being 
ignited in the air, at temperatures between 940° and 1100°, it 





* Cf. R. J. Meyer and K. Retgers, Z. anorg. Chem., 57, 104 (1908), at 
530° the oxides of manganese are slowly but quantitatively changed into 
Mn,0O,. 


126 GRAVIMETRIC ANALYSIS. 


follows that this method for the determination of manganese is 
quite generally applicable. It is nearly as accurate as the 
methods described under 1 and 2, if the ignition of the precipitate 
takes place in an electric furnace at about 1000°, but very good 
results are obtained if, as recommended by Gooch, * the porcelain 
crucible (containing the carbonate, manganous manganite, or 
sulphide) is entirely surrounded by the oxidation flame of a 
Teclu burner, whereby a moderately high heat is obtained with- 
out too much free access of air.t 

After the ignition, the crucible and its contents are cooled in 
a desiccator and then weighed. From the weight p of the 
oxide, the amount of manganese can be calculated according to 
the equation 


Mn,O0,:3Mn=p:2 


3Mn 


*“Nin,0, ? 


4. Determination of Manganese as Manganese Pyrophosphate, 
Mn»P,07. 


This excellent method was recommended by W. Gibbst and 
subsequently studied by Gooch and Austin.§ 

The slightly acid solution, containing an amount of manganese 
corresponding to not over 0.5 gm. Mn2P207, and no other metals 
except alkalies, is treated with 20 gm. ammonium chloride, 5 to 10 
c.c. of a cold saturated solution of sodium phosphate, and ammonia, 
drop by drop, until a slight excess is present. The solution is heated 





* Zeit. f. anorg. Chem., X VII (1898), p. 268. 

+ To illustrate the accuracy of the three methods just described for the 
determination of manganese, the following results obtained by H. Weitnauer 
are given. He obtained after making six determinations by each method 
the following mean values: 50 c.c. of a pure manganese: sulphate solution 
treated with ammonium carbonate and changing the precipitate to sulphate 
gave 0.1025 gm. Mn; by precipitating as sulphide and weighing as such, 
0.1027 gm. Mn; and by changing the sulphate to Mn,O,, 0.1029 gm. Mn. 

. ~Am: J. Saptre 46, 216; Z. anal. Chem., a 101 (1868). 
§ Z. anorg. Chem., 18, 339 (1898). 


COLORIMETRIC DETERMINATION OF MANGANESE. 127 


to boiling and kept at this temperature for three or four minutes, 
or until the precipitate assumes a silky, crystalline appearance. 
After cooling, the precipitate is filtered through a Gooch or Munroe 
crucible, washed with cold ammonium nitrate solution, dried, 
and ignited within a larger crucible or in an electric furnace. 

The results are excellent. 

Manganese can be determined very accurately by volumetric 
methods (see Volumetric Analysis). 


5. Colorimetric Determination of Manganese. 


Small amounts of manganese may be accurately and quickly 
determined by the colorimetric method. ‘This is chiefly used for 
the estimation of the manganese present in iron and steel. If 
more than 1:5 per cent. of manganese is present, the results are 
unreliable. The method depends upon the oxidation of the 
manganese to permanganic acid, bringing the solution to a definite 
volume and comparing its color with another solution containing 
a known amount of manganese. If the solutions are colored 
exactly the same shade, then the amounts of manganese which 
they contain are the same. 

Procedure.—A standard solution of potassium permanganate 
is first prepared by dissolving 0.072 gm. of the crystallized salt in 
500 c.c. of water; 1 c.c. of this solution contains 0.05 mgm. of 
manganese. 

Exactly 0.2 gm. of the iron or steel is dissolved in 15-20 c.c. 
of nitric acid (sp. gr. 1.2) in a 100-c.c. measuring-flask. The acid 
is heated to boiling to effect complete solution, after which the 
solution is allowed to cool and diluted up to the mark with water. 
After thoroughly mixing, 10 c.c. of the liquid are brought by means 
of a pipette into a small beaker, 2 ¢.c. of nitric acid (sp. gr. 1.2) 
are added, and the liquid is heated until it begins to boil, when the 
flame is removed, 0.5 gm. of lead peroxide is added, the mixture 
is shaken and then heated for two minutes to boiling. After 
standing some time, the warm, violet-colored solution is filtered 
through a small asbestos filter * into a glass-stoppered test-tube 





*The asbestos must have been previously ignited, treated with KMn0O, 
solution, and finally washed with water. 


128 GRAVIMETRIC ANALYSIS. 


about 20 em. high, and graduated in cubic centimeters. The 
filter is washed with as little water as possible, the tube is stoppered 
and shaken until the solution is thoroughly mixed. Into a 
second tube of the same size, and also graduated in cubic centi- 
meters, 1s placed 1-5 c.c. of the standard manganese solution, 
and this is carefully diluted with water until the two liquids have 
exactly the same shade when viewed horizontally. The height 
of the liquid in each tube is then carefully read. 

Assuming that 1 c.c. of the standard solution were placed 
in the cylinder and diluted to 7 c.c. in order to obtain the same ~ 
shade produced by ¢ c.c. of the other solution, then as the con- 
centrations of the two liquids are directly proportional to their 
heights, 

T :t=0.05 mgm.:2 


__ t-0.05 mgm. 
es 7 ; 





‘This amount of manganese is contained in 0.02 gm. of the 
iron, so that the percentage of manganese present is 


0.02: ita asiadli = 100 
T 
pee er cent. Mn 
=ap=P ; ‘ 


Rather more accurate results are obtained if, instead of using 
a standard solution obtained from potassium permanganate, a 
sample of steel is used containing a known amount of manganese 
and treated in exactly the same way as the sample to be analyzed, 
a fresh standard being prepared for each analysis. 

An even better colorimetric method has been devised by M. 
Marshall * and H. E. Walters.{ 

Although manganese is precipitated as manganous acid, from 
solutions slightly acid with nitric or sulphuric acid, by the 
addition of alkali persulphates, the oxidation goes farther and 





* Chem. News, 83, 76 (1904). 
} Ibid., 84, 239 (1904). 


DETERMINATION OF NICKEL AS NICKEL GLYOXIME. 129: 


permanganic acid is formed within a short time if a catalytic agent 
is present, such as silver nitrate. 


2Mn(NO3z)2+5(N H4) 28208 + 8H20 =. 
5(NH4)2804+5H2S04+4HNO3+2HMn0O,. 


Procedure.—0.2 gm. of steel is placed in a 100-c.c. flask and 
dissolved in 20 c.c. of cold nitric acid (sp. gr. 1.2). Thereupon 
10 c.c. of silver nitrate solution (1.388 gm. AgNOs3 in a liter of 
water), are added, the solution made up to exactly 100 e.c. and 
mixed. Of this solution 10 ¢.c. are placed in glass-stoppered, 
graduated test-tube. After adding 2.5 c.c. of ammonium per- 
sulphate solution (200 gm. in a liter of water) the test-tube is 
placed in water at 80° to 90° and allowed to remain there until 
the bubbles of gas arising become more numerous and remain at 
the top for a few seconds. The solution is then cooled by placing 
the tube in cold water, and the color is compared with a standard 
solution containing a known amount of permanganic acid. * 


NICKEL, Ni. At. Wt. 58.68. 


Forms: Nickel Dimethyl Glyoxime, NiCgH,4,N,0,; Nickel, Ni; and 
Nickel Oxide, NiO. 


1. Determination as Nickel Glyoxime, Ni(C,H,N,0O,),. 


Dimethyl glyoxime, CH,-CNOH-CNOH-CH,, was recom- 
mended by L. Tschugaeff + as a reagent for nickel and used by 
K. Kraut { for detecting the presence of traces of nickel in ashes. 
O. Brunck § and others have also studied the reaction and found 
it to furnish a most rapid and accurate method for the quanti- © 
tative estimation of nickel either by itself or in the presence of 
cobalt, zinc and manganese. If the solution contains tartaric 
acid enough to prevent the precipitation of iron by ammonia, the 








* Or better a solution of a steel of known manganese content. 
t Z. anorg. Chem., 46, 144 (1905); Ber., 38, 2520 (1905). 

t Z. angew. Chem., 19, 1793 (1906); zbid., 20, 1844 (1907). 

§ Ibid., 20, 834 (1907). 


- 130 GRAVIMETRIC ANALYSIS. 


nickel in a sample of nickel steel can be determined accurately 
within two hours and without the removal of any other metal. 

When a dilute, neutral solution of a nickel salt is treated with 
an alcoholic solution of dimethyl glyoxime, a red, crystalline 
precipitate of nickel dimethyl glyoxime is formed. 


NiClo+2(CH3) 2C2(NOH)2 = (CH3)2C2(NO)2Ni - (CH3)2C2(NOH)> 


Dimethyl glyoxime. Nickel dimethyl glyoxime. ace qa 


The salt is soluble in mineral "acids so that precipitation is 
incomplete because of the acid set free in the reaction. It be- 
comes quantitative, however, if the mineral acid is neutralized 
by ammonia or if sodium acetate is added, whereby the mineral 
acid is replaced by acetic acid in which the precipitate is prac- 
tically insoluble. Large quantities of ammonium salts or of — 
alkali acetate do no harm, but an excess of ammonia tends to 
prevent the formation of the precipitate. The precipitate is 
distinctly soluble in absolute alcohol, but only traces dissolve 
in 50 per cent. alcohol, and in more dilute alcohol it is even less 
soluble. When'thrown down in the cold or in the presence of 
much free ammonia the precipitate is very voluminous and hard 
to filter. 

Procedure-—The neutral or slightly acid* solution is diluted 
so that not more than 0.1 gm. of cobalt is present in 100 c.c., 
heated nearly to boiling and treated with somewhat more than 
the theoretical amount of an alcoholic 1 per cent. solution of 
dimethyl glyoxime.t Ammonia is then cautiously added until 
the solution smells slightly. While still hot, the precipitate is 
filtered into a Gooch or Munroe crucible, washed with hot water 
and dried at 110° to 120° for 45 minutes. It contains 20.31 per 
* cent. Ni. 

The nickel salt of dimethyl glyoxime is red and crystalline. 
It contains no water of crystallization and sublimes at 250° 
without decomposition. 





* If strongly acid, the solytion is nearly neutralized with caustic potash, 
then the reagent is added, etc. 
_| The volume of the alcoholic solution should in no case be more than 
half that of the nickel solution, as the precipitate is soluble in alcohol 


DETERMINATION OF NICKEL AS METAL BY ELECTROLYSIS. 131 


2. Determination of Nickel as Metal by Electrolysis. 


From strongly acid solutions nickel is not deposited upon 
stationary electrodes by a current of 1-3 amperes. From slightly 
acid solutions the deposition is not quantitative. 

From ammoniacal solutions nickel is readily deposited, and on 
account of its simplicity and accuracy this method is to be 
strongly recommended for the determination of nickel. 


- (a) Method of Gibbs. * 


Nickel sulphate or chloride (but not the nitrate) is dissolved in 
an ammoniacal solution of ammonium sulphate and electrolyzed. 

The nickel is deposited upon a weighed cathode and, at the 
end of the electrolysis, the gain in weight represents the quantity 
of nickel. 

Requirements and Procedure.—For this, as well as for all other 
electrolytic determinations, the apparatus shown in Fig. 31 may 
be used. | 

B represents a storage-battery, which is provided with the 
binding posts MM. The current is led first through the variable 
resistance W, then through the known resistance W’ (a resistance 
of 1 ohm is most convenient to use t), and from here to the 
decomposition cell finally back to the battery. 

If at any time it is desired to measure the voltage between the 
electrodes of the cell, the voltmeter V is connected with the 
binding posts of the decomposition cell, by. throwing the switch 
Q,t Fig. 32, so that cis connected with b and c’ with b’. The 


* Z. anal. Chem., 3, 334 (1864). Cf. Fresenius and Bergmann, Z. anal. 
Chem., 19, 320 (1880). 

+ The resistance w’=1Q can be made from nickelin wire. The resistance 
- of the wire is measured with the aid of the Wheatstone bridge and a length 
cut off corresponding to one ohm. This wire is wound round a wooden block 
and the ends fastened to binding posts. 

tIf a commutating switch is not available, one can be prepared by 
taking the cover of a pasteboard box, about 2 cm. deep, filling it with melted . 
paraffin, and then, after cooling, making little cavities at aa’bb’cc’ by 
pressing a test-tube, which is filled with hot water, against the cold wax. 
These cavities are filled with mercury and the switch finished with copper 
wire. Cf. Fig. 36, p. 178. 





132 GRAVIMETRIC ANALYSIS. 

















Seperate) inte 

~~" 

. M a a Ne eee One NS 
] ; nW—~8 

z o fo o3 a o— ——___ 

: Lae 



































a ee a 
' 


! 
I 
—— Bites? | 
t 





























Cc’ 
! 
i 
J 
| ig 
r 
1 
SZ (o) (e) ° O 
eo / (S) oO fo) ° O 








Fre, 31. 


DE TERMINATION OF NICKEL AS METAL BY ELECTROLYSIS. 133 


strength of the current, on the other hand, is obtained by placing 
the switch in the opposite position with a united to b and a’ to 
b’, as shown in Figs. 31, 32. 





Since, according to Ohm’s law, the 


Electromotive force 
Resistance : 





Strength of current = 


then if the strength of the current is expressed in amperes, the 
electromotive force in volts, and the resistance in ohms, we have 


E 
A= R: 

In case W’=1 ohm, then 
2 A=E, 


and the voltmeter will show directly the strength of the current 
(amperes). * 

It is arranged so that the current may be taken from different 
points along MM, and it is thus possible to carry out a number 
of electrolytic determinations at the same time, and the volt- 
meter V serves as measuring instrument for all the analyses that 
are in progress. By means of the SS it is possible to connect 
easily the voltmeter with the different cells. While a measure- 





* With weaker currents, the known resistance can be made W’=10Q 
so that the voltmeter will show ten times the actual current. 


134 _ GRAVIMETRIC ANALYSIS. 


ment is being made at any cell, all other switches must be cut 
out of circuit. | 
The decomposition cell consists of a glass beaker in which is 
placed as cathode a wire gauze electrode (first recommended by 
Cl. Winkler) and as anode a platinum spiral. The electrodes - 








>; = 
=| = 














must always reach to the bottom of the beaker and the top of 
the gauze electrode should be nearly covered by solution. In 
some cases it is desirable to use a platinum dish as cathode, as 
recommended by Classen. (See Fig. 36, p. 178.) 

The electrodes are usually connected with two electrode 
stands on which metal arms are attached to an upright glass rod 
(Fig. 33). To prevent serious loss of electrolyte by spattering, 


DETERMINATION OF NiCKéL AS METAL BY ELECTROLYSIS. 135 


the beaker is covered with two halves of a watch-glass. This is 
not entirely satisfactory, as when much gas is evolved a little of 
the liquid is still carried off mechanically. This method of 
fastening the electrodes, moreover, has the disadvantage that 
when the electrolysis is carried out in a hot solution, acid or 
ammoniacal vapors, as the case may be, condense on the brass 









x) 





lh 


: \\ 














Fia. 34. 


arms of the electrode support and in some cases the liquids thus 
condensed dissolve some brass and the resulting solution may 
drop into the beaker, and spoil the analysis. To prevent this 
misfortune, the author bends the ends of the electrodes to a 
right angle and connects them with an electrode stand designed 
as shown in Fig. 34. 

This electrode holder consists of two brass rods insulated 
from one another by means of an intervening layer of mica, and 
the rods are fastened to the ring r through a piece of ebonite, e. 


136 GRAVIMETRIC ANALYSIS. 


The openings to hold the wires are cut wedge-shaped, so that any 
shape of wire can be inserted. 

Since the ends of the electrodes leave the beaker in a horizontal 
direction, the beaker can be covered tightly by means of a whole 
watch-glass, and not only are losses by spattering avoided, but 
there is absolutely no danger of contamination from the outside. 


The Electrolysis of Nickel. 


For. every 0.25-0.30 gm. nickel, present as sulphate or chloride, 
but not as nitrate, * 5-10 gm. of ammonium sulphate and 30-40 c.c. 
of concentrated ammonia are added, and the solution diluted with — 
distilled water to a volume of 150 ¢.c._ This solution is electrolyzed 
at the room temperature with a current of 0.5-1.5 amperes and an 
- electrode potential of 2.8-3.3 volts. The electrolysis is finished 
after three hours, as can be shown fairly satisfactorily by adding 
a little water and allowing the current to pass through the solution 
for fifteen or twenty minutes longer. If at the end of this time 
no nickel has deposited upon the electrode surface which was 
wet for the first time by the last dilution, the determination is 
finished. If the solution is kept at a temperature of from 
50°-60° C. only about one hour is necessary for the deposition. 
The deposited metal adheres firmly to the electrode, is bright, 
and possesses almost the color of platinum. 

As soon as the electrolysis is finished, the watch-glass is 
removed, the electrode holder is raised so that only the bottoms 
of the electrodes remain in the liquid, and the upper parts of 
the electrodes are washed thoroughly with water from a wash- 
bottle. The electrodes are then raised entirely out of the solution 
and the bottoms washed immediately with water. The current 
is then turned off and the cathode rinsed with absolute alcohol, 
after which it is dried by holding it high above a gas flame. After 
cooling in a desiccator, it is weighed. 

To clean the cathode, place it in a small beaker, add enough 
nitric acid (1:1) to wet all the nickel, and heat for at least fifteen 
minutes. This treatment is absolutely necessary to remove the 
last traces of nickel. If this is not done, the electrode on being 


* Page 131. 





THE ELECTROLYSIS OF NICKEL. 137 


ignited becomes discolored, and it is then very difficult to clean 
the electrode by repeated treatment with acid followed by ignition. 
The discolored electrode, however, can be used for another 
electrolytic determination, To make sure that all the nickel 
has been deposited from the electrolyzed solution, the ammonia. 
is almost wholly neutralized with hydrochloric acid and a few 
cubic centimeters are added of 1 per cent. solution of dimethyl 
glyoxime in alcohol. When less than a tenth of a milligram of 
nickel is present, it will take several minutes for a yellow coloration 
to appear, and soon afterward the red crystals of nickel salt 
will be precipitated. 

' The nickel not deposited by an electrolysis may be estimated 
accurately by shaking the solution thoroughly and comparing 
the color produced by the addition of dimethyl glyoxime with 
that produced with a dilute nickel solution containing a known 
quantity of nickel. Naturally such a colorimetric test can be 
used only with very small quantities of nickel. 

Remark.—The electrolysis of nickel from an ammoniacal 
solution should not be continued for too long a time, because the 
cathode slowly gains in weight even after all the nickel has been 
deposited from the solution. The anode is attacked, causing 
platinum to go into solution, which is deposited upon the cathode, 
partially, at least. 

The presence of too little ammonia often results in the forma- 
tion of black Ni(OH), at the anode; the analysis then comes out 
too low. : 

Classen’s method for depositing nickel from a solution of 
- ammonium oxalate apparently gives too high results* and 
cannot be recommended. 


3. Determination as Nickelous Oxide. 


The nickel solution is heated in a porcelain dish with bromine 
water and an excess of pure potassium hydroxide, whereby the 
nickel is precipitated as brownish-black nickelic hydroxide, 
Ni(OH),. The precipitate is filtered off, washed by decantation 





* A. Windelschmidt, Dissertation, Minster, 1907. W. D. Treadwell, Dis- 
sertation, Ziirich, 1909. 


138 GRAVIMETRIC ANALYSIS. 


with hot water, dried, and, after burning the filter, ignited 
and weighed as NiO. The grayish-green oxide thus obtained 
always contains small quantities of silicic acid and alkali,* 
whereby the results are too high. By treating the ignited mass 
with hot water, the greater part of the alkali can be removed. 
Drying and again igniting gives the weight of NiO+Si0O,. The 
oxide is treated in a porcelain crucible with hydrochloric acid, 
evaporated completely to dryness, the dry residue moistened 
with concentrated hydrochloric acid and then with hot water, 
filtered through a small filter, washed with hot water, and the 
filter together with the residue ignited wet in a platinum 
crucible. The weight of this silica, SiO,, subtracted from the 
former weight of NiO +Si0,, gives good results. 

Remark.—It is possible to precipitate nickel quantitatively — 
as Ni(OH), by means of caustic potash alone and the precipitate 
is changed to NiO by ignition. This method is open to the same 
objections as the above and, furthermore, Ni(OH), is not so easily 
filtered and washed as Ni(OH),. 

- These two methods are more tedious to carry out and the 
results are not as accurate as in the case of the first two methods 
described and will probably not be used much in the future. 

Besides the methods described, it has been proposed to pre- 
cipitate nickel as the sulphide, and weigh it as the oxide by 
ignition in air.t| The method is good but hardly comparable 
with the dimethyl glyoxime method, the electrolytic method, or 
the volumetric titration with potassium cyanide. 


COBALT, Co. At. Wt. 58.97. 
Forms: Co, CoSO,. 
1. Determination as Metal. 
(a) By Electrolysis. 
The most accurate method for the estimation of cobalt is by 


electrolysis and the details of the process are precisely the same 
as have been given under nickel, i. e., from a strongly ammoniacal 
* Cf. A. Windelschmidt, loc. cit. and W. D. Treadwell, loc. cit. 


+ Hi. Cormimboef, Ann. chim. appl., II, 6 (1906). Cf. A. Windelschmidt, 
loc. cit. : 





DETERMINATION OF COBALT AS METAL. 139 


solution containing ammonium salts and the sulphate or chloride 
(preferably the former), of cobalt. It is customary to use a little 
more ammonia than in the determination of nickel, because 
cobalt has a greater tendency to deposit as black Co(OH), at the 
anode. The duration of the electrolysis is the same as with 
nickel, rather than somewhat longer. At the end of the deter- 
mination, after the electrodes have been removed, the entire 
solution is tested for cobalt by adding ammonium sulphide or 
potassium sulphocarbonate. ~ 


(b) By Reduction of the Oxide in a Stream of Hydrogen. 


The cobalt solution is heated to boiling in a porcelain evaporat- 
ing-dish, and the cobalt is precipitated as black cobaltic hydroxide 
by the addition of caustic potash and bromine water. The pre- 
cipitate is filtered off,* dried, and ignited. After cooling it 
is treated with water in order to remove the small.amount of 
alkali which is always present, and then the residue is ignited 
in a stream of hydrogen and weighed as metal. After weighing, 
the metal is dissolved in hydrochloric acid, evaporated to dryness, 
the dry mass moistened with hydrochloric acid, then treated with 
water, and the small residue of silicic acid filtered off. This resi- 
due is ignited and its weight subtracted from that obtained after the 
ignition in hydrogen. Cobalt may also be precipitated as cobaltous 
hydroxide by caustic potash alone, but the resulting precipitate 
is not so easy to filter and wash as the cobaltic hydroxide. The 
precipitation by means of sodium carbonate is not so satisfactory. 

The oxides of cobalt when ignited in air yield a mixture of CoO 
and Co,0, in varying proportions, so that they are not suited for 
the quantitative determination of cobalt. 

Remark.—The results obtained by this method are usually a 
little higher than by electrolysis. 





* Cobaltic hydroxide, unlike nickelic hydroxide, has the tendency of 
giving a turbid filtrate on washing. If, however, Schleicher & Schiill’s 
filter-paper No. 589 (blue band) is used, none of the precipitate passes 
through. 


140 _ GRAVIMETRIC ANALYSIS. 


2. Determination as Sulphate. 


The method is the same as was described under Manganese — 


(p. 104). 
Zinc, Zn. At. Wt. 65.37. 


Forms: ZnNH,PO,, Zn,P,0,, ZnO, ZnS, Zn. 


1. Determination as Zinc Ammonium Phosphate or Zinc 
Pyrophosphate. 


This excellent method, first recommended by H. Tamm, * 
has been studied and improved by G. Lésekann and T. Meyer,+ 
M. Austin,t and especially H. D. Dakin.§ 

Procedure.—The cold acid|| solution of the zine salt is treated 
with ammonia, in a platinum or porcelain dish, until it is left 
barely acid. Care is necessary at this point, as zinc ammonium 
phosphate is soluble both in acids and ammonia. It is then 
diluted with water to a volume of 150 c.c. and heated on the water- 
bath. To the hot solution, ten times as much ammonium phos- 
phate is added as there is zine present. (If the diammonium 
phosphate contains some monoammonium phosphate, the salt 
should be dissolved in cold water and dilute ammonia added until 
the solution just becomes pink with phenolphthalein.) The 
precipitate that first forms is amorphous, but it soon changes into 
fine crystals of zinc ammonium phosphate. The transformation 
takes place more rapidly in proportion to the quantity of ammo- 
nium salts present. After the heating has continued for about 
fifteen minutes, the dish is removed from the water-bath and after 
being allowed to settle for a short time the precipitate is filtered 
through a Gooch or Munroe crucible, washed with hot, 1 per 
cent. ammonium phosphate solution § until free from chlorides 

* Chem. News, 24, 148.. 

+ Chem. Ztg., 1886, 729. 

{ Am. J. Sci., 1899; Z. anorg. Chem., 22, 212 (1900). 

§ Z. anal. Chara’ 39, 273 (1900). 

|| If the solution is neutral, 2 or 3 gms. of ammonium chloride are added 
and then the analysis carried out. 


{ According to Voigt, Z. angew. Chem., 1909, 2282, the precipitate is 
washed immediately with hot water. 





DETERMINATION OF ZINC. T41I 


etc., then twice with cold water, then with 50 per cent. alcohol, 
dried at 110—120° for an hour and weighed as ZnNH,PO,, which 
contains 36.64 per cent. Zn. 

Or, the precipitate may be weighed as the pyrophosphate, 
Zn,P,O7, in which case the dried zine ammonium phosphate is 
heated very slowly in an electric oven to 900°-1000°. If such 
an oven is not at hand, the Gooch or Munroe crucible is placed 
in a larger platinum crucible and heated over the gas flame. 
The temperature is gradually raised until finally the full heat of 
the Teclu burner or of the blast lamp is reached. The crucible 
is heated until its weight is constant. Zn,P,O7 contains 42.90 
per cent. Zn. 

The determination as pyrophosphate is to be recommended 
when the zinc solution contains a very large quantity of ammonium 
salts because it requires long washing to remove these and this 
renders the results a little low. When the precipitate is weighed 
as pyrophosphate, the ammonium salts are volatilized and it is 
not necessary to remove them by washing. 

Remark.—In some cases, as when magnesium or aluminium 
is present, the procedure of K. Voigt is followed. The solu- 
tion of the zinc salt, containing ammonium salts as well, is 
treated with an excess of ammonia and then with ammonium phos- 
phate. After standing some time, the precipitate of magnesium 
ammonium phosphate and aluminium phosphate is filtered off, 
the zine ammonium phosphate being soluble in ammonia. 
The filtrate is received in a platinum or porcelain dish and is 
heated on the water-bath until all the free ammonia has been 
expelled, whereby zine ammonium phosphate separates out 
quantitatively in the form of the crystalline precipitate. It is 
‘treated as described above. If some of the precipitate should 
adhere firmly to the sides of the dish, it may be dissolved in a few 
drops of hydrochloric acid, the solution immediately neutralized 
with ammonia, and heated a few minutes on the water-bath 
before filtering. 


46° 


142 GRAVIMETRIC ANALYSIS. 


2. Determination as Zinc Oxide. 


The carbonate, nitrate, acetate, and oxalate of zine are readily 
and quantitatively changed to zinc oxide by ignition in the air; 
in the case of the sulphate, when present in relatively large amounts, 
the transformation into oxide is difficult. Small amounts of the 
sulphate may be changed to oxide by igniting over the blast-lamp. 
It is advisable, however, in case the zinc is present as sulphate, to 
precipitate it from the aqueous soliition as sulphide and weigh it 
as such according to 3; or to dissolve the sulphide on the filter in 
dilute hydrochloric acid, receiving the solution in a weighed plati- 
num dish, evaporating to dryness on the water-bath, and changing 
to oxide by the method of Volhard as described below, and weigh- 
ing as such. 

The chloride is readily changed to oxide, according to Volhard, 
by gentle ignition with pure mercuric oxide. The process is as 
follows: The neutral solution of the chloride, contained in a 
platinum dish, is treated with a large excess of pure yellow mer- 
curic oxide *, suspended in water, and evaporated to dryness on 
the water-bath, whereby mercuric chloride and zine oxide are 
formed, 


ZnCl, + HgO=Zn0+ HegCl,, 


both of which are white substances. Enough mercuric oxide 
should be used so that the residue obtained after the evaporation 
is noticeably yellow. : 

The dry mass is ignited under a hood with a good draft (on 
account of the mercury vapors being poisonous), at first gently and 
finally strongly, and the residue of zinc oxide is weighed, both 
mercuric chloride and oxide being volatile. The results are 
excellent. 





* The mercuric oxide is prepared by precipitating a solution of mercuric 
chloride with pure caustic potash. The precipitate is allowed to settle, 
washed by decantation with water until free from chloride, and kept sus- 
pended in water in a bottle with a wide neck. A considerable amount of 
the mercuric oxide, say 5-10 gm., should leave no weighable residue after 
ignition. 


DETERMINATION OF ZINC AS SULPHIDE. 143 


If the solution contains, besides zinc, also alkalies, the zine can 
be precipitated as carbonate and changed to oxide upon ignition. 
The precipitation of the zine carbonate should take place in a 
porcelain dish and the sodium carbonate solution should be added 
drop by drop to the cold, barely acid solution free from ammonium 
salts. The sodium carbonate is added until the zine solution 
becomes turbid, when it is heated to boiling, whereby the greater 
part of the zinc is precipitated as granular zinc carbonate. Two 
drops of phenolphthalein solution are then added and enough 
sodium carbonate solution to impart a distinct pink color. In 
this way a precipitate of zinc carbonate is obtained free from alkali, 
which is not the case if the hot solution is at once precipitated 
by the addition of an excess of sodium carbonate.* The precipitate 
is filtered from the hot solution and washed with hot water until 
20 drops of the filtrate leave no residue on evaporation. The pre- 
cipitate is dried, the greater part transferred to a weighed porce- 
lain crucible, the filter burned by itself in a platinum spiral, and 
the ash added to the main part of the precipitate in the crucible, 
which is ignited, at first gently and finally strongly, over a Teclu 
burner and weighedt after cooling in a desiccator. 


3. Determination as Sulphide. 


This determination is chosen when the zine is present in a 
solution containing ammonium salts, or when it is necessary to 
separate zinc from alkaline earths, alkalies or metals of this group. 
Zine sulphide may be precipitated from ammoniacal solutions, or 
from solutions containing free acetic, formic, citric, or sulphocyanie 
acids. 





*In case considerable amounts of ammonium salts are present there 
may be no precipitation. Sodium carbonate should then be added tntil 
the solution is slightly alkaline and the solution boiled until all the ammonia 
is expelled. 

{ If the solution contains sulphate, the precipitate produced by sodium 
carbonate always contains more or less basic zine sulphate, which may 
easily lead to high results. In the presence of sulphates, therefore, it is 
advisable to precipitate the zinc as sulphide and determine it as such accord- 
ing to 3.. 


144 GRAVIMETRIC ANALYSIS. 


(a) Precipitation of ZnS from Ammoniacal Solutions, 


The slightly acid solution is placed in an Erlenmeyer flask 
and treated with sodium carbonate solution until a permanent 
precipitate is obtained. This is dissolved by the addition of a 
few drops of ammonia, after which for every 100 c.c. of the solution 
5 gms. of ammonium acetate (or, better, ammonium thiocyanate) 
are added, followed by a slight exeess of freshly prepared ammo- 
nium sulphide, the flask is nearly filled with boiled water, stoppered 
and allowed to stand twelve to twenty-four hours. Without dis- 
turbing the precipitate, the clear upper liquid is poured through a 
Schleicher & Schiill’s filter No. 590. The precipitate is covered 
with a solution containing in every 100 c.c. 5 gms. of ammonium ace- 
tate (or ammonium thiocyanate) and 2 ¢c.c. of ammonium sulphide 
solution, shaken, allowed to settle, and the turbid upper liquid is 
poured through the filter, taking care to receive the filtrate in a 
fresh beaker; in case it comes through turbid it is poured through 
the filter again. The decantation is repeated three times, after 
which the precipitate is transferred to the filter and washed com- 
pletely with the above solution, taking pains to keep the filter full 
of the wash liquid during the entire operation, finally washing with 
water containing ammonium sulphide only. The precipitate 
is then dried, transferred as completely as possible to a 
weighed Rose crucible, the filter burned by itself and the ash 
added to the main portion of the precipitate. The precipitate is 
now mixed with the aid of a platinum wire, with one-third as 
much pure sulphur, covered with a layer of sulphur and heated, 
as described under Manganese (page 125) in a current of hydro- 
gen. The crucible is finally allowed to cool in the stream of 
hydrogen and from the weight of the zinc sulphide the weight 
of zinc present is calculated, 


ZnS: Zn=p:s 


Zn 


5 Tes? 


ELECTROLYTIC DETERMINATION OF ZINC. 145 


and if a is the amount of the original substance, then the 
per cent. of zinc is 


Zn 


a: ZnS p=100: a 
100 Zn p - ; 
t= Ss ERG zinc. 


(b) Precipitation of ZnS from Acid Solutions. 


The solution, which has been nearly neutralized with ammonia, 
is treated with ammonium chloride or sulphate and a little 
ammonium or sodium acetate; and is then saturated with hydro- 
gen sulphide. After the precipitate has settled completely, 
the supernatant solution is poured through a filter, and the pre- 
cipitate washed with 2 to 4 per cent. acetic acid which has been 
saturated with hydrogen sulphide. When thoroughly washed 
it is treated as described above. It is to be noted that the 
zine sulphide shows less tendency to form colloidal solutions when 
it is thrown down from a slightly acid solution than when it is 
precipitated from alkaline solutions. 


/ 


4. Electrolytic Determination of Zinc. 


In the presence of acid, zinc is not deposited by an electric 
current of 0.5 to 1 ampere, although it may be deposited even 
then by stronger currents. 

From the solution of potassium or sodium zincate, or from the 
complex alkali zine cyanides, it is easy to deposit the zinc quan- 
titatively. 

(a) Method of F. Spitzer.* 


The solution of zinc sulphate (chlorides and nitrates should 
be absent) is treated with a drop of phenolphthalein and with 
sodium hydroxide solution until a permanent coloration is obtained. 
- Then 20 to 25 c.c. of normal caustic soda solution are added, the 





* Z. Elektrochem., 11, 401 (1905). 


146 GRAVIMETRIC ANALYSIS. 


solution is diluted to 150-200-c.c. and electrolyzed, using a plat- 
inum gauze cathode, with a current of 0.8 to 1 ampere and 3 to 
4 volts electrode potential. At the end of three hours the elec- 
trolysis is finished, provided not more than 0.5 gm. of zine was 
present. Without breaking the current, the electrodes are 
raised nearly out of the bath, the upper portions are washed 
quickly with water, then the electrodes are taken entirely out of 
the solution and rinsed with water. The current is then turned 
off, the cathode washed with absolute alcohol, dried above a 
flame, cooled in a desiccator, and weighed. When deposited 
in this way, zinc forms a bluish-gray layer that adheres firmly to 
the electrode. To make sure that all the zinc was deposited, the 
electrodes are cleaned and the solution electrolyzed for thirty 
minutes longer. A slight increase in weight will be obtained in 
every case because the anode is attacked slightly by the alkaline 
solution so that the cathode slowly continues to gain in weight 
from deposited platinum. If at the end of half an hour the 
gain in weight is not over 0.3 gm. then the deposition of the 
zine was complete the first time, as can be shown by testing with 
sodium sulphide. The results are always a little high.* 

To clean the electrodes, they are boiled thoroughly with 
hydrochloric acid (1:2) washed well with distilled water, and 
ignited. It is not necessary to cover the platinum gauze with a 
thin coating of copper or of silver, as has been recommended 
when a platinum dish is used as the cathode. . 

Remark.—If too little caustic soda is present, a spongy deposit 
of zinc is obtained which does not adhere well to the electrode. | 

For this reason the above directions should be followed closely. 

In the presence of ammonia the determination is not successful. 
If, therefore, it is desired to analyze a solution containing an 
ammonium salt, it must be boiled with caustic soda until all the 
ammonia has been expelled. If, moreover, chlorides or nitrates 





* Ellwood B. Spear, J. Am. Chem. Soc., 32, 530 (1910). The experiments 
have been repeated in the author’s laboratory by Janini, who obtained as 
an average from fourteen determinations with 50 c.c. of a zine sulphate. 
solution, the value 0.1014 gm. Zn instead of 0.1008 gm. Zn,’a difference of 
about 0.6 per cent. 


SEPARATION OF MANGANESE, ETC., FROM ALKALINE EARTHS. 147 


are present they must be removed by evaporation with sulphuric 
acid. The solution is evaporated on the water-bath and finally 
heated over the free flame until dense vapors of sulphuric acid 
are expelled. The solution is then diluted and analyzed in the 
usual manner. 


(b) The Potassium Cyanide Method.* 


A drop of phenolphthalein is added to the solution of zinc 
sulphate, caustic soda solution until a permanent pink coloration 
is obtained, and then potassium cyanide solution until a clear 
solution results. This is diluted to a volume of 150-200 c.c. and 
electrolyzed with a current of 0.5 ampere. At first the electrode 
potential is about 5.8 volts, but it falls during the analysis on 
account of the current heating the solution. The electrolysis 
is finished in two or three hours. 

Other methods for the electrolytic estimation of zine are 
given in A, Classen’s book Quantitative Analysis by Electrolysis. 


SEPARATION OF MANGANESE, NICKEL, COBALT, AND ZINC 
FROM THE ALKALINE EARTHS. 


The separation depends upon the insolubility of the sulphides 
of the metals of this group and the solubility of the sulphides of 
the alkaline earths. | 

Procedure.—The neutral solution of the chlorides, contained 
in an Erlenmeyer flask, is treated with ammonium chloride (in 
case it is not already present) and freshly-prepared colorless 
ammonium sulphide solution is added drop by drop until no 
further precipitation takes place and the liquid has a distinct 
odor of ammonium sulphide. The flask is then almost completely 
filled with boiled water, corked, and allowed to stand twelve hours. 
The precipitate is filtered antl washed as described in tne Deter; 
mination of Zine (p. 144). 

If only a small amount of alkaline-earth metals are present and 





+ Luckow, Z. anal. Chem., 19, 1 (1880). 


148 GRAVIMETRIC ANALYSIS, 


the ammonium sulphide solution is entirely free from ammonium 
carbonate, the separation is usually complete after one precipita- 
tion; in the presence of considerable calcium, strontium, barium, 
or magnesium the sulphide precipitate will always be more or less 
contaminated with these substances, so that the precipitation 
must be repeated. For this purpose the washed precipitate is 
dried, transferred as completely as possible to a porcelain crucible, 
the filter-paper burned in a platinum spiral and the ash added to the. 
main part of the precipitate in the crucible, which is now covered 
with a watch-glass, treated with dilute hydrochloric acid, and 
heated to boiling after the evolution of hydrogen sulphide has 
ceased, in order to remove all of the hydrogen sulphide. A very 
little concentrated nitric acid is now added and the mixture warmed 
until the precipitate is completely dissolved; the solution is evapo- 
rated to dryness, treated with a little concentrated hydrochloric 
acid, and again evaporated to dryness in order to change to chloride 
any nitrate that may have been formed. The dry mass is moistened 
with a few drops of concentrated hydrochloric acid, dissolved in hot 
water, and the slight residue of sulphur filtered off, which, in case 
barium is present, always contains a small amount of barium 
sulphate, and is therefore washed with hot water, dried, ignited 
in a porcelain crucible, and weighed. The filtrate is then 
precipitated exactly as before by the addition of ammonium 
sulphide. 

In case nickel is present, -a too great excess of ammonium 
sulphide must be carefully avoided, as otherwise the nickel sulphide 
will pass through the filter (cf. Vol. I). In all cases, however, 
the filtrate should be tested for nickel by acidifying with acetic 
acid, heating to boiling, and passing hydrogen sulphide into 
the solution. If a slight black precipitate is produced by this — 
treatment, it is filtered off and combined with the main precipi- 
tate (cf. p. 156 et seq). The filtrate containing the alkaline-earth 
metals is freed from ammonium salts by evaporating to dryness, 
dissolved in hydrochloric acid, and examined as described on p. 
76 et seq. ee 

Remark.—The ammonium sulphide solution used in the above 
separation must be free from ammonium carbonate. As, however, 
all commercial ammonia contains this salt, it must be freed from 


PREPARATION OF AMMONIA FREE FROM CARBONATE, 149 


carbonate before being used for the preparation of ammonium 
sulphide solution. 


Preparation of Ammonia Free from Carbonate. * 


About 10 gms. of freshly slaked lime are added to 500 e.c. of 
concentrated ammonia contained in a liter flask that is connected 
with a condenser. The condenser is closed by means of a tube 
containing soda-lime, and the contents of the flask are allowed 
to stand for a day with frequent shaking. After this, from 300- 
400 c.c. of water are placed in a flask and boiled, meanwhile 
passing through the water a current of air that has been freed 
from all traces of carbon dioxide by passing through concentrated 
caustic potash solution and then through a tower filled with 
soda-lime. The water is allowed to cool in this air-stream. 
The flask containing the ammonia is then placed on the water- 
bath in such a position that the condenser-tube is inclined slightly 
upward, and this is connected with the delivery-tube, through 
which the air previously passed into the flask of boiling water. 
By warming the water-bath the ammonia is now distilled over 
into the flask containing the boiled water, by which it is completely 
absorbed. By saturating a part of this ammonia with hydrogen 
sulphide, a solution of ammonium sulphide is prepared suitable 
for the above-described separation. 


SEPARATION OF THE BIVALENT FROM THE OTHER METALS 
OF THE AMMONIUM SULPHIDE GROUP. 


This separation is often designated as that of the protoxides 


from the sesquioxides; this designation is not applicable in the 
case of titanium and urany| derivatives. | 


The Barium Carbonate Method. 


This method depends upon the fact that ferric, aluminium 
and chromic salts (as well as titanic and uranyl salts) are preci- 





* The distillation of ammonia also serves to free it from silica, which it 
always contains when kept in glass bottles for any length of time. 


150° GRAVIMETRIC ANALYSIS. 


pitated in the cold by barium carbonate, while manganese, 
nickel, cobalt, zinc, and ferrous salts are not. The trivalent 
metals are strongly decomposed hydrolytieally: 


FeCls +HOH ~2(HCl) +Fe (OH)Cle. 


Free acid and a basic salt ares formed by this hydrolysis, the 
composition of the latter depending upon the quantity of 
the water and the temperature. If the free acid is removed by 
the addition of barium carbonate, the equilibrium is disturbed 
and the hydrolysis goes further until finally the insoluble hy- 
droxide is formed: 


Fe(OH)Cl,-+ 2HOH =2HCl + Fe(OH),. 


The barium carbonate, then, serves only to neutralize the acid 
set free by the hydrolysis, and the total reaction is expressed 
by the following equation: 

Gee 142 Fw see ei B Cl oF OH 3H,CO, 
2FeCl, + 6HOH + 3BaCO, = 3BaCl,+2Fe(OH),+ 3H,0 +300," 


The salts of the bivalent metals are not subject to this hy- 
drolysis in the cold, consequently they are not precipitated by 
the addition of barium carbonate. On warming, however, they 
are hydrolyzed to an appreciable extent and are then precipitated 
by barium carbonate. 

Procedure.—Sodium carbonate solution is added drop by drop 
to the slightly acid solution of the chlorides or nitrates, but not 
the sulphates,* of the metals, in an Erlenmeyer flask until a slight, 
permanent turbidity is produced, which is then redissolved by 
the addition of a few drops of dilute hydrochloric acid. The 
solution is diluted and treated with pure barium carbonateT 
(suspended in water) until after thoroughly shaking an excess of the 








* Barium carbonate will precipitate the bivalent metals when sulphates 


are present, €.g.: 
ZnSO, + BaCO,;= ZnCO,-+ BaSO,. 


+ The barium carponate must be free from alkali carbonate. 


THE BARIUM CARBONATE METHOD. ISI 


latter remains on the bottom of the flask. The flask is closed 
and allowed to stand for several hours with frequent shaking. The 
clear liquid is then decanted off, the residue treated with cold 
water and again decanted. This decantation is repeated three 
times, after which the precipitate is transferred to the filter and 
completely washed with cold water. The precipitate contains all 
of the iron, aluminium, chromium, titanium, and uranium in the 
presence of the excess of barium carbonate. The filtrate contains 
the bivalent metals and barium chloride. 

The precipitate is dissolved in dilute hydrochloric acid, boiled 
to remove the carbon dioxide, and the iron, aluminium, chromium 
(titantum and uranium) are separated from the barium * by double 
precipitation with ammonium sulphide as described on p. 147. 
The iron, aluminium, chromium (titanium and uranium) are sepa- 
rated from one another as described on pp. 107-120. 

The filtrate from the barium carbonate precipitation is freed 
from barium by the addition of sulphuric acidt to the boiling 
solution after it has been made acid with hydrochloric acid. The 
barium sulphate is filtered off and the monoxides are separated 
from one another as described on p. 156. 

Remark.—The above separation of the scsquioxides from the 
protoxides is not absolutely certain in the presence of nickel and 
cobalt. In this case, particularly when considerable iron is pres- 
ent, the precipitate produced by barium carbonate contains small 
amounts of nickel and cobalt. This difficulty can be overcome, 
however, by adding ammonium chloride to the solution (3-5 gms. 
for each 100 c.c. of solution) before precipitating with barium 
carbonate; the separation is then satisfactory. 





* Most authorities recommend precipitating the barium first with sul- 
phuric acid and then separating the iron, aluminium, etc. The precipitate 
of barium sulphate always contains small amounts of the heavy metals, 
so that the author prefers the above procedure. 

+ Or, better, by double precipitation of the other metals with ammonium 
sulphide. 2 


152 GRAVIMETRIC ANALYSIS. 


SEPARATION OF IRON, ALUMINIUM, AND TITANIUM (BUT NOT 
CHROMIUM AND URANIUM) FROM MANGANESE, NICKEL, 
COBALT, AND ZINC. 


Basic Acetate Method. 


This classic method depends upon the fact that ferric, alu- 
minium and titanium acetates are hydrolyzed in hot, dilute 
solutions much more readily thfin the acetates of the bivalent 
metals. From the equation 

Fe (CoH302) 2 + 2HOH@2HC2H302 +Fe (OH) 2° C2H30z, 


it is evident that acid is set free which tends to stop the reaction, 
due to the solvent action of hydrogen ions. The concentration 
of free hydrogen ions, however, is kept low by the addition of 
sodium acetate. Then, as a rule, some manganese is likely to 
be precipitated, so that it is advisable to dissolve the precipitate 
and repeat the precipitation. Hydrated manganese dioxide is more 
insoluble than manganous hydroxide, Mn(OH,), and hence long 
boiling in the air tends to increase the quantity of manganese 
precipitated. The method is somewhat tedious, but gives excel- 
lent results. 

Procedure——The slightly acid solution of the chlorides, con- 
tained in a small beaker, is treated with sodium carbonate solu- 
tion in the cold until a slight permanent opalescence is obtained, 
which is then redissolved by the addition of a few drops of dilute 
hydrochloric acid. Meanwhile a boiling, dilute solution of sodium 
or ammonium acetate is prepared in a large round-bottomed 
flask, containing for each 0.1 to 0.2 gm. of iron or aluminium, 
‘1.5 to 2 gm. of acetate and 300 to 400 c.c. water. When the 
iron solution is ready, the lamp is taken away from beneath the 
flask, the iron solution is added, and then the boiling is con- 
tinued for one minute, the flame removed (the precipitate 
becomes slimy on long boiling), the precipitate allowed to settle 
and filtered immediately while the liquid is hot, through a fluted 
filter, washing three times by decantation with boiling water 
containing ammonium or sodium acetate. The filter together 
with the precipitate is spread upon a glass plate, the bulk of the 
precipitate rinsed into a porcelain dish, and that remaining on 


SEPARATION OF IRON FROM MANGANESE. 153 


the filter dissolved. by alternately treating with concentrated 
hydrochloric acid and hot water. The resulting solution is 
evaporated nearly to dryness on the water-bath and the basic 
acetate precipitation is repeated exactly as before. The filtered 
and washed precipitate is dissolved in hydrochloric acid and the 
iron separated from aluminium according to page 107. The 
combined filtrates containing the protoxides are acidified with 
10-20 e.c. of concentrated hydrochloric acid, in order to prevent 
the precipitation of hydrated manganese dioxide, evaporated 
almost to dryness, dissolved in a little water, the manganese, 
nickel, cobalt and zine precipitated by ammonium sulphide as 
described on p. 147, and analyzed according to p. 156. 
Remark.—This procedure requires practice. It is especially 
suited for the separation of iron and titanium from the protoxides; 
the separation is usually less satisfactory with aluminium, so that 
in case considerable amounts of the latter are present, the barium 
carbonate separation is to be preferred. If it is merely a case of the 


Separation of Iron from Manganese, 


the following modifications of the basic acetate process give 
satisfactory separations with only a single precipitation. 


(a) O. Brunck’s Method. * 


The acid solution, containing not more than 0.3 gm. of iron, 
is treated with 0.35 gm. of potassium chloride or 0.26 gm. ammo- 
nium chloride for each 0.1 gm. of iron present. The solution is 
evaporated to dryness on the water-bath, the residue pressed 
with a glass rod, and heated five or ten minutes longer. By this 
time practically all the mineral acid is expelled. The residual 
salts are dissolved in 10 to 20 c.c. of water and to the resulting 
solution there is added 1.5 gm. of sodium acetate for each 0.1 
gm. of iron present.t The solution is diluted with boiling water 
to a volume of 400 to 500 c.c. for each 0.2 gm. of iron present; 
it is heated, with constant stirring, until boiling begins, and then 

* Chem. Ztg., 1904, I, 513. Cf. W. Funk, Z. anal. Chem., 45, 181 (1906). 

+ The sodium acetate crystals often contain a little sodium carbonate, so 


that they should be dissolved in a little water and the solution made barely 
acid before adding it to the iron solution. 





154 GRAVIMETRIC ANALYSIS. 


the flame is removed and the precipitate allowed to settle. The 
solution is decanted through a fluted filter and the precipitate 
washed with hot water. The precipitate is dissolved in as’ 
little hydrochloric acid as possible, the iron precipitated by 
ammonia, filtered, dried, and ignited as described on page 87. 
The filtrate from the basic acetate precipitation, or better the 
combined filtrates from both precipitations,* is acidified with 
hydrochloric acid, evaporated ngarly to dryness, the residue 
dissolved in a little water and the manganese, nickel, cobalt 
and zine precipitated with ammonium sulphide according to 
the directions on page 147, and separated according to page 156. 


(6) Method of A. Mittasch.} — 


The slightly acid solution, containing not more than 0.3 gm. 
of iron and having a volume of not over 100 c.c., is carefully 
neutralized, while stirring constantly, by adding ammonium. car- 
bonate solution (200 gm. of the commercial salt in 1 liter of water) 
from a pipette or burette. When the precipitate that is first pro- 
duced begins to dissolve very slowly, the neutralization is finished 
with an ammonium carbonate solution, which is prepared by tak- 
ing 50 c.c. of the first solution and diluting to 1 liter, the dilute 
reagent being added until the precipitate produced will not 
dissolve within one or two minutes of stirring. At this point, 
3 c.c. of double normal acetic acid are added, and the solution 
stirred until the precipitate disappears. The solution is diluted 
with 400 ¢c.c. of hot water and heated until it begins to boil, when 
the greater part of the iron will have been precipitated. Then 
20 c.c. of ammonium acetate solution (60 gm. of the commercial 
salt in 1 liter of water){ are added and the boiling continued for 





* The ammoniacal filtrate from the Fe(OH), precipitate is acidified with 
5 e.c. of concentrated hydrochloric acid before adding it to the filtrate from 
the basic acetate precipitation, otherwise manganese is likely to be pre- 
cipitated when the two filtrates are mixed. 

+t Z. anal. Chem., 42, 508 (1903). 

+ Commercial eenxaontien acetate has the symbol NH,C,H,0,-HC,H,O,. 
If none of it is on hand, £00 c.c. 2N. solution ammonia are mixed with 
50 c.c. 2N. acetic acid; the mixture must be faintly acid. Of this solution 
10 c.c.+5 e.c. 2N. acetic acid are used for the precipitation of the iron, 
and 10 c.c. of 2N. acetic acid are added to dissolve the precipitate pro- 
duced by ammonium carbonate. 


SEPARATION OF IRON AND ALUMINIUM FROM MANGANESE. 155 


a minute longer. Without waiting for the precipitate to settle, 
it is filtered off and washed with hot water until free from 
chlorides. ! 

The small quantity of precipitate adhering to the sides of the 
vessel in which the precipitation took place is dissolved in a 
few drops of hydrochloric acid, the iron precipitated by ammonia 
and the ferric hydroxide filtered off through a separate filter. 
Both filters are now dried, burned and the iron weighed as Fe,O,- 


SEPARATION OF IRON AND ALUMINIUM FROM MANGANESE, 
NICKEL, COBALT, AND ZINC. 


Sodium Succinate Method. 


This method, applicable for the separation of large quantities 
of iron from small quantitics of manganese, nickel, etc., is based 
upon the fact that ferric iron is quantitatively precipitated from 
neutral solutions as light-brown ferric succinate by the addition of 
neutral alkali succinate solution, While manganese, nickel, etc., 
remain in solution. 

Procedure.—In case the solution contains free acid and all the 
iron is in the ferric form, it is neutralized with ammonia until a 
reddish-brown coloration is formed, when sodium or ammonium 
acetate is added until the color becomes a deep brown, and then the 
solution of alkali succinate, after which the mixture is warmed 
gently, allowed to cool, filtered, and washed at first with cold water, 
then with warm water containing ammonia, until 20 drops of the 
filtrate leave no residue when evaporated to dryness on platinum. 
By means of the washing with ammonia, the ferric succinate is 
changed to ferric hydroxide which is dried and weighed as ferric 
oxide after ignition.in a porcelain crucible. If aluminium is present, 
the ignited residue is further analyzed as described on p. 107. The 
bivalent metals in the filtrate are best precipitated by the addition 
of ammonium sulphide and analyzed as follows: 


156 - GRAVIMETRIC ANALYSIS. 


SEPARATION OF THE BIVALENT METALS OF THE AMMONIUM 
SULPHIDE GROUP FROM ONE ANOTHER. 


Separation of Zinc from Nickel, Cobalt, and Manganese. 


All methods for this separation rest upon the slight solubility - 
of zine sulphide and the ready solubility of the remaining sulphides 
in their state of formation.* At this point it may be well to say 
a few words with regard to the most recent explanation of the 
process that takes place in the salution of electrolytes. 


Solubility Product. 


Inasmuch as no substance is absolutely insoluble in water, it 
follows that in every case where a precipitate is produced the solu- 
tion is saturated with the substance and (according to Ostwald) in 
the case of difficultly soluble substances the dissolved portion is 
practically completely dissociated electrolytically. The binary sub- 
stance A, consisting of the elements B and C, is decomposed in 
aqueous solution according to this scheme: 


AZB+C. 
If the concentrations of the ions B and C are designated by 
[B] and [C], and that of the undissociated portion by [A], then 


according to the mass-action law the following relation holds 
for any given temperature: 


[B].{C] 
[A] 





= constant. 


Every increase of [B] or [C] causes, therefore, an increase of 
[A], and, as the solution is already saturated with A, this will 
produce precipitation of the substance. 

This product [B.C], which if exceeded causes a supersatura- 
tion of the solution, and consequently precipitation, is called the 





* Nickel and cobalt sulphides when once formed are insoluble in dilute 
acids. These substances probably exist in two allotropic modifications, 
of which one is soluble and the ‘other insoluble. The soluble form has 
never been isolated. 


EXPLANATION OF THE SOLUTION OF SULPHIDES IN ACIDS. 157 


solubility product. If, therefore, in any solution the solubility 
product is already reached, then the solution is saturated with respect 
to the substance A, and if the solubility product is not reached, then 
the liquid exerts a solvent action upon the solid substance. 


Explanation of the Solution of Sulphides in Acids. 


According to the above theory, the solution of a sulphide (e.g., 
zinc sulphide) in acid is conceived to take place as follows: 

On treating the solid sulphide with water, a part of the 
salt is dissolved until the solubility product is reached. This 
almost inappreciable amount is practically completely dissociated 
into ions. On adding acid to the solution, the positive hydrogen 
ions unite with the negative sulphur ions to form neutral hydrogen 
sulphide, which being a very weak acid is only dissociated to a 
slight extent, so that sulphur ions disappear from the solution 
and the solubility product of zine sulphide is no longer reached: 

ZnS +2HCl=H,8-+ ZnCl. 

The liquid, therefore, dissolves more of the solid zine sulphide and 
the above reaction again takes place and this process is repeated 
until all of the zine sulphide is brought into solution. The solu- 
bility of a sulphide in acid, therefore, is proportional to its solubility 
product and to the concentration of the hydrogen ions. If we, 
then, desire to precipitate zine by means of hydrogen sulphide 
from a neutral solution of an inorganic compound, the following 
consideration shows us how this may be accomplished: If hydro- 
gen sulphide is conducted into a solution containing zinc com- 
bined with a mineral acid, the zinc is indeed precipitated, but as 
the amount of zinc sulphide formed increases, there is an increase 
in the concentration of the hydrogen ions: 


oe C4 e 4 PS 
ZnCl, +2HSH = Zn 
\sH 


The precipitation is, therefore, incomplete. It can be made 
complete, however, if we can avoid this increase in the concentra- 





* The Zn(SH), is at once decomposed into ZnS and H,S. 


158 GRAVIMETRIC ANALYSIS. 


tion of the hydrogen ions. This can take place by replacing the 

mineral acid formed by. a weaker acid, i.e. one which is only slightly 

dissociated, so that the solution will contain fewer hydrogen ions. * 
The following methods depend upon this principle. 


Method of Smith and Brunner.+ 


Procedure.—The hydrochloric acid solution of the four metals 
is treated with sodium carbonate yntil a permanent precipitate is 
formed, which is redissolved by the addition of a few drops of very 
dilute hydrochloric acid. Into this almost neutral solution hydro- 
gen sulphide is passed for five minutes, then a few drops of a very 

dilute solution of sodium or ammonium acetate are added and the 
solution is saturated with hydrogen sulphide, allowed to stand 
overnight, filtered, and washed with hydrogen sulphide water which 
contains in every 100 c.c. 2 gms. of ammonium salt (either the chlo- 
ride, sulphate, or sulphocyanate). The zinc is then determined 
cither as oxide or sulphide according to the methods described 
on pp. 142 and 143. 

Remark.—Inasmuch as the exact amount of acid to be set free 
is unknown, it is impossible to tell exactly how much alkali acetate 
is necessary, and herein lies the chief difficulty. If too much alkali 
acetate is added, some nickel or cobalt sulphide may be precipitated 
(shown by the gray color of the zine precipitate). If not enough 
alkali acetate is added, the zinc will not be completely precipitated. 
The following separation is more certain. 


Method of Cl. Zimmerman.}{ 


Procedure.—The weakly acid solution is treated with sodium 
carbonate solution until a permanent precipitate is formed, which 
is redissolved by the addition of a few drops of very dilute hydro- 
chloric acid, then for every 80 c.c. of the solution 10, or at the most 
15, drops of double-normal hydrochloric acid,§$ and 10 c.c. 0 





* Concerning the equilibrium conditions in the precipitation of sulphides 
by hydrogen sulphide, see Bruner and Zawadzki, Chem. Zentr., 1910, 5. 

+ Chem. Centrabl., 1895, 26. 

{ Ann. d. Chem. u. Pharm, 199, (1879) p. 3; 204 (1880), p. 226. 

§ The addition of hydrochloric acid is in all cases necessary, because other- 
wise nickel sulphide will be precipitated with the zine accra especially 
when considerable nickel and little zine are present. 


METHOD OF CL. ZIMMERMAN. 3 159 


ammonium sulphocyanate (1:5) solution are added, after which the 
solution is heated to about 70° C. and is saturated with hydrogen 
sulphide. At first the solution becomes only slightly turbid,* but 
after some time pure white zinc sulphide is thrown down in clouds, 
constantly becoming denser. After the solution has become sat- 
urated with hydrogen sulphide, the beaker is covered and allowed 
to stand in a moderately warm place until the precipitate has set- 
tled and the uppcr liquid is clear, after which the precipitate is 
filtered and washed, as described in the method of Smith and 
Brunner. 

From the filtrate nickel, cobalt, and manganese are precipitated 
by means of ammonium sulphide, filtered and separated according 
to the following methods. 

Remark.—What is the part played by the ammonium sul- 
phocyanate in this determination? Certainly it cannot act the 
same as the ammonium acetate in the Smith-Brunner method, for 
sulphocyanic acid is not, liké acetic acid, a weak acid, but a very 
strong one, almost as strong as hydrochloric acid itself, and the 
dissociation of strong acids is only slightly influenced by the addi- 
tion of their neutral salts. : 

Ammonium sulphocyanate probably simply ‘‘salts out” the 
zine sulphide (cf. Vol. I). 

By the action of hydrogen sulphide upon the zine salt, zine sul- 
phide is produced both in the hydrogel and hydrosol forms and the 
‘ammonium sulphocyanate changes the latter into the insoluble 
hydrogel. If this explanation is correct, the separation of zine 
from nickel, etc., will succeed equally well if the ammonium sul- 





* There are at the start but few zinc ions in the solution. The four 
metals are present for the most part in the’ form of complex thiocyanates 
of the general formula [R(CNS),](NH,).. The zine salt, like carnallite 
(see Vol. I) is slightly dissociated, 


(Zn(CNS),](NH),Zn(CNS), +2NH,CNS, 


and the zine thiocyanate is converted into insoluble sulphide by the action 
of hydrogen sulphide. When the zinc begins to precipitate as sulphide, 
the equilibrium is disturbed and eventually all the zinc becomes precip- 
itated. 


160 GRAVIMETRIC ANALYSIS. 


phocyanate is replaced by ammonium chloride or ammonium sul- 
phate. That this is the case is shown by the following method. 


*‘Salting-out Method.” 


Experiments were performed by G. H. Kramers in order to 
determine whether the separation of zinc from nickel and cobalt 
could be accomplished in weakly acid solutions by hydrogen sul- 
phide after the addition of any ammonium salt of a strong acid.* 
The results obtained showed this t& be possible. 

Procedure.—The neutral solution + containing the nickel and 
zine either in the form of sulphate or chloride (the sum of the 
oxides present amounting to about } per cent. of the weight of the 
solution) is treated with 8-10 drops of double-normal hydrochlorie 
acid and about 2 per cent. of ammonium sulphate (referred to the 
total amount of liquid) and the solution is saturated at 50° C. with 
hydrogen sulphide; the warm solution is allowed to stand until the 
pure white precipitate of zinc sulphide has settled out and is then 
treated exactly as described under the Method of Zimmerman. 

Results.—In the following experiments a zine sulphate solution 
containing 5.890 gms. zine to the liter and a solution of nickel sul- 
phate containing 5.320 gms. nickel to the liter were used. 






































a a . = 1 Led a r=] M Sloe! ao 
S g Q SRO tele Pe ee No N28 Ard Z2 
e RQ ae =, 19 + Tas * = ws ‘sg HB 
mn |e ae (a te ba oe tes ch a Pn ee ie a SS Fae 
é é q ee Ehebaaty paw ti BaP kasi Pe red = <4 SH | es 
rs) S) xs £ 2 3 3 = BS e BS 
A e J 
mf 20} 90) Fis. 3 fel GRRE TRS Ne ash 0.1188 | 0.1178 | 0.1072 |0.1066 . 
a 66) OO. diya 3 Tees Ea ARS NAY 0.3533 | 0.3534 | 0.1051 |0.1066 
aC a Bae Ne Rn ar 10 1) Bie peters wn nae 0.1184 | 0.1178 | 0.3206 |0.3192 
eS ane oe es 15 IG ie ee 0.1182 | 0.1178 
& | 20 GOUT ee, 30 51s Bah RAWAL EM oats Sori 0.1089 | 0.1178 
aR che aaa is GN BY Rae rage ge EE 0.1173 | 0.1178 
ro} | 8 gee! Melee aaa aed Bae ee 10. (a Rees: 0.3536 : 0.3534 | 0.1082 10.1066 
120 | 60 |...... Be Ores. 10 | nats ee 0.1184 ; 0.1178 
+1901 60° |.2-2%. iE ee wetietees 20) aes 0.1168 | 0.1178 
= | 20 20 60 BA ul Beem. 0.1184 | 0.1178 | 0.1064 |0.1066 
2l6o!] 20 | 110 ee ecient is ese acta 0.3542 | 0.3534 
ls0 | 60 | 100 ye ees 77 Pay ater 0.1168 | 0.1178 ; 
-S(20] 20 60 Biduprncs grees 0.1182 | 0.1178 | 0.1074 |0.1066 
60 | 20 | 110 be BOTS Presets 10 | 0.3552 | 0.3534 | 
-& (20| 60 | 100 2 es ee a aS 20 | 0.1190 | 0.1178 




















* Or any other salt, e.g., a potassium salt. 
+ If the solution is acid, it is neutralized by sodium carbonate as described 
under the preceding methods. 


SEPARATION OF MANGANESE FROM NICKEL AND COBALT. 161 


Separation of Manganese from Nickel and Cobalt. 


The solution of the chlorides or sulphates is treated with an 
excess of sodium carbonate, strongly acidified with acetic acid, 
and for each gram of nickel or cobalt present 5 gms. of ammonium 
acetate are added, the solution is diluted to 100-200 c.c., heated to 
70-80° C., saturated with hydrogen sulphide, filtered, and washed 
with hot water. The manganese is in the filtrate, and the nickel 
and cobalt are in the precipitate. 

Remark.—tThe filtrate often contains small amounts of nickel 
and cobalt. In order to remove these metals, the solution should be 
concentrated and colorless ammonium sulphide added. It is then 
made slightly acid with acetic acid, warmed, and filtered. In 
case a precipitate of nickel or cobalt sulphides is formed by 
this treatment, the filtrate is again tested in the same way and the 
process repeated until no further precipitation is produced. 


Separation of Cobalt from Nickel. 
(a) Method of Tschugaeff-Brunck.* 


This method is probably the quickest and most accurate for 
the estimation of nickel in the presence of cobalt. It depends 
upon the fact that nickel is quantitatively precipitated by 
means of dimethyl glyoxime, from a barely ammoniacal solution 
or from a slightly acid solution containing sodium acetate. Cobalt, 
under these conditions, is not precipitated. 

Procedure.—lf the quantity of cobalt present does not exceed 
the quantity of nickel, the procedure is exactly the same as when 
nickel alone is present; with larger quantities of cobalt two or 
three times as much of the dimethyl glyoxime reagent is added 
and the precipitation is accomplished exactly as described on 
page 129. For the determination of both nickel and cobalt, 
the original solution is divided into two portions. In one por- 
tion the nickel is determined as outlined above, and in the other 
the two elements are deposited electrolytically as described on 
page 136, and the cobalt found by difference. If only a little 
of the substance is available, the two metals are deposited 





* O. Brunck, Z. angew. Chem., 1907, 1848. 


162 GRAVIMETRIC ANALYSIS, 


together by electrolysis, the weighed deposit dissolved in nitric 
acid (the electrodes must be completely immersed in the acid 
and the solution boiled for at least 20 minutes), the resulting 
solution concentrated to a small volume, and the nickel deter- 
mined as described above. The method can be recommended 
strongly. 


(6) The Potassium Nitrite Method of N. W. Fischer.* 
> 
Brunck’s Modvfication.+ 


The solution containing an excess of acid is evaporated to 
dryness in a porcelain dish and the residue treated with one or 
two drops of dilute hydrochloric acid and 5 to 10 c.c. of water. 
Pure caustic potash solution is then added drop by drop 
until the reaction is barely alkaline. The resulting precipitate 
is dissolved in as little glacial acetic acid as possible, half of the 
solution’s volume of 50 per cent. potassium nitrite solution is 
added, and 10 drops more of acetic acid; the mixture is stirred 
well and allowed to stand twenty-four hours. At the end of 
this time the precipitation is almost always complete. It should 
be tested, however, by removing a little of the undiluted solution 
with a pipette, adding to it a little more potassium nitrite solution, - 
and allowing to stand a little longer. If at the end of an hour 
no further precipitation results, then all the cobalt has been 
precipitated. If a precipitate is formed, the whole solution is 
treated with more potassium nitrite and again allowed to stand. 
The clear liquid is poured through a filter, the residue trans- 
ferred to the filter and washed with a 10 per cent. potassium acetate 
solution until 1 c.c. of the filtrate on being acidified with acetic 
acid and boiled with 1 ¢.c. of a 1 per cent. solution of dimethyl 
glyoxime will show no test for nickel. This is usually the case 
after washing four times. As much of the precipitate as possible 
is now transferred to a small porcelain dish, which is covered 
with a watch-glass, cautiously acidified with sulphuric acid, and 
heated on the water-bath until no more brown vapors are evolved. 





* Pogg. Ann., 71, 545 (1847). 
t Z. angew. Chem., 1907, 1847. 


SEPARATION OF COBALT FROM NIGKEL., 163 


The small quantity of precipitate remaining on the filter is dis- 
solved: by pouring hot, dilute sulphuric acid through the filter 
and this acid is added to the main solution of the cobalt. After 
evaporating as far as possible on the water-bath, the heating 
is continued on an air-bath until dense vapors of sulphuric acid 
are evolved. After cooling, the residue is dissolved in water and 
the cobalt determined electrolytically, as described on p. 138. 
ff it is not convenient to carry out an electrolysis, the nitrite 
precipitate is dissolved in hydrochloric acid and the cobalt 
determined according to p.139 (0). 

The filtrate containing the nickel can be treated with hydro- 
chloric acid until the nitric acid is completely decomposed, and the 
nickel then precipitated as black nickelic hydroxide by caustic 
potash and bromine water, filtered, washed, and weighed as the 
oxide, according to p. 137. 

Remark.—This method gives reliable results provided the 
solution is free from alkaline earths. In the latter case the nickel 
and alkaline-earth metals are precipitated with the cobalt. (Cf. 
¥o1,/1,): 


(c) Liebig’s Potassium Cyanide Method. * 


This method is based upon the different behavior of the com- 
plex cyanogen compounds of both metals towards bromine or 
chlorine in alkaline solution. (Cf. Vol. I.) 

Procedure-—The neutral solution, which may contain only 
nickel, cobalt, and the alkalies, is treated with an excess of purest 
98 per cent. potassium cyanide and 5 gm. of pure potassium hy- 
droxide, after which bromine water is added, with constant stirring, 
until no more nickelic hydroxide is precipitated. Care must be 
taken that the solution remains strongly alkaline until the end 
of the process; upon this point depends the success of this excellent 
method. When the precipitation is complete, the solution is 
diluted with cold water and the nickel determined as oxide, as 
described on p. 137. 

The cobalt remains in the filtrate as potassium cobalticyanide. 
After the addition ot dilute sulphuric acid, the solution is evapo- 





* Ann. d. Chem. u. Pharm., 65, 244; 87, 128. 


164 » GRAVIMETRIC ANALYSIS. 


rated as far as possible on the water-bath, a little concentrated 
sulphuric acid is added, and the residue is heated over a free flame 
until dense, white fumes are evolved and the effervescence has 
ceased : 


2K,Co(CN), +12H,S0,+12H,0= 
2CoSO, +3K,SO, +1100 +CO, +6(NH,),SO, +803. 


The cold, blue mass is dissolved in water and the cobalt 
deposited electrolytically; or, thé*cobalt may be precipitated by 
the addition of bromine water and potassium hydroxide, saeeiaties: 
dried and determined as metal according to p. 139. 


(d) Liebig’s Mercuric Oxide Method. 


In this method advantage is taken of the fact that potassium 
nickelocyanide, like almost all other complex cyanogen compounds, 
is decomposed by mercuric oxide, whereas potassium cobalti- 
cyanide, on the contrary, is unaffected: 


K,Ni(CN), +2HgO +2H,0 = Ni(OH), +2KOH + 2H¢(CN),. 


Procedure.—A slight excess of pure potassium cyanide is added 
to the neutral solution, which is then heated on the water-bath 
for at least one hour in order to change the potassium cobalto- 
cyanide to. potassium. cobalticyanide (cf. Vol. I). The solution is 
then treated with a suspension of mercuric oxide in water and 
heated for a long time, with frequent stirring, upon the water- 
bath. The decomposition is complete after one or two hours. 
The solution is diluted somewhat with hot water, and the pre- 
cipitate, consisting of nickelous hydroxide and the excess of 
mercuric oxide, is filtered off, dried, ignited under a hood with a 
good draft, and the residue of nickel oxide weighed as described 
on p. 137. It is better, however, to dissolve the nickel oxide 
in sulphuric acid and determine it electrolytically according to 
p. 136, or as the salt of dimethyl glyoxime, according to p. 129. 

The filtrate containing potassium cobalticyanide and mercuric 
cyanide is treated with sulphuric acid exactly as described 
under (6) andthe cobalt .determined as metal, Sebel by 
electrolysis. 


SEPARATION OF NICKEL FROM ZINC. 165 


The author has also tested and found satisfactory the method 
of Ilinsky and Knorre;* but it seems to have no advantages over 
the above-described procedures. 

Recently Rosenheim and Huldschinsky + have applied Vogel’s 
qualitative test for cobalt (cf. Vol. I, p. 137) to the quantitative 
separation of this metal from nickel, and have obtaimed excellent 
results. $ 


Separation of Nickel from Zinc. Method of Tschugaeff-Brunck.{ 


The solution is treated with ammonium chloride, and enough 
ammonia to make it slightly ammoniacal; no precipitate will be 
formed if sufficient ammonium chloride has: been added. The 
solution is then just acidified with hydrochloric acid, heated to 
boiling and the nickel precipitated with an alcoholic 1 per cent. 
dimethyl glyoxime solution exactly as outlined on p. 129. 

In the filtrate, it is best to precipitate the zinc as sulphide 
by acidifying with acetic acid and saturating the hot solution 
with hydrogen sulphide (cf. p. 145). 

Remark.—When considerable zinc is-present it is necessary to 
add more dimethyl glyoxime to precipitate the nickel. 


Separation of Nickel from Manganese. Method of Tschugaeff- 
Brunck.§ 


The analysis is carried out exactly as described above with 
the only difference that the final precipitation takes place in an 
acetic acid solution.. The greater part of any mineral acid 
present is neutralized carefully with ammonia, the barely acid 
solution is treated with 1 per cent. dimethyl glyoxime solution 
and then, after the precipitate has formed, sodium acetate is 
added and the analysis continued according to p. 129. If the 
alkali acetate is added before the dimethyl glyoxime, a very 
voluminous precipitate is formed which, to be sure, can be filtered 





* Berichte, 18, 669. 

t Ibid., 34, 2050. | 
¢t Z. angew. Chem., 1907, 1849.. 
§ Ibid. 


166 GRAVIMETRIC ANALYSIS. 


with suction, but even then the filtration is tedious. Thus when 
possible it is best to add the sodium acetate after the dimethyl 
glyoxime. When, on the other hand, iron has been removed by 
a basic acetate separation and nickel and manganese are to be 
determined in the filtrate, the precipitation must take place in a 
solution already containing sodium acetate. In the filtrate 
from the nickel dimethyl glyoxime precipitation, the manganese 
is precipitated with ammonium, sulphide and determined as 
described on p. 125, 


Separation of Nickel from Iron. 


If the iron is present in the ferrous condition it is oxidized 
by boiling with nitric acid. Then from 1 to 3 gm. of tartaric 
acid are added and the solution made slightly ammoniacal in 
order to find out whether enough tartaric acid has been added 
(the solution must’ remain perfectly clear). After making barely 
acid with hydrochloric acid, the nickel is precipitated with 
dimethyl glyoxime, the acid just neutralized with ammonia, 
and the analysis continued according to p. 129. 


Determination of Nickel in Steel. * 


The sample, weighing about 0.5 gm., is dissolved in 10 c.c. of 
concentrated hydrochloric acid, enough nitric acid is added to 
completely oxidize the iron, and if there is any separation of 
silica at this point some hydrofluoric acid is added. Two or three 
ems. of tartaric acid are introduced, and the solution diluted to a 
volume of 300¢.c. It is then carefully tested to see whether enough 
tartaric acid is present to prevent any precipitation of iron when 
the solution is made alkaline with ammonia, more tartaric acid 
being added if necessary. The solution, which is left slightly acid, 
is heated nearly to boiling and treated with 30 c.c. of a 1 per cent. 
alcoholic solution of dimethyl glyoxime. The acid is finally very 
carefully neutralized with ammonia, Iéaving the solution so that 
it barely smells of this reagent. The precipitate is filtered through 





* QO, Brunck, Stahl und Eisen, 28, 331. 


REMOVAL OF FERRIC CHLORIDE BY ETHER. 167 


a Gooch or Munroe crucible, washed with hot water, dried at 
110°-120° for 45 minutes and weighed as Ni(C,H,N,0.),. | 

By this method the nickel in a sample of steel can be deter- 
mined within about two hours. The results are accurate, but 
lower than is often obtained in practice, because the cobalt is 
usually determined with the nickel, which is not the case in this 
method. 


Removal of Ferric Chloride by Ether, Method of Rothe. 


The fact that ferric chloride dissolved in hydrochloric acid, 
sp. gr. 1.1, is more soluble in ether than in this acid is often taken 
advantage of in the determination of metals such as nickel, copper, 
vanadium and chromium in samples of steel. It has also been 
used for the determination of sulphur in steel after oxidation to 
sulphuric acid, which does not dissolve in the ether. The under- 
lying principle is the same as that governing the distribution of 
iodine between water and carbon disulphide (see pp. 658 and 659, 
footnote). An example will be given of such a process in the 
Blair method for estimating vanadium, molybdenum, chromium 
and nickel in steel. (See p. 313.) 


168 GRAVIMETRIC ANALYSIS. 


METALS OF GROUP II. 


MERCURY, LEAD, BISMUTH, COPPER, CADMIUM, ARSENIC, 
ANTIMONY, TIN (PLATINUM, GOLD, SELENIUM, TELLURIUM, 
MOLYBDENUM, GERMANIUM, TUNGSTEN, AND VANADIUM). 


A. SULPH®@-BASES. 
MERCURY, LEAD, BISMUTH, COPPER, CADMIUM, 


MERCURY, Hg. At. Wt. 200.6. 
Forms: HgS, Hg,CL, and Hg. 
Determination as Sulphide. 

(a) By Precipitation with Hydrogen Sirokaie 


The solution containing no oxidizing substances (FeCl,, Cl, 
much HNO,, etc.) and the mercury entirely as mercuric salt is 
saturated with hydrogen sulphide in the cold, the precipitate 
allowed to settle, filtered through a Gooch crucible, washcd with 
cold water, dried at 105°-110° C. and weighed. 

Remark.—This method affords excellent results and should be 
used whenever possible. Unfortunately, howevcr, it is not always 
applicable, for in most cases the solution to be analyzed contains 
strong nitric acid (obtained by the solution of impure mercuric 
sulphide in aqua regia, by the decomposition of organic mereury 
compounds by the method of Carius, or by the oxidation of mcreurous 
salts). It is not possible to expel the excess of nitric acid by 
evaporating the solution with hydrochloric avid, because consider- 
able amounts of mercuric chloride are thereby volatilized with 
the escaping steam. Thus 50 c.c. of a mercuric chloride solution 
containing 0.5235 gm. of the salt, treated with 10 c.c. of nitric 
acid and evaporated on the water-bath five times almost to dryness, 
with the addition each time of 50 c.c. concentrated hydrochloric 
acid, yiclded in separate experiments 0.3972 gm. mercuric sulphide 
=88.56 per cent. and 0.3695 gm. mercuric sulphide=82.39 per 
cent., or, in other words, a loss of 11-17 percent. Insucha case the 
following procedure suggested by Volhard should be used: 


DETERMINATION OF MERCURY AS SULPHIDE. 169 


(b) By Precipitation with Ammonium Sulphide. 


The acid solution of the mercuric salt is almost neutralized 
with pure sodium carbonate and is treated with a slight excess of 
freshly-prepared ammonium sulphide. Pure sodium hydroxide 
solution (free from Ag, Al,O,, and SiO,) is then added, meanwhile 
rotating the solution until the dark liquid begins to lighten, when 
it is heated to boiling and more sodium hydroxide is added until 
the liquid is perfectly clear. The solution now contains the mer« 


cury as sulpho-salt, He SNe Ammonium nitrate is then 


added and the solution boiled until the ammonia is almost entirely 
expelled, and the precipitate is allowed to settle, which it will 
do much more quickly than if it were produced by hydrogen 
sulphide directly. By means of the boiling with ammonium 
nitrate, the sulpho-salt is decomposed according to this equation: 


Hg(SNa),+2NH,NO,=2NaNO,+ (NH,).S+HgS. 


The clear liquid is poured through a Gooch crucible, and the 
precipitate washed by decantation with hot water until the 
wash water no longer reacts with silver nitrate solution. The 
precipitate is then transferred to the crucible, dried at 110° C., and 
weighed. In case the precipitate contains free sulphur, it should 
be boiled with a little sodium sulphite before filtering.* 

H. Rauschenbach tested this method, analyzing pure mercuric 
chloride with the addition of nitric acid, and obtained as a mean of 
two experiments 73.80 per cent. Hg instead of the theoretical value, 
73.85 per cent. 

A still better way of removing free sulphur from the precipitate 
consists of extracting with carbon bisulphide. In this case the 
mercuric sulphide, together with the sulphur, is filtered through 
a Gooch crucible, completely washed with water and then three 
times with alcohol. The crucible is now placed upon a glass 
tripod in a beaker containing some carbon bisulphide (Fig. 35) ;+ 





* By boiling with sodium sulphite, the sulphur is changed to auto 
thiosulphate, Na,SO,+S=Na,S.0O,. 

+ G. Vortmann, Helwan timepicle aus der quantitativen chemischem 
Analyse, p. 28, Vienna, 1899. 


170 GRAVIMETRIC ANALYSIS. 


the beaker is supported over a vessel filled with hot water and 
covered with a round-bottomed flask con- 
taining cold water which serves as a condenser. 
After about an-hour the sulphur will be com- 
pletely extracted. The carbon bisulphide is = 
removed from the precipitate by washing [===== 
once with alcohol and once with ether. The LS 
ether is driven off by gently wagming, and 
the precipitate then dried at 110° C. and 
weighed. | 

H. Rauschenbach analyzed pure mercuric 
chloride by this method and obtained as a 
mean of eight experiments 73.79 per cent. 
Hg instead of 73.85 per cent., and in the case 
of eight further experiments made without gel om 
removing the sulphur he obtained 74.17 per 
cent. instead of the theoretical value, 73.84 per cent. 




















Determination of Mercury in Non-Electrolytes. 

If it is desired to determine mercury in an organic non-electro- 
lyte, the compound is decomposed by the method of Carius (see 
Elementary Analysis) by heating in a closed tube with concen- 
trated nitric acid, and the mercury precipitated as sulphide by the 
method of Volhard; or the acid solution is treated with pure sodium 
hydroxide solution to alkaline reaction and then with pure potas- 
sium cyanide until the mercuric oxide has dissolved, after which 
the solution is saturated with hydrogen sulphide, ammonium 
acetate added, the solution boiled until the ammonia is almost 
entirely expelled, the precipitate allowed to settle, filtered, and 
washed first with hot water, then with hot dilute hydrochloric acid, 
and finally with water. After drying at 110° C. the precipitate of 
mercuric sulphide is weighed. 


Determination as Mercurous Chloride. 


For the analysis of a solution containing a mercurous salt, the 
solution is treated with sodium chloride, diluted considerably with 
water, filtered, after standing twelve hours, through a Gooch cruci- | 
ble, dried at 105° C., and weighed. If the solution contains a mer- 


DETERMINATION OF MERCURY AS METAL. I7I 


curic salt, it is first. reduced, by the method of H. Rose, by means 
of phosphorous acid in the presence of hydrochloric acid. 

Procedure.—The mercury solution (which almost always con- 
tains nitric acid) is treated with hydrochloric acid, diluted con- 
siderably with water, an excess of phosphorous acid is added, and 
after standing for twelve hours the precipitate is filtered through 
a Gooch crucible, dried at 105° C., and weighed. 

Remark.—The results obtained by this method are always 
about 0.4 per cent. too low, but in spite of this fact the method is 
to be recommended. 

The phosphorous acid necessary for this method is obtained by 
the oxidation of phosphorus in moist air or by the decomposition 
of phosphorus trichloride with water, evaporating the solution to 
remove the hydrochloric acid and dissolving the residue in water. 


Determination as Metal. 


Almost all mercury compounds are quantitatively decomposed 
on heating with lime according to the equation 


HgX+CaO=CaX +Hg+0. 


The iodide alone is not readily acted upon. 

To carry out this determination, a glass tube 50 cm: long and 
1.5 em. wide, open at both ends, is taken and in one end an asbes- 
tos plug is placed, followed by 8 cm. of pure lime; then an intimate 
mixture of a weighed amount of substance with lime, finally a layer 
of lime 30 em. long and at the other end of the tube another asbes- 
tos plug. After the tube has been filled, the end nearest this sec- 
ond asbestos plug is drawn out until it is only 4 cm. wide, and is 
connected by means of rubber tubing with the empty narrower 
arm of a Péligot tube. The other wider end of the Péligot tube is 
loosely filled with pure gold-leaf. The glass tube is placed in a 
combustion-furnace and illuminating-gas (carbon dioxide is less 
suited) is passed through it for half an hour. The tube is heated, 
at first where the 30 cm. layer of lime is, then the other burners are 
lighted one after another until finally the entire contents of the 
tube is subjected to gentle ignition. During the whole of the opera- 
tion illuminating-gas is being passed through the apparatus at the 
rate of about three bubbles a second. The greater part of the 
mercury collects in the lower empty end of the Péligot tube and 


172 GRAVIMETRIC ANALYSIS. 


the mercury vapors that are carried further amalgamate with the 
gold. A small amount of the mercury condenses in the drawn-out 
tube. After cooling the apparatus (in a current of illuminating-gas) 
the narrow part of the tube is cut off both sides of the condensed 
mercury and weighed. It is then heated gently while air is passed 
through it to volatilize the mercury and again weighed. The dif- 
ference in weight gives the amount of mercury condensed in the 
tube. The Péligot tube is usually gnoist; dry air is, therefore, con- 
ducted through it for some time, after which it is weighed. 

The results obtained by this method * are perfectly satisfactory. 
Winteler found in the analysis of pure mercuric chloride 73.81, 
73.88, 73.74 per cent. instead of the theoretical value, 73.85 per 
cent. 

Experiments made attempting to condense the mercury under 
water invariably gave too low values (about 1-2 per cent.). 

Although it is easy to obtain good results by this method, it 
can. be dispensed with, for the sulphide method affords just as 
exact results in much less time. 

In case it is dcsired to determine the amount of mercury vapor 
present in a given space, it is only necessary to aspirate the gas 
through a calecium-chloride tube filled with gold-leaf. The gain 
in weight of the latter shows the amount of mercury present in 
the gas. 


Electrolytic Determination of Mercury.{ 


Mercury can be determined satisfactorily by the electrolysis 
of acid, neutral, or alkaline solutions. The metal is deposited in 
the form of little drops, which, when the quantity is small, adhere 
to the electrode, or, when larger amounts are present, the mercury 
may collect at the bottom of the platinum dish used as cathode. 
The use of silver-plated electrodes is also advised. 

The electrolysis takes place to advantage in solutions slightly 
acid with nitric acid. 





* First proposed by Erdmann and Marchand, J. prakt. Chem., 31, 385. 

+ Luckow, Z. anal. Chem., 19, 15 (1880); Smith and Knerr, Am. Chem, 
J., 8, 206; F. W. Clarke, Ber., 11, 1409 (1878); Riderff, Z. angew. Chem., 
1894, 388; Classen and Ludwig, Ber., 19, 324 (1886); G. Vortmann, Ber., 
24, 2750 (1891). cates oe | 


ELECTROLYTIC DETERMINATION OF MERCURY. ee eg 


Procedure.—The neutral or slightly acid solution of the mer- 
curous or mercuric salt is placed in a beaker, diluted with water 
to 150 ¢.c., treated with 2 or 3 ¢.c. of concentrated nitric acid, 
and electrolyzed with a platinum gauze cathode at the ordinary 
temperature with a current of 0.055-0.10 ampere. The voltage 
under these conditions’ corresponds to 3.5-5 volts. If the 
electrolysis is started at night, it will be finished next morning, 
provided the amount of mercury does not exceed 1 gm. By using 
a current of 0.6-1 ampere the electrolysis is finished at the 
end of two or three hours. At the end of the electrolysis, the 
metal is washed with water without interrupting the current, 
then with alcohol* and dried. The metal is further dried by 
touching it with filter paper, and then placing it in a desiccator + 
over fused potassium hydroxide and a small dish of mercury. 
In this way correct results are obtained. Drying at 100° and then 
over sulphuric acid in a desiccator gives rise to low results because 
the acid absorbs considerable mercury vapor. 

During the electrolysis of mercuric chloride { the solution 
often becomes turbid in consequence of the formation of insoluble 
mercurous chloride; this does no harm, however, as the metal 
is subsequently deposited on the cathode. 

Mercury can also be electrolyzed from a solution in potassium 
cyanide in the presence of some caustic alkali, and similarly from 





*Tt is usually stated that alcohol is not to be used, but with gauze 
electrodes it does no harm. | 

} Private communication from A. Miolati, cf. Borelli, Revisto tecnica, 
V, Part 7 (1905). Even at 20° the tension of mercury vapor is considerable. 
It amounts to 0.00133 mm. 

t In the electrolysis of the chloride, it is better to use a platinum dish 
with dull, unpolished inner surface (Classen) because then any mercurous 
chloride will certainly be reduced to metal, which is not always the case 
with gauze electrodes. When a dish is used as cathode, the electrode is 
washed with water, without breaking the current, by pouring water into it 
from a wash-bottle while the solution is being siphoned off. As soon as 
the ammeter (or voltmeter used as an ammeter) reaches the zero mark, the 
washing is finished. The current is then turned off, the water carefully 
poured off, the rest of it removed by touching it with filter-paper, and 
the electrode dried as above and weighed. The drying requires several 
hours. 


174 GRAVIMETRIC ANALYSIS. 


a solution formed by dissolving mercuric sulphide in 50-60 c.c. 
of concentrated sodium sulphide solution. 

The great advantage. of the electrolytic determination of 
mercury lies in the fact that good deposits are obtained irrespective 
of the nature of the acid radical, or element, which is combined 
with mercury. ’ 


LEAD, Pb. A®. Wt. 207.1. 
Forms: PbO, PbSO,, PbO,, and in rare cases PbCl,.* 


1. Determination as Lead Oxide, PbO. 


If the lead is present as carbonate, nitrate, or peroxide, it is 
only necessary to ignite a weighed portion in a porcelain crucible 
over.a small flame and weigh the residue. The treatment of the 
nitrate requires care, because on rapid ignition the mass decrepi- 
tates. 

2. Determination as Lead Sulphate, PbSO,. 


If the lead is present in solution in the form of its chloride or 
nitrate, it is placed in a porcelain dish, an excess of dilute sulphurie 
acid is added} and the mixture evaporated on the water-bath as 
far as possible, then over a free flame until dense white fumes of 
sulphurie acid are evolved, and afterwards allowed to cool. A little 
water is added, the mixture stirred, allowed to stand some hours, 
filtered through a Gooch crucible, washed at first with 4 per cent. 
sulphuric acid, then with alcohol, and dried at 100°C. The dried 
precipitate is placed in a larger porcelain crucible, provided with 
an asbestos ring, and ignited over the full flame of a Teclu burner. 

If it is desired to use an ordinary filter, the precipitate is finally 
washed with alcohol until the wash liquid no longer gives the sul- 
phuric acid reaction, dried, as much of it as possible is transferred 
to a weighed porcelain crucible, the filter ignited in a platinum 
spiral (p.22),and the ash added to the contents of the crucible. 
By means of the reducing. action of the burning filter, some of the 
lead sulphate adhering to it is always reduced to lead, which must 





* See Analysis of Vanadinite. 
+ The solution at the time of filtering should contain about 5 per cent. 


of free sulphuric acid. 


DETERMINATION OF LEAD AS LEAD SULPHATE. 175 


be changed back to sulphate before weighing. For this purpose 
the precipitate in the crucible is moistened with dilute nitric acid, 
evaporated on the water-bath to dryness, a few drops of concen- 
trated sulphuric acid added and the crucible heated over a free 
flame until no more fumes are given off, when it is gently ignited and 
weighed. 3 

In case the lead is originally present as acetate, the solution 
is treated with an excess of dilute sulphuric acid and twice its 
volume of alcohol, filtered after standing some hours, and the 
precipitate of lead sulphate treated exactly as described above. 

In order to determine the amount of lead present in organic 
compounds, the substance can be placed in a large porcelain crucible, 
treated with an excess of concentrated sulphuric acid, and very 
cautiously heated in the covered crucible over a free flame until 
the sulphuric acid is completely expelled. The crucible is then 
gently ignited, and if the residue is white it is ready to-be weighed; 
otherwise more sulphuric acid is added and the process repeated 
until finally a white residue is obtained. 

In case the organic lead compound is soluble in water, it is 
preferable to first precipitate the lead by means of hydrogen 
sulphide and then transform the precipitate into sulphate. For 
this purpose, as much as possible of the washed and dried pre- 
cipitate is placed upon a watch glass, the filter and remainder 
of the precipitate are heated in a large porcelain crucible, which is 
supported in an inclined position, and heated carefully over a 
small flame until the filter-paper is completely consumed. The 
main part of the precipitate is added to the crucible, which is 
then covered with:a watch-glass and treated with concentrated 
nitric acid at the temperature of the water-bath. When the 
main reaction is over, the treatment with fuming nitric acid is 
repeated until the contents of the crucible are pure white in 
color. The watch-glass is then removed, five or ten drops of 
dilute sulphuric acid are added, the liquid is evaporated as far 
as possible on the water-bath, the excess of sulphuric acid is re- 
moved by heating on the air-bath (cf. Fig. 11, p. 27) and the lead 
sulphate is weighed: Should the precipitate be dark colored 
after the ignition, it is moistened with concentrated sulphuric 
acid and the excess of acid again expelled. 


176 GRAVIMETRIC ANALYSIS. 


If the lead is present in an organic compound which jis not 
capable of dissociation, the compound should be decomposed in a 
closed tube with strong nitric acid according to the method of 
Carius (see page 287), finally washing out the contents of the tube, © 
adding sulphurie acid, and treating the precipitate as above de- 
scribed. 


Separation of Lead Sulphate from Barium Sulphate and Silicic 
Acid. 


In the analysis of sulphide ores containing lead, it is customary 
to dissolve the finely powdered ore in nitric acid, or aqua regia, 
and to remove the volatile acids by evaporation with sulphuric 
acid, eventually heating over the free flame until fumes of sul- 
phuric acid come off thickly. The sulphuric acid should be diluted 
with an equal volume of water before adding it to the original 
solution; usually 5 c.c. of the diluted acid:is sufficient. After the 
evaporation the moist residue is allowed to cool, then water is 
added and the precipitate filtered and washed with 1 per cent. 
sulphuric acid. The precipitate contains all the lead as sulphate 
but often contains silica and barium sulphate (also strontium 
sulphate and sometimes calcium sulphate). It is purified by 
redissolving the lead in hot ammonium acetate solution (made by 
neutralizing acetic acid, sp. gr. 1.04, with ammonia, sp. gr. 0.96, 
and leaving the mixture barely ammoniacal). When the precip- 
itate is large in amount it is best to wash it into a beaker or flask 
and heat it with about 20 c.c. of the ammonium acetate solution 
(or enough to dissolve all the lead sulphate), then filter through 
the original filter and wash with hot ammonium acetate solution, 
and finally with hot water until the filtrate gives no blackening 
with ammonium sulphide. Small amounts of lead sulphate are 
dissolved on the filter. The silica and barium sulphate will all 
remain behind. 

In order to obtain lead from the acetate solution, it is precip- 
itated as sulphide by hydrogen sulphide, and transformed, after 
drying, into sulphate as described on page 175. oe 

Or, the ammonium acetate solution may be treated with 10 c.c. 
of 50 per cent. sulphuric acid, the acetic acid removed by evap- 
oration, the residue allowed to cool, diluted with water, and the 


ELECTROLYTIC DETERMINATION OF LEAD. 477 


lead sulphate filtered into a Gooch crucible, washed with dilute 
sulphuric acid, heated in an air bath and weighed. 

If the amount of ammonium acetate solution used is not too 
large, the lead may be precipitated by adding enough sulphuric 
acid to the acetate solution to make the solution contain from 
5-10 per cent. sulphuric acid. Sometimes the precipitate is not — 
pure lead sulphate, in which case it should be redissolved in 
ammonium acetate and the precipitation as sulphate repeated. 


3. Electrolytic Determination of Lead as Peroxide (PbO2). 


Many neutral solutions of complex lead salts, a neutral solu- 
tion of lead acetate, also alkaline lead solutions yield deposits of 
metallic lead on the cathode when subjected to electrolysis; but 
lead is never determined this way; partly because of the round- 
about process necessary, and partly on account of the fact that 
the deposited lead is oxidized so readily. If a neutral or slightly 
acid (nitric acid) solution of lead nitrate is electrolyzed, the 
lead is deposited partly as metal upon the cathode and partly as 
brown peroxide on the anode. if, however, the solution contains 
sufficient free nitric acid, it is easily possible to deposit the lead 
quantitatively upon the anode as firmly-adhering lead peroxide. 

Procedure.—The solution of lead nitrate, containing not more: 
than 0.5 gm. lead, is placed in a platinum dish whose inner sur- | 
face is unpolished (as recommended by Classen), 20-30 c.c. of 
pure nitrie acid, sp. gr. 1.4, are added, the solution is diluted to 
150-200 ¢e.c. and electrolyzed in the cold with a weak current of 
about 0.5-1 ampere and 2-2.5 volts. When the electrolysis is 
carried out in the cold, all the lead will be deposited as the peroxide 
at the end of two hours and a half or three hours. Only an hour 
or an hour and a half is required if the temperature of the cell 
is kept at 50°-60°. If it is desired to let the electrolysis run over 
night, a current of 0.05 ampere is sufficient. 

A suitable arrangement of the electrolytic apparatus is shown . 
in Fig. 36, but the dish should serve as anode and the platinum 
spiral as cathode. The resistance Wis made by taking about 
10 m. of nickel wire of about 0.5 mm. diameter, fastening it to a 
board as shown in the drawing and connecting the wires in pairs 
by means of a brass hook, of which only one is shown in the 
sketch. By suitably moving these hooks it is possible to vary 


178 GRAVIMETRIC ANALYSIS. 


the resistance at will. Instead of this arrangement, that shown 
in Fig. 31, p. 132, may be used; such an apparatus is more 
convenient but also more expensive. At the end of the elec- 
trolysis, which is shown by the fact that dilution with a little 
water so as to expose a fresh surface of platinum causes no yel- 
lowish-brown coating to appear at the end of half an hour, the 
dish is washed without breaking the current. This is accom- 
plished by introducing distilled water while the solution is being 
siphoned off. It is important in this operation to keep the deposit 





gs oe 
ETE 














Fig. 36. 


of lead peroxide completely covered with liquid. When the 
solution that is being siphoned off no longer reacts acid, or at 
least only barely acid, the washing is complete and the circuit can 
be broken. The dish is finally washed once more with distilled 
water, dried at 180° C., and weighed. The results obtained are 
always slightly high on account of the lead peroxide not being 
completely anhydrous when dried at this temperature, so that it 
seems to the author to be advisable to gently ignite the dish 
before weighing, thereby readily converting the peroxide into 
lead oxide.* The results obtained in the author’s laboratory 
leave nothing to be desired. 





* Cf. W. C. May, Z. analyt. Chem., 14, 347 (1875). 


DETERMINATION OF BISMUTH AS BISMUTH OXIDE, ETC. 179 


Results.—{(a) 10 c.c. lead nitrate solution containing 0.0631 
gm. lead yielded deposits of PbO, weighing 0.0734, 0.0731, 0.0735, 
0.0733 gm.; mean 0.07332 corresponding to 0.0635 gm. lead. After 
ignition the lead monoxide formed weighed respectiveiy 0.0679, 
0.0678, 0.0679, 0.0681; mean 0.0679 gm. corresponding to 0.0630 
instead of 0.0631 gm. lead. 

(b) 10 c.c.. of a lead nitrate solution containing 0.1898 gm. 
lead yielded deposits of PbO, weighing 0.2202, 0.2200, 0.2203, 
0.2202; mean 0.2202 corresponding to 0.1907 gm. lead. After 
ignition the weights of lead oxide obtained were 0.2042, 0.2046, 
0.2043, 0.204:; mean 0.2044, corresponding to 0.1897 gm. Pb 
instead of 0.1898 gm. These experiments were performed by M. 
Stoffel. 

Remark.—By employing a stronger current and keeping the 
solution warm during the electrolysis, the deposition is complete 
in much less time, but according to the author’s experience the 
results obtained are not so satisfactory. By rotating one of the 
electrodes and using a stronger current, the deposition can be made 
to take place in a short time. If a little lead deposits on the 
cathode, this is remedied by stopping the current for a short time, 
toward the end of the electrolysis. 

Besides the above-mentioned forms, lead is also determine 
as the chromate and as the chloride. The latter method is 
sometimes used in the analysis of bearing metal, cf. p. 252. 


BISMUTH, Bi. At. Wt. 208.0. 
Forms: Bi203, Bi2S3, Bi. 
1. Determination as Bismuth Oxide, Bi,O,. 


Solid bismuth nitrate or carbonate is readily changed to the 
oxide by gentle ignition. When bismuth, however, is present 1m 
solution as the nitrate, it should be first precipitated as the basic 
carbonate and this changed by ignition to the oxide. ens 

Procedure.—-The solution is diluted with water (if a turbidity 
ensucs it makcs no diffcrence) a slight excess of ammonium 
carbonate is added, and after heating to boiling the precipitate 


180 GRAVIMETRIC ANALYSIS, 


is filtered off. washed with hot water, dried, ignited,*and weighed | 
as Bi,O,. If the solution from which the bismuth is to be pre- 
cipitated contains besides nitric acid other acids (HCl, H,SO,, etc.), 
the precipitate produced by ammonium carbonate always con- 
tains basic salts of these acids which cannot be converted to the 
oxide by ignition. In this case, which is most frequent in analysis, 
the bismuth should be determined according to one of the follow- 
ing methods. ° 


2. Determination as Sulphide, Bi,S,. 


The slightly acid solution is saturated with hydrogen sulphide, 
filtered through a Gooch crucible (or a filter that has been dried 
at 100° C. and weighed), washed with hydrogen sulphide water, 
then with alcohol to remove the water, and afterwards with freshly- 
distilled carbon bisulphide ¢ to remove any sulphur that may be 
mixed with the precipitate. 

The washing with carbon bisulphide is eontinued until a few 
drops of the filtrate leave no residue on being evaporated to dry- 
ness on a watch-glass. The precipitate is then washed with 
alcohol to remove the carbon bisulphide and finally with ether, 
dried at 100° C., and weighed as BigS3. 

The distillation of the carbon bisulphide should be performed 
as follows: Ordinary commercial carbon bisulphide is placed in 
a long-necked, round-bottomed flask, provided with a closely fit- 
ting cork (not rubber) stopper which is bored once. Through the 
hole in the cork is placed a glass tube bent twice at right angles, 
whose further end leads into a dry flask (without using a stopper 
for this receiver). Two large beakers are placed upon the table, 
one filled with water at about 60-70° C. and the other with cold 
water. If the flask containing the carbon bisulphide is placed 
in the beaker containing the warm water, and the other flask in the 
beaker of cold water, the carbon bisulphide will distil rapidly from 
one flask to the other. Care must be taken during this operation 











* If the precipitate is large in amount, the greater part is placed on a 
watch-glass, the remainder adhering to the filter is dissolved in hot, dilute 
nitric acid, the solution evaporated to dryness in a weighed platinum dish, 
and the main portion of the precipitate added. ‘The dish and its contents 
are heated at first gently but finally over the full flame of a Bunsen burner. 

t As described on p. 169 or on p. 223. 


DETERMINATION OF BISMUTH AS METAL. 181 


that there is no lighted gas-burner in the immediate vicinity, for 
otherwise there is danger of the vapors of carbon bisulphide taking 
fire. 


3. Determination as Metal. Method of H. Rose.* 


The bismuth is first precipitated as basic carbonate as described 
under 1, and the dried precipitate, together with the ash of the filter, 
is placed in a porcelain crucible and ignited gently. Five times 
as much of 98 per cent. potassium cyanide is added to the con- 
tents of the crucible and the mixture is fused, whereby the oxide 
and basic salt are changed to metallic bismuth: 


Bi,O,+3KCN =3KCNO + Bi, 
2BiOC]+4KCN =2KCNO + 2KCl+ (CN), + Bi,. 


Since bismuth melts at 268° C., but boils at- 1600° C., it is possi- 
ble to perform this operation with a Bunsen flame of about half the 
usual height without running any risk of losing some of the bismuth 
by volatilization. The reduction is usually complete at the end of 
twenty minutes. After cooling, the melt is treated with water, 
which dissolves the salts and leaves the metallic bismuth behind 
in the form of a fused metallic globule. Frequently, however, the 
fusion will have loosened some of the glaze of the porcelain crucible, 
which will remain behind with the bismuth after the treatment 
with water. Consequently the aqueous solution is filtered through 
a filter that has been dried at 100° C. and weighed with the empty 
erucible. After washing first with water, then with absolute 
alcohol and ether and drying at 100° C., the filter is again placed 
in the crucible and weighed. The gain in weight represents the 
amount of metallic bismuth. 

Bismuth sulphide can also be reduced by potassium cyanide, 
but in this case a longer and stronger heating is necessary. 


4. Determination as Metal. Method of Vanino and Treubert.; 


In this method the bismuth is precipitated as metal by means of 
formaldehyde in alkaline solution. The slightly acid bismuth solu- 





* Pogg. Ann., 110, p. 425. 
+ Berichte, 31 (1898), 1303. 


182 GRAVIMETRIC ANALYSIS, 


tion is treated with formaldehyde and a considerable excess of pure 
10 per cent. caustic soda solution and warmed on the water-bath until 
the liquid above the precipitate has become perfectly clear; more for- 
maldehyde and caustic soda solution are then added and the mixture 
heated over a free flame,* decanted repeatedly with water to which 
a little aldehyde has been added, again boiled, and by pressing with 
a glass rod the partly spongy, partly pulverulent precipitate is made 
to collect together. The precipitate is then filtered through a fil- 
ter that has been previously dried at 105° C. and weighed, washed 
with absolute alcohol, dried at 105° C. and weighed. 

Remark.—Results obtained in the author’s laboratory by this 
method were as a rule too high. Thus W. Urech obtained from 
pure bismuth nitrate solution, as a mean of four experiments, 100.78 
per cent. instead of 100 per cent. 

The high results are caused by the difficulty in removing 
the last traces of alkali. Absolutely accurate results may be 
obtained by dissolving the precipitated bismuth in nitric acid, 
precipitating by ammonia and ammonium carbonate and weighing 
as the oxide according to (1). Naturally this roundabout process’ 
would only be chosen when the bismuth solution contained other 
acids (HCl, H,SO,, or H,PO,); the necessity of fusing with potas- 
sium cyanide would then be avoided. 


COPPER, Cu. At. Wt. 63.57. 
Forms: CuO, Cu,S, Cu, Cu,(CNS).. 


1, Determination as Copper Oxide, CuO. 


The solution, which must be free from organic substances and 
ammonium salts, is heated to boiling in a porcelain dish and pure 
caustic potash solution is added, drop by drop, until the precipi- 
tate becomes dark brown and is permanent, while the solution 
itself shows an alkaline reaction towards litmus-paper. After the 
precipitate has settled, the upper liquid is carefully poured through 
a filter and the precipitate washed by decantation with hot water 
until the wash water no longer shows an alkaline reaction, when the 





~~ * Frequently, particularly on long boiling, the liquid becomes colored 
yellow or frown. This has no influence upon the results, 


DETERMINATION OF COPPER AS COPPER OXIDE. 183 


precipitate is transferred to the filter and completely washed. 
Usually a small amount of copper oxide adheres to the porcelain 
dish so firmly that it can be removed only by vigorous rubbing 
with a glass rod covered at the end with a piece of rubber 
tubing, and finally when the precipitate is removed from the dish 
some will then remain on the rubber. Consequently it is better to 
. proceed as follows: As much of the precipitate as possible is removed 
by a stream of water from the wash-bottle, then two drops of dilute 
nitric acid are added, and by inclining the dish and rubbing with 
the glass rod, the whole of the precipitate remaining on the dish is 
moistened with the acid. Two drops of the acid are sufficient, with 
correct manipulation, to dissolve all of the copper oxide. A small 
fresh filter is prepared and the dish is held in an inclined position, 
so that the liquid remains near its lip, the sides are washed once 
with hot water and the contents of the dish (which is continually 
maintained in this inclined position) are heated to boiling over a 
small flame and precipitated by the addition of caustic potash, 
drop by drop. (A large excess of alkali is to be avoided on account 
of its solvent action upon the precipitate.) * The whole contents 
of the dish are then quickly poured through the small filter and 
the dish is immediately washed once with water. The copper oxide 
is now all on the filter. The precipitate is washed with hot water, 
both filters are dried, and the most of the precipitate transferred 
to a porcelain crucible, the filters ignited in a platinum spiral, and 
the ash added to the contents of the crucible. The crucible is cov- 
ered and ignited, at first gently, and finally with the full heat of the 
Bunsen burner, then weighed. If the process is carried out care- 
fully, the results obtained are almost the theoretical values but 
as a rule they are a trifle high. 


2. Determination as Cuprous Sulphide, Cu,S. 


The solution, which contains for every 100 c.c. about 5 c.c. of 
concentrated acid (best sulphuric acid), is heated to boiling and 
hydrogen sulphide is introduced until the solution becomes cold. 
If the right amount of acid was present, the precipitate settles 
quickly in large flocks and the-upper liquid appears completely 





aioe Se as ee 38 


184 GRAVIMETRIC ANALYSIS. 


colorless. Before filtering, the wash liquid is prepared by passing 
hydrogen sulphide through the long tube of a wash-bottle for one 
minute, then closing the short tube with a piece of rubber tubing 
and shaking vigorously. As soon as no more bubbles pass through 
the liquid, the water is saturated; this takes about a minute at the 
most. 

A filter is now placed in a funnel containing a platinum cone, 
the funnel: is fitted to a suction-bottle and the filtration is begun 
at first without using suction, taking care that the filter is con- 
stantly kept full. When all the precipitate is on the filter, it is 
washed with the hydrogen sulphide water containing acetic acid, 
and, at this point also, the filter must be kept full of liquid. The 
washing is continued until 1 c.c. of the filtrate shows no test for 
mineral acid.* The filter is now for the first time allowed to 
drain completely, and it is dried as much as possible by means 
of gentle suction, then completely by heating in the drying 
closet at 90°-100° C. 

As much of the precipitate as possible is now transferred to a 
weighed Rose crucible (of unglazed porcelain),t the filter is 
burned in a platinum spiral and the ash allowed to fall at first 
upon an unglazed crucible cover, where it is heated gently till it 
glows, in order to make sure that it contains no unburned car- 
bonaceous matter; the ash is then added to the main portion 
of the precipitate in the crucible. A little sulphur that has been 
recrystallized from carbon bisulphide is added to the contents 
of the crucible, the perforated cover is now placed on the crucible 
(Fig. 37), a stream of hydrogen is passed through it (the wash- 
bottle shown contains concentrated sulphuric acidt), and the cru- 
cible is heated at first over a small flame and finally so that the 





* The test for sulphuric acid is made with barium chloride. To test for 
hydrochloric acid, the solution is boiled until the hydrogen sulphide is ex- 
pelled and is then treated with silver nitrate. 

+ A quartz crucible is more desirable, as the transformation of CuS into 
Cu,S can then be watched. 

-{ If the hydrogen is prepared from zine and hydrochloric acid, the gas 
should be passed first through water and then through a wash-bottle con- 
taining concentrated sulphuric acid. — 


DETERMINATION OF COPPER AS CUPROUS SULPHIDE. 185 


bottom of the crucible glows faintly, at which temperature the 
cupric sulphide is changed to cuprous sulphide, 


2CuS =Cu,S +S. 


Too strong heating is inadvisable according to Hampe. * 
When the excess of sulphur has been driven off (which can be 
readily ascertained by removing the cover of the crucible and 





finding no blue flame to be perceptible and no odor of burning 
sulphur), the current of hydrogen is increased so: that eight 
bubbles per second pass through the wash-bottle (at first, not 
more than four bubbles per second should have been the 
rate), and the flame is removed. The crucible is allowed to 
cool in the current of hydrogen and weighed after remaining 
in the desiccator for fifteen minutes. The cuprous sulphide 
should be brownish black or black, and should show no reddish- 
brown stains (due to Cu or Cu,O); this is the case if the cur- 
rent of hydrogen was too slow during the cooling. In this case 





+ Z. anal. Chem., 38, 465 (1894). 


186 GRAVIMETRIC ANALYSIS. 


a little sulphur must be added to the precipitate and the process 
repeated. 

Remark.—It is sviaest that the sulphur used for ae experi- 
ment should leave on ignition no weighable residue. This is why 
the sulphur used should be recrystallized from carbon bisulphide. 

The reason why it is necessary to keep the funnel filled with 
liquid during the filtration and washing of the cupric sulphide is this: 
If moist copper sulphide is exposed«o the air it is quickly oxidized 
and the hydrogen sulphide wash water acts upon the salt formed by 
the oxidation, (CuS,O,-CuSO,), and transforms it into colloidal 
cupric sulphide, which forms a pseudo-solution, passes through the 
filter, and on coming in contact with the acid filtrate is coagu- 
lated. If, however, the precipitate is not exposed to the air during 
the filtration there is no oxidation and the filtrate remains clear. 

Instead of changing the cupric sulphide into cuprous sulphide, 
it has been proposed to convert it to oxide by ignition in the 
air and weighing the copper in this form. If, however, the highest 
degree of accuracy is desired, this should not be done, for the 
ignited product always contains some sulphate. When this 
method is chosen, the cupric sulphide should be heated in a glazed 
porcelain crucible, at first over a small flame, so that the mass 
does not melt, and the heat gradually increased until finally a 
blast-lamp is used and the copper weighed as CuO. The results 
are about 0.1 per cent. too high when not more than 0.2 gm. of 
precipitate is present. Holthof* states that copper oxide abso- 
lutely free from sulphate can be obtained if the precipitate is 
ignited wet in an inclined porcelain crucible. 


3. Determination as Cuprous Sulphocyanate, wire? 
Method of Rivot.t 


The solution, slightly acid with sulphuric or mae acid 
(oxidizing agents must not be present), is treated with an excess 
of sulphurous acid,{ after which ammonium sulphocyanate is 





* Z,. anal. Chem., 28, 680 (1889). 

+ Compt. Rend., 38, 868; see also R. G. van Name, Zeit. f. anorg. Chem., 
26, 230, and Busse, Zeit. f. anal. Chem., 17, 53, and 30, 122. 

+ Instead of sulphurous acid, ammonium bisulphite may be used. The 
latter is prepared by saturating aqueous ammonia with SO,,. 


ELECTROLYTIC DETERMINATION OF COPPER. 187 


added drop by drop with constant stirring, whereby at first a 
ereenish precipitate of cupric and cuprous -sulphocyanate is pre- 
cipitated, which after stirring becomes pure white. The precipitate 
is allowed to settle completely (this requires several hours); it is 
then filtered and washed with cold water until the filtrate shows 
only a slight reddish coloration when ferric chloride is added, 
after which it is washed several times with 20 per cent. alcohol, 
dried at 110-120° C., and weighed. R. Philipp found by this 
method 99.95 per cent. instead of 100 per cent. copper, as a mean 
of twelve experiments. The cuprous sulphocyanate can be dried 
at a temperature as high as 160° C., but at 180° C. it begins to 
decompose. The Munroe crucible can be used to advantage in - 
this determination. The precipitate permits rapid filtration, and 
a turbid filtrate is never obtained. After the determination is 
finished, the greater part of the precipitate is shaken out of the 
crucible, and the remainder dissolved in hot nitric acid. 


4. Electrolytic Determination of Copper. 


This most accurate and most convenient of all methods for 
the determination of copper was first proposed by W. Gibbs in 
1864.* : 

Copper may be deposited by means of the electric current 
from acid, alkaline, and neutral solutions, but for analytical pur- 
poses only the use of acid solutions is of importance. 

Procedure—The safest way, according to F. Forster,* is to 
deposit the copper from a sulphuric acid solution. To the neutral 
solution containing the copper in the form of sulphate, 10 c.c. 
of twice normal sulphuric acid are added, the solution is diluted to 
a volume of 100 c.c. and electrolyzed with exactly two volts 
potential at the electrodes and this potential is kept constant 
during the electrolysis. These conditions are fulfilled by simply 
connecting the electrodes with the poles of a single storage cell. 
The electrolysis requires at least eight hours if done at the ordi- 
nary ‘temperature, but by keeping ‘the solution at 70°—80°, 0.2 
gm. of copper is deposited in 60-80 minutes. If, therefore, it 





* Z. anal. Chem., 3, 334. 


188 3 GRAVIMETRIC ANALYSIS. 


‘is desired to carry on the electrolysis over night, it is done in the — 
cold. It is very easy to decide when the electrolysis is finished 
by adding a little water and noticing whether there is any more 
copper deposited upon the freshly exposed electrode surface. 
The cathode is then washed with water, without breaking the 
circuit, exactly as was described under the electrolytic determina- 
tion of nickel (p. 136). Finally, the cathode is rinsed with alcohol, 
dried by holding it high above*a flame, cooled in a desiccator, 
and weighed. ; 

If these directions are followed closely, the copper is never 
‘deposited in a spongy condition. The presence of Ni, Co, Fe, 
Zn and Cd does not influence the analysis and the copper may be 
separated from these elements by means of such an electrolysis. 

If the solution to be analyzed contains copper and some of 
the above-mentioned base metals, it is evaporated to dryness, 
heated with a little sulphuric acid until dense fumes are evolved, — 
cooled, treated with 10 c.c. of 2 N. sulphuric acid, diluted to 100 
c.c. and electrolyzed as described above. 

If, however, only copper is present in the solution, it may 
be deposited very nicely in the following manner. The solution 
should contain 4-5 c.c. of concentrated nitric acid in 100 c.c. If, 
originally, i; contained more nitric acid than this, it is either 
evaporated to dryness or neutralized with ammonia, and then the 
required quantity of nitric acid added. The solution is heated 
to 50°-60° and electrolyzed with a current of 1 ampere and elec- 
trode potential of 2-2.5 volts. The electrolysis is over at the 
end of two hours, when not more than 0.3 gm. of copper is present. 
The analysis is finished as above but there is more danger of 
traces of copper being dissolved while the electrodes are being 
removed. 

Remark.—The copper may be deposited electrolytically much 
more rapidly by the use of a rotating electrode or any stirring: 
arrangement. The use of a gauze cathode has also been rec- 
ommended. The solution should not be diluted too much, as 
spongy deposits are obtained from very dilute solutions unless 
a very weak current is used. As a general rule, the more con- 
centrated the copper solution, the stronger the current that can 


be used. 


ELECTROLYTIC DETERMINATION OF CADMIUM. 189 


CapDMIuM, Cd. At. Wt. 112.4. 
Forms: Cd, CdSO,, CdO. 


1. Electrolytic Determination of Cadmium. 


Of all the methods for the determination of cadmium the electro- 
lytic method is not only the most convenient, but by far the most. 
accurate, and of the many methods proposed that of Beilstein and 
Jawein* can be recommended. From the experience obtained in the 
author’s laboratory the best procedure is as follows: To the solution 
of the sulphate a drop of phenolphthalein is added and then pure 
caustic soda solution until a permanent red color is obtained. A so- 
lution of 98 per cent. potassium cyanide is now added with constant 
stirring until the precipitate of cadmium hydroxide produced by the 
caustic soda has completely dissolved (an excess of potassium cya- 
nide should be scrupulously avoided), the solution is diluted with 
water to 100-150 c.c. and electrolyzed in the cold, using a gauze 
cathode, for from five to six hours with a current of 0.5—0.7 ampere 
and an electromotive force of 4.8-5 volts; at the end of this time 
the current is increased to from 1—-1.2 amperes and the solution 
is electrolyzed for one hour more. If these directions are followed, 
all of the cadmium (if not more than 0.5 gm. is present) will be 
deposited as a: firmly adhering dull deposit of almost silver- 
white metal. The current is then stopped, the liquid is quickly 
poured off and the deposited metal washed first with water, 
then with alcohol and finally with ether; it is dried and 
weighed. Experiments performed by von Girsewald gave fault- 
less results. 

After the electrolysis is finished, the solution should always 
be tested for cadmium. For this purpose, it is saturated with 
hydrogen sulphide. If much cadmium is present, a yellow pre- 
cipitate is obtained, but if very little, a yellow coloration results. 
The latter is due to the formation of colloidal cadmium sulphide, 
and the color is so intense that R. Philip estimates the quantity 
of cadimum not precipitated, by comparing the shade with that 
produced in a solution containing a known quantity of cadmium 
and the same amounts of potassium cyanide and caustic potash 
as in the solution tested. 





* Berichte, 12, 446. 


390 GRAVIMETRIC ANALYSIS. 


Remark.—lIf for the electrolysis a current of 0.5 ampere were 
used, the cadmium will not be all deposited at the end of twelve 
hours; if, however, the current is increased at the end, as above 
stated, to 1 ampere, the electrolysis will be surely finished at six 
to seven hours. To work with the stronger current from the 
beginning is not to be recommended unless a gauze cathode is 
used, or one of the electrodes is rotated, for otherwise the metal 
is deposited in a spongy form and on washing some of it is likely 
to be lost. < 

A solution containing 0.4568 gm. Cd., 3 gm. KCN, 1 gm. NaOH, 
and diluted to 125 ¢c.c. with water, can be electrolyzed in fifteen 
minutes with a current of 5 amperes and 5.5 volts if one of the 
electrodes be rotated.* 

From neutral and weakly acid solution:, cadmium can be 
deposited electrolytically, but not from strongly acid solutions. 


2. Determination as Cadmium Sulphate, CdSO,. 


Next to the electrolytic method, the determination of cadmium 
as the sulphate is the best. If the cadmium is combined with a 
volatile acid, the compound is treated in a weighed porcelain cru- 
cible with a slight excess of dilute sulphuric acid, the solution evapo- 
rated on the water-bath as far as possible, and finally the excess 
of sulphuric acid is removed by heating in an air-bath (the crucible 
is placed in a larger crucible that is provided with an asbestos ring).f 
The heat is applied at first slowly, and the temperature is raised 
gradually until finally no more fumes of sulphuric acid are evolved. 
The outer crucible can even be heated with the full flame of a Teelu 
burner without running any risk of decomposing the cadmium sul- 
phate; it is, however, not necessary to heat it so strongly. As soon 
as the fumes of sulphurie acid cease to come off, the operation is 
ended and the crucible and its contents are weighed after cooling 
in a desiccator. The cadmium sulphate should be pure white 
and should dissolve in water to form an absolutely clear solution. 

If the cadmium has been precipitated from a solution as the 
sulphide, the greater part of the precipitate is placed in a large 
porcelain crucible, covered with a watch-glass, and treated with 
hydrochloric acid (1:3) on the water-bath. After the precipitate 





* See Edgar F. Smith’s Electro-Analysis. 
+ Cf. Fig. 11, p. 27. 


THE PRECIPITATION OF CADMIUM AS SULPHIDE. Igi 


has dissolved and the evolution of hydrogen sulphide has ceased, 
the lower side of the watch-glass is washed, the crucible is placed 
under the funnel, and the precipitate which adhered to the filter- 
paper is dissolved by dropping hot hydrochloric acid (1:3) upon it, 
finally washing the filter with hot water, evaporating the solution 
upon the water-bath, and proceeding as above described. 

The results obtained by this method. are excellent. 


The Precipitation of Cadmium as Sulphide. 


The frequently recommended determination of cadmium as the 
sulphide must be rejected; it is useless. It is not possible to precipi- 
tate pure cadmium sulphide from acid solutions by means of hydro- 
gen sulphide; the precipitate is always contaminated with a basic 
salt (Cd,CLS—Cd,SO,S, ete.) whether the precipitation takes place 
in cold or hot solutions, whether under atmospheric pressure or 
under increased pressure (in a pressure-flask), and in fact the amount 
of basic salt formed increases with the amount of free acid present. 
Results are obtained as much as 5 per cent. too high. Follenius * 
attempted to make the method possible by igniting an aliquot part 
of the dried and weighed precipitate in a stream of hydrogen sul- 
phide. If the sulphide was contaminated with sulphate, he suc- 
ceeded in changing it all to sulphide and obtained results that were 
acceptable. If, however, chloride was present, a considerable 
part was lost by sublimation, so that the results obtained were too 
low. It is, furthermore, not possible to ignite the cadmium sul- 
phide with sulphur in a current of hydrogen, as was described 
under Zinc and Copper, for cadmium sulphide is so volatile that 
some of it is lost. 

On the other hand, the method of precipitating the cadmium as 
sulphide from solutions containing 2-7 c.c. of concentrated sul- 
phuric acid in 100 ¢.c. is to be recommended, for by this means a 
precipitate is obtained which can be readily filtered and which by 
solution in hot hydrochloric acid (1:1) and evaporation with sul- 
phuric acid can be changed without loss to the sulphate and weighed 
as such, 


Ske 





* Zeit. f. anal. Chem., XIII, 422. 


192 GRAVIMETRIC ANALYSIS. 


3. Determination as Cadmium Oxide, CdO. 


Cadmium carbonate and cadmium nitrate can be changed to 
the oxide by strong ignition. 

The cadmium is precipitated from its solutions at the boiling 
temperature by the addition of a slight excess of potassium car- 
bonate, and after standing for some time on the water-bath, and 
when the precipitate has completely settled, it is filtered off, washed 
with hot water, and dried. As much of the dried precipitate as 
possible is transferred to a watch-glass and set aside for the time 
being. The filter is washed with dilute nitric acid to dissolve 
the small amount of the precipitate which still adheres to it and 
the solution is received in a weighed porcelain crucible and evap- 
orated to dryness. The main portion of the precipitate is now 
added, and the crucible is at first very gently heated by placing 
the open crucible high above a small flame from a Teclu burner, 
until the whole mass has become a uniform brown throughout- 
The temperature is now gradually raised until finally the full heat of 
the burner is reached. It is important during this operation to 
take care that the inner flame-mantle does not touch the cruci- 
ble, for otherwise reducing gases may enter the crucible and reduce 
a part of the oxide to metallic cadmium, which is volatile at this 
temperature.* The cadmium oxide is obtained as a brown 
powder which is infusible, insoluble in water, but readily soluble 
in dilute acids.t 

Remark.—It is not advisable to precipitate the cadmium by 
means of sodium carbonate solution, for in that case it is difficult 
to wash the precipitate free from alkali. 


SEPARATION OF THE SULPHO-BASES FROM THE METALS OF 
THE PRECEDING GROUPS. 


Hydrogen stlphide precipitates only the metals of the “hydro- 
gen sulphide group” from acid solutions. It is to be noted that 
zine precipitates with this group if the solution is not acid enough; 





* Tf the cadmium carbonate is filtered into a Munroe crucible, and ignited 
in an electric oven, the transformation takes place readily without danger 
of any volatilization. 

+ The oxide after ignition is a black, crystalline powder. 





ANALYSIS OF BRASS. 193 


while if the solution is too acid lead and cadmium are often 
incompletely precipitated. A suitable concentration is 5-7 c.c. 
of concentrated hydrochloric acid to 100 c.c. of liquid. 


EXAMPLE. 


Analysis of Brass (Alloy of Copper and Zinc with Small 
Amounts of Lead, Iron, and Nickel). | 


About 0.4-0.5 gm. of the alloy, in the form of borings,* 
is dissolved in about 20 c.c. of nitric acid, sp. gr. 1.2, in a 200-c.c. 
casserole which is covered with a watch-ylass. After the reaction 
begins to slacken, complete solution is effected by warming on the 
water-bath. The solution is then evaporated to complete dryness, 
moistened with a little nitric acid, dissolved in about 50 c.c. of 
hot water, and any metastannic acid present is allowed to 
settle, is filtered off, washed with hot water, dried, and the 
tin determined according to p. 228. To the cold filtrate 3 c.c. 
of pure, concentrated sulphuric acid are added, the solution is 
evaporated on the water-bath as far as possible, and then heated 
cautiously over a free flame until dense white fumes of sulphuric 
acid are evolved. After cooling the residue is treated with 50 c.c. 
of water and 15 c.c. of alcohol, stirred well, filtered, washed, and 
the lead sulphate determined according to p. 174. ‘The filtrate 
is evaporated until the alcohol is completely removed, 100 c.c. cf 
water are added, the solution is heated to boiling, and hydrogen 
sulphide is conducted into it until it becomes cold, when the copper 
sulphide is filtered off, washed first with hydrogen sulphide water 
containing in every 100 c.c. 20 c.c. of double-normal sulphuric acid 
and at the end with 5 per cent. acetic acid, which is saturated with 
hydrogen sulphide, until the filtrate gives no precipitate on being 
treated with barium chloride. The copper is determined, accord- 
ing to p. 183, as Cu,8. : 

The filtrate from the copper sulphide is evaporated to a small 
volume in order to remove completely the excess of hydrogen 





* The borings are usually somewhat greasy.. They should be washed with 
ether before weighing. Cf. p. 236, foot-note. 


194 GRAVIMETRIC ANALYSIS. 


sulphide, the iron is then oxidized by the addition of bromine 
water, precipitated by ammonia, and filtered. In order to make 
sure that the precipitate of ferric hydroxide contains no zine, it 
is dissolved in a little hydrochloric acid and the precipitation with 
ammonia is repeated. The filtered and washed precipitate is 
ignited in a porcelain crucible and weighed as ferric oxide cof 
p. 87). 

The combined filtrates from the ferric hydroxide are acidified 
with a little sulphuric acid, heated to about 50° C., and the zinc 
determined as zine sulphide according to the “ salting-out ” 
method described on p. 160. For the determination of nickel, 
the filtrate from the zine sulphide precipitation is boiled to expel 
the hydrogen sulphide and the nickel determined as the salt of 
dimethyl glyoxime according to p. 129. 


SEPARATION OF THE SULPHO-BASES FROM ONE ANOTHER. 


1. Separation of Mercury from Lead, Bismuth, Copper, 
and Cadmium. 


Method of Gerhard v. Rath. 


Principle-—This separation is based upon the insolubility of 
mercuric sulphide in boiling, dilute nitric acid (sp. gr. 1.2-1.3) 
and the solubility of the remaining sulphides. 

Procedure.—The solution (containing the mercury entirely in 
the mercuric form) is precipitated by means of hydrogen sulphide, 
the precipitate filtered off, washed with hydrogen sulphide water, 
transferred to a porcelain dish and boiled for a considerable length 
of time with nitric acid of the above concentration, then diluted 
with a little water and washed with water containing nitric acid. 
The residue of mercuric sulphide thus obtained always contains 
sulphur, and in case considerable lead were present it will also 
contain lead sulphate. It is, therefore, dissolved in a little aqua 
regia, diluted with water, filtered from the separated sulphur and 
lead sulphate and the mercury precipitated according to the method 
of Volhard, with ammonium sulphide (cf. p. 169). If some of the 
lead sulphate should go into solution with the mercury on treating 





SEPARATION OF BISMUTH FROM LEAD. 195 


with aqua regia, it will be converted by the ammonium sulphide 
and potassium hydroxide into insoluble lead sulphide, while the 
mercury will be in the form of its soluble sulpho-salt. In this 
case the lead sulphide is filtered off, washed with dilute potassium 
hydroxide solution, and the mercury then precipitated as sulphide, 
as described on p. 169. 


2. Separation of Bismuth from Lead. 


(a) Method of Lowe. 


Principle —Bismuth nitrate is changed by the action of water 
into an insoluble basic salt, while lead nitrate undergoes no such 
transformation. 

Procedure.—The solution of the two metals in nitric acid is 
evaporated on the water-bath until it reaches a syrupy consist- 
ency, water is added, and after thorough stirring with a glass 
rod the evaporation is repeated and the process continued until 
the addition of the water fails to produce any further turbidity; 
a sign that the bismuth has been completely converted into the 
basic salt Bi,O,NO,OH. A cold solution of ammonium nitrate 
(1 NH,NO,:500 H,O) is now added, and after standing some time, 
with frequent stirring, in order to make sure that the lead 
nitrate is completely dissolved, the solution is filtered. The 
precipitate is washed with the dilute ammonium nitrate solution 
and dried. As much of it as possible is transferred to a weighed 
porcelain crucible and together with the ash of the filter is ignited,* 
at first gently, and finally with the full flame of a Bunsen burner. 
It is weighed as Bi,Q,. 

- From the filtrate the lead is precipitated according to p. 174, 
as sulphate, and weighed as such. It is less satisfactory to pre- 
cipitate the lead as sulphide and weigh it in this form after 
gentle heating with sulphur in a Rose crucible. 


(b) Method of Jannasch.f 


Principle.—The separation depends upon the different vola- 
tility of the two bromides. Bismuth bromide is fairly readily 
volatile; lead bromide is only difficultly so. 


* It is still better to proceed as inthe detateninntion of cadmium oxide, p.192. 
+ Praktischer Leitfaden der Gewichtsanalyse. 





196 GRAVIMETRIC ANALYSIS, 


Procedure.—The solution of the nitrates is evaporated to dry- 
ness, 100 c.c. of water, sufficient hydrochloric acid to afford a 
clear solution, and a few drops of fuming nitric acid are added,* 
after which hydrogen sulphide is introduced. The precipitated 
sulphides are immediately filtered, the precipitate is dried at 100° C. 
in a stream of carbon dioxide, after which as much of the precipitate 








Fig. 38. 


as possible is placed in an agate mortar and the ash of the filter added 
to it. The whole of the precipitate is ground fine and transferred 
without loss to a weighed porcelain boat, which is then introduced 
into the decomposition tube Rf (Fig. 38), made of difficultly-fusible 
glass. At first a stream of dry carbon dioxide is passed through the 
apparatus and the substance is gently heated by means of a small 
flame, in order to completely dry it. The water condensing in the 
front part of the tube is driven over into E by careful heating. 
The bottle A containing bromine { is now connected with the 
apparatus and the stream of carbon dioxide is passed through 
it; the gas, carrying bromine vapors with it, is passed through 
the vertical calcium chloride tube filled with pieces of calcite, 





* By the addition of the fuming nitric acid the precipitated sulphide is 
contaminated with considerable sulphur; such a precipitate is more readily 
decomposed by the action of bromine. 

+ In this determination, the bulb of the tube is unnecessary; it should be 
replaced by one such as is shown in Fig. 38, JZ. For other analyses it is 
better to have the bulb. 

t For this experiment the bromine used must be absolutely free from 
chlorine and is prepared as follows: 50-60 c.c. of commercial bromine are 
treated, in a tightly stoppered separatory funnel, witha 10 per cent. potassium 
bromide solution. The funnel is shaken vigorously, and the bromine sepa- 
rated from the aqueous alkali solution. After washing two or three times with 
water it is ready for use. 





SEPARATION OF BISMUTH FROM LEAD. 197 


then through the concentrated sulphuric acid contained in B, after 
this through the tube C containing glass beads moistened with 
sulphuric acid, and finally through the tube D filled with glass 
wool, and from this the dry bromine vapors reach the sub- 
stance. The latter is heated over a small flame (kept in con- 
‘stant motion) and the yellow bismuth bromide distils off and 
condenses partly in the narrow part of the tube and partly in the 
zeceiver E, which contains dilute nitric acid (1 HNO3:2 H2O). The 
substance is heated hotter, whereby more bismuth bromide is vola- 
tilized, and this is again distilled as completely as possible into the 
receiver. Finally the substance is heated more strongly still, until 
the lead bromide begins to melt. When no more of the yellow 
sublimate is formed, the decomposition is shown to be complete 
and the substance is allowed to cool in a stream of carbon dioxide. 
The bromine that escapes from the tube K is passed into aleohol 
contained in the beaker F. When the apparatus has become cold, 
the bromine bottle is removed, and the bromine is removed from 
the apparatus by passing carbon dioxide through it for some 
time. The boat filled with lead bromide is then weighed, and 
from the weight of the PbBr, that of the lead is computed. To 
check this, the lead bromide is dissolved in freshly-prepared 
chlorine water, an excess of dilute sulphuric acid is added, and 
the solution is evaporated to remove the hydrochloric acid, at 
first on the water-bath and finally over a free flame until dense 
fumes of sulphuric acid are evolved. 

After cooling, water and alcohol are added, the precipitate 
filtered off and the weight of the lead sulphate determined as 
described on p. 174. For the bismuth determination, the nitric 
acid solution contained in E and K is poured into a beaker, filtered 
if necessary from any sulphur, evaporated to a small volume, and 
the bismuth precipitated by the addition of ammonium carbonate 
and determined as metal as described on p. 181. 

There have been many other methods proposed for the separa- 
tion of lead and bismuth,* all of which are less satisfactory than 
the two methods just described, so that they will not be discussed 
in this book. 





* Cf. O. Steen, Z. angew. Chem., 1895, p. 530. 


198 GRAVIMETRIC ANALYSIS. 


3- Separation of Bismuth from Copper. 


The solution is treated with an excess of ammonium carbonate, 
warmed gently, and filtered. The precipitate of basic bismuth 
carbonate almost always contains small quantities of copper, so 
that it is dissolved in nitric acid and the separation by means of 
ammonium carbonate is repeated. The basic bismuth salt is 
fused with potassium cyanide and weighed as metal, according 
to p. 181, 

For the copper determination, ‘the two filtrates are coianell 
evaporated to remove the excess of ammonium carbonate, acidified 
with sulphuric acid, and the copper precipitated by means of 
hydrogen sulphide, being determined as cuprous sulphide accord- 
ing to p.183, or the sulphuric acid solution is subjected to elec- 
trolysis as described on p. 187. 

According to Fresenius and Haidlin, bismuth can be separated 
from copper very nicely by means of potassium cyanide. For 
this purpose the acid solution is precipitated by the addition of a 
slight excess of sodium carbonate, potassium cyanide is added, 
and the solution warmed and filtered. All of the copper is found ~ 
in the filtrate, while the precipitate contains bismuth oxide con- 
taminated with alkali. The residue is, therefore, dissolved in 
nitric acid, the bismuth precipitated by means of ammonium car- 
bonate and determined as metal according to p. 181. The 
filtrate containing the copper is evaporated with nitric acid, in 
order to destroy the cyanide, and the copper determined electro- 
lytically according to p. 187. 


4. Separation of Lead from Copper by Means of Electrolysis. 


This separation depends upon the fact that the electric current 
deposits lead quantitatively as PbO2 upon the anode from 
solutions containing a definite amount of nitric acid, while 
the copper is either not deposited at all under these conditions 
or is found upon the cathode to some extent. After the lead is 
completely deposited, the copper solution is poured into a second 
weighed platinum dish, the excess of the acid is neutralized with 


SEPARATION OF LEAD FROM COPPER. 199 


ammonia, and the solution again electrolyzed. The copper will 
now deposit quantitatively upon the cathode. 

Procedure.—The solution of the two nitrates is placed in a 
platinum dish (of the form recommended by Classen wit. te 
inner surface unpolished) and 15 ¢.c. of nitric acid (sp. gr. 1.35- 
1.38) are added, after which the solution is diluted to 150 c.c. and 
electrolyzed at 50°-60° C. with a current of 1-1.5 amperes and 
an electrode potential of 1.4 volts. After 1-1.5 hours practically 
all the lead will be deposited upon the anode (dish) in the form of 
a firmly adhering, brown coating of lead peroxide, PbO,. At 
the cathode (a plate electrode) a considerable part of the copper 
will be deposited, but the remainder will still be in solution. The 
circuit is broken and the solution poured as quickly as possible 
into a second weighed platinum dish, and the washings added 
to this dish. After washing the electrodes with water, the first 
dish with the PbO, deposit is dried at 180° and weighed. The 
solution in the second dish contains a little lead and some copper, 
It is made slightly ammoniacal, 4 c.c. of concentrated nitric acid 
are added, and the solution electrolyzed at 60°. The platinum dish 


- now serves as the cathode, while the plate electrode * serves as 


the anode; in case traces of lead remain in solution after the first 
electrotysis, it will now be deposited. After an hour or two 
with a current of one ampere all the remaining copper and lead 


will be deposited. When the electrolysis is complete the elec- 


trodes are washed without breaking the circuit and the weight | 
of the copper and PbO, is determined. 

If only small amounts of lead and copper are present, the 
electrolysis should take place under the conditions described on 


'p. 187, except in this case a weighed plate electrode should be 


employed as the anode. Under these conditions the lead will be 
deposited as the peroxide upon the anode, while the copper will 
separate out upon the dish. 





*The plate electrode with copper upon it was weighed, cleaned, and 
then weighed again. 


200 GRAVIMETRIC ANALYSIS. 


5. Separation of Lead from Copper and Cadmium, 


(From Bismuth less satisfactorily.) 


The solution of the nitrates or chlorides is treated with an excess 
of sulphuric acid, evaporated to remove the nitric or hydrochloric 
acid, and the lead determined as sulphate as described on p. 
174. 


6. Separation of Copper from Cadmium. 


(a) Method of A. W. Hofmann.* 


_ A. W. Hofmann states that copper and cadmium can be sepa- 
rated from one another by boiling their sulphides with sulphurie 
acid (1:5) whereby cadmium sulphide is dissolved while copper 
sulphide is unacted upon. Hofmann seems to have tested this 
separation only qualitatively and not quantitatively, but neverthe- 
less this method is given in all early text-books without submitting 
any analyses to prove its accuracy. Experiments performed in the 
author’s laboratory showed that in the form proposed by Hofmann 
this method cannot be used for the quantitative separation of 
the two metals; on the other hand, if it is carried out according to 
the following modifications, excellent results are obtained. 
Procedure.—Sufficient sulphuric acid is added to the solution 
of the sulphates so that one part of the acid is contained in four 
parts of the solution. The latter is now heated to boiling, and 
during the boiling hydrogen sulphide is passed through it for 
twenty minutes, after which the solution is boiled for fifteen 
minutes longer. The solution is filtered while hot through a 
funnel kept filled with carbon dioxide and the precipitate is washed 
with boiled, hot water to the disappearance of the acid reaction. 
The copper sulphide thus obtained is easy to filter and wash; it 
however, always contains small amounts of cadmium, so that the 
separation must be repeated. The copper sulphide ic, therefore, 
transferred to a porcelain dish by means of a stream of water from 





* Ann. d. Chem. und Pharm., 115, 286. 





SEPARATION OF COPPER FROM CADMIUM. 201 


the wash-bottle, where it is dissolved in nitric acid, the solution 
evaporated to dryness, the dry mass treated with sulphuric acid 
(1:4) and again evaporated on the water-bath as far as possible 
to remove the greater part of the nitric acid. After this, without 
regard to the separated sulphur, the mass is washed with as little 
water as possible into an Erlenmeyer flask, for every 0.3-0.5 gm. 
of copper about 150-200 c¢.c. of sulphuric acid (1:4) are added, 
and the separation by means of hydrogen sulphide is repeated 
exactly as above described. The pure copper sulphide that is 
finally obtained is dried and the copper determined as cuprous 
sulphide as described on p. 183, or it is dissolved in nitric acid 
and the solution electrolyzed as described on p. 187. 

For the cadmium determination, hydrogen sulphide is passed 
into the cold filtrate, the precipitated cadmium sulphide after 
being washed is transferred by means of a spatula to a porcelain 
dish, hydrochloric acid (1:3) is poured over it, the dish covered 
with a watch-glass and heated on the water-bath until the precipi- 
tate is dissolved and until the hydrogen sulphide is all expelled. 
The dish is now placed under the funnel and the cadmium sulphide 
which remained upon the filter is dissolved by dropping hot hydro- 
chloric acid (1:3) upon it, finally washing the filter with water. 
The contents of the dish are evaporated to dryness, the dry mass 
dissolved in a little sulphuric acid, washed into a weighed porcelain 
crucible, and treated with 1 c¢.c. of concentrated nitric acid* and 
a little more sulphuric acid. After this the contents of the crucible 
are evaporated as far as possible upon the water-bath, the excess of 
sulphuric acid removed by heating in an air-bath, and the cadmium 
determined as sulphate according to p. 190. 

The above method was tested by Oberer in the author’s labora- 
tory and the following results obtained: 





* The nitric acid is added to oxidize the fibres of filter-paper; if these 
are not destroyed they will cause a partial reduction of the cadmium sul- 
phate. 


202 GRAVIMETRIC ANALYSIS. 


























* | Amount Found in 

Amount Taken. Found. Difference. Per Cent. of the 

Theoretical Value. 
1. - OCax0 3126 om. 0.3130 gm. +0.0004 100.12 
Cod) 2504. th 0.2506 +0.0002 100.03 
2: Lape 31968 ceo 0.3125.“ —0.0001 99 .97 
Cd==0 2502 -"  e. 0.250%. = —0.0003 99.88 
3... Cus 0.3126.°° 2S ca 0.3134 “. +0.0003 100.25 
Cd=0: 2508." ~~. 0.2496 “ —0.0003 99.63 
4, Cand 3106 0.3120 “ —0.0006 99.81 
Cone. Gee 0.6252 “ —0.0007 £9.88 
5. Ci=6.3142. 6 ooo 0.3147. +0.0095 100.16 
Cd= 06.6250 82 0.6248 ‘ —0.0011 99.82 
@. Cux0.3149. 4s. 0.3150 “ +0.0008 100.25 
Cam 0 . 6268 Ss aks 0.6240 “ —0.0019 99 .69 











(6) Method of Rivot-Rose. 

The copper is precipitated as sulphocyanide according to p. 
186, and from the filtrate the cadmium is precipitated as sulphide 
by means of hydrogen sulphide and determined as sulphate 
according to p. 190. The results are good. 


(c) Method of Fresenius and Haidlen. 
(The Potassium Cyanide Method.) 


The neutral solution containing salts of both metals is treaied 
with potassium cyanide until the precipitate that is first formed 


redissolves, after which more potassium cyanide is added (about 
three times as much as was necessary for the precipitation and so- 


lution of the precipitate) and either ammonium or hydrogen sul- 
phide is added to the cold solution. The cadmium is precipitated 
as the yellow sulphide, while the copper remains in solution. * 





* The copper, however, remains entirely in solution only when more than 
enough potassium cyanide is present than is required to form the complex salt 
K.,Cu(CN),. If the pure potassium cuprocyanide is dissolved in consider- 
able water and hydrogen sulphide passed into the solution, there is a partial 
precipitation of Cu,S; the more dilute the solution, the more the precipi- 
tation. By the addition of an excess of potassium cyanide, the precipitation 
is prevented. A cold, concentrated solution of the above salt is not precipi- 
tated by hydrogen sulphide (v. Girsewald, Zurich, 1902) 





SEPARATION OF COPPER FROM CADMIUM. 203 


The cadmium sulphide thus precipitated shows a great tendency 
of passing through the filter-paper even when a “‘hardened”’ filter 
is used, so that it is ‘‘salted out.”’ A considerable amount of pure, 
solid potassium chloride is stirred into the solution, the precipitate 
is allowed to stand overnight, and in the morning it is filtered 
through a Schleicher & Schill “hardened filter.” The precipi- 
tate is washed first by decantation with concentrated potassium 
chloride solution, it is then transferred to the filter and washed 
with the same solution. For the cadmium determination this 
precipitate cannot be used on account of the potassium chloride 
which adheres to it, and it is not advisable to wash the salt out 
with water, for in this case a turbid filtrate will be obtained. It is, 
therefore, dissolved in hot hydrochloric acid (1:3) from a wash- 
bottle, the solution is evaporated to dryness, the residue dissolved 
in water, filtered if necessary from separated sulphur, and for 
every 100 c.c. of the solution 5-7 ¢.c. of concentrated sulphuric 
acid are added, and the cadmium is precipitated by passing hydro- 
gen sulphide into the cold solution. This time the cadmium 
sulphide is easily filtered. The cadmium is determined as sulphate 
according to p. 190. 

The filtrate is evaporated with nitric acid until the odor of 
hydrocyanic acid can no longer be detected, and the copper is most 
conveniently determined according to p. 183 as cuprous sulphide. 

Remark.—The results obtained by this method are good, but 
considerable time and patience are required. 


(d) By Electrolysis. 


The experiments of R. Philipp in the author’s laboratory show 
that a very accurate separation can be made, as recommended by 
Neumann, by electrolyzing the nitric acid solution. 

The solution, containing not more than 0.2 gm. cadmium, is 
treated with 4-5 ¢.c. of concentrated nitric acid or 10 c¢.c. nitric 
acid sp.gr. 1.2, and diluted to 150 c.c. in a platinum dish. The 
anode, a disk electrode, is placed so that it only dips into the 
liquid a short way. Under these conditions, 0.2 gm. of copper 
is deposited perfectly free from cadmium, within 12 or 14 hours 


204 GRAVIMETRIC ANALYSIS. 


by a current of 0.2-0.3 ampere and a voltage of 1.9-2.3 volts. 
with a current of 1 to 1.5 amperes and 2.5-2.6 volts electrode 
potential, the cadmium is deposited in about five hours. The 
solution is siphoned off, while pure water is poured into the dish 
without breaking the current; the dish is finally rinsed with alcohol, 
dried and weighed with the deposited copper. The solution is 
treated with sufficient sulphuric acid, evaporated to expel the 
nitric acid, cooled, diluted and the cadmium. electrolyzed from 
cyanide solution as described on p. 189. 

Remark.—If{ considerably more than 0.2 gm. Cd is present in 
150 c.c. of the solution, there is danger of small amounts of 
cadmium separating out upon the copper during the washing of 
the deposit, especially when the anode extends well into the 
solution. This is because the concentration of the acid becomes 
less during the washing. In analyzing a solution containing a 
large amount of cadmium and small amount of copper, therefore, 
it is best to wash at first with 2 per cent. nitric acid rather than 
with distilled water. 

The separation requires but a few minutes with a rotating 
anode or cathode, and a stronger current. 


DETERMINATION OF ARSENIC. AS TRISULPHIDE, ETC. 205 


B. DIVISION OF THE SULPHO-ACIDS. 
Arsenic, Antimony, Tin. 


SELENIUM, TELLURIUM, GOLD, PLATINUM, TUNGSTEN, 
MOLYBDENUM, VANADIUM.) 


ARSENIC, As. At. Wt. 74.96. 
Forms: As,S,, As,S,, Mg,As,0,. 


1. Determination as Arsenic Trisulphide, As.S.. 


For the determination of arsenic in this form, it must be present 
in its trivalent state, i.e., as arsenious acid or as arsenite. 

The solution is made strongly acid with hydrochloric acid and 
the arsenic precipitated in the cold with hydrogen sulphide. The 
excess of the latter is removed by passing a stream of carbon 
dioxide through the solution, which is then filtered through a 
Gooch crucible that has been previously dried at 105° C. The 
precipitate is washed with hot water, dried at 105° C. to constant 
weight, and weighed as As,S,. 


2. Determination as Arsenic Pentasulphide, As,S,, according to 
Bunsen.* 


Modified by Fr. Neher.f 


The solution, which must contain all of the arsenic as arsenic 
acid, is treated with hydrochloric acid little by little (it is best to 
keep the solution cooled by surrounding the flask with ice) until 
the solution contains at least two parts of concentrated hydro- 
chloric acid for each part of water. A very rapid stream of hydro- 
gen sulphide is conducted into this solution (contained in a large 
Erlenmeyer flask) until it is saturated with the gas, after which 





* Ann. d. Chem. und Pharm., 192, 305. ; 

+ Z. anal. Chem., 32, 45; see also Brunner and Tomicek, Monatshefte, 
8, 607; McCay, Z. anal. Chem., 27, 682, and J. Thiele, Ann. d. Chem. u.+ 
Pharm., 265, 65. 


206 eo GRAVIMETRIC ANALYSIS. 


the flask is stoppered and allowed to stand two hours. The arsenic 
pentasulphide is then filtered through a Gooch crucible which has 
been dried at 105° C., and the precipitate is washed completely 
with water, then with hot alcohol (to hasten the subsequent dry- 
ing). After drying at 105° C. the precipitate is weighed as As,§,. 
It is not necessary to wash it with carbon bisulphide. 

Remark.—lf the above directions are conscientiously followed, 
this method gives faultless results. If, on the other hand, the 
directions are deviated from in the slightest respect, the precipitate 
is likely to contain some arsenic trisulphide, whereby low results 
will be obtained. If the solution is not kept cool and the hydro- 
chloric acid is added too rapidly, the heat of the reaction suffices 
to change a part of the arsenic chloride (this compound probably 
exists in solution) to arsenious chloride and chlorine, so that 
on passing hydrogen sulphide into the solution a mixture of arsenic 
trisulphide and arsenic pentasulphide will be obtained. 


3. Determination of Arsenic as Magnesium Pyroarsenate, 
according to Levol. 


The solution, which must contain all of the arsenic as arsenate, - 
and have a volume of not over 100 c.c. per 0.1 gm. arsenic, is 
treated drop by drop, under constant stirring, with 5 c.c. of 
concentrated hydrochloric acid and then, for each 0.1 gm. of 
arsenic, there is added 7-10 c.c. of magnesia mixture * and a 
drop of phenolphthalein solution. Now, with constant stirring, 
10 per cent. ammonia is added from a burette until the phenol- 
phthalein imparts a permanent red color to the solution, and then 
enough more of the 10 per cent. ammonia is added to make one- 
third the volume of the neutralized solution. After standing 
twelve hours the liquid is filtered through a Gooch or Monroe 
crucible. The precipitate in the beaker is transferred to the 
crucible by squirting upon it some of the original solution from 
a small wash bottle. The precipitate is then washed with 2.5 
per cent. ammonia until free from chloride. It is drained as 





_* Prepared by dissolving 55 gms. crystallized magnesium chloride and 
70 gms. ammonium chloride in 650 c.c. water and diluting this-to a volume 
of one liter with ammonia, sp.gr. 0.96. 





DETERMINATION OF ARSENIC AS MAGNESIUM PYROARSENATE, 2°07 


completely as possible by suction, dried at 100° and heated in 
an electric oven quite gradually to a temperature of about 400° 
to 500°, until there is no more ammonia evolved. Then the 
temperature is raised to 800° to 900° and kept there for about 
10 minutes. The crucible is then cooled in a desiccator and 
the precipitate weighed with the precipitate in the form of 
Mg,P.,0:. | 

If an electric oven is not available the crucible with the 
precipitate is placed in an air-bath (cf. Fig. 11, p. 27), having 
the bottom of the Gooch crucible fcome within about 2-3 mm. 
of the bottom of the outer crucible. A thin layer of ammonium 
nitrate powder * is added to the precipitate, which is then 
heated, at first gently, gradually increasing the temperature 
until a light-red- glow on the outer crucible is obtained, after 
which the precipitate is allowed to cool in a desiccator and 
is weighed as Mg.As,O;. The results obtained are excellent. 

Remark.—The precipitate produced by the magnesia mixture 
has the formula MgNH,AsO,+6H,0 and loses 53 molecules of water 
at 102° C.; it has, therefore, been proposed to dry the precipitate 
at this temperature and to compute the amount of arsenic present 
as follows: 7 


[MgNH,AsO,+4H,0]: As=p:2. 


It is, however, impossible to obtain a constant weight at this 
temperature, so that the procedure is not to be recommended. 
If the precipitate is dried at 105-110° C. the salt is obtained almost 
entirely free from water and at a slightly higher temperature it 
begins to decompose. The only form in which the precipitate 
should be weighed is as magnesium pyroarsenate. 


— 





* Instead of using ammonium nitrate, the crucible may be provided with 
#; perforated cover and heated in a current of oxygen. 


wo 


208 GRAVIMETRIC ANALYSIS. 


Solubility of Magnesium Ammonium Arsenate, according to 
Levol. 


600 parts of water dissolve 1 part of the salt. 

In 23 per cent. ammonia it is almost entirely insoluble. Accord- 
ing to J. F. Virgili,* 1 part of anhydrous magnesium ammonium > 
arsenate dissolves in 24,558 parts of ammonia water. 


Colorimetric Deterfhination of Arsenic. 


Small quantities of arsenic, such as are present in wall papers, 
may be estimated very accurately by means of the Marsh apparatus, 
comparing the mirror with a series of standards formed with 
known quantities of arsenic.t It is just as accurate, however, 
to use the much simpler apparatus used for the Gutzeit test. 
Treadwell and Comment{ allow the arseniuretted hydrogen 
to react with disks containing silver nitrate and compare the 
resulting color with a standard which, unfortunately, must be 
produced freshly with each analysis, as it does not keep very 
well. Almost equally accurate, and much more convenient 
is the method of F. Hefti§ and that of C. R. Sanger and O. F. 
Black || in which the arseniuretted hydrogen is allowed to act upon 
mercuric chloride paper. | 


(a) Method of Hefti. 


In the first place, all the organic matter is destroyed by heat- 
ing the sample in a tube with fuming sulphuric and nitric acids 
(see Vol. I), both of which must be free from arsenic. The 
resulting liquid is evaporated with sulphurous acid on the water- 
bath in order to reduce the arsenic acid to arsenious acid and 
when all the excess of SO, has been expelled, the solution is 
poured into the graduated tube 7 of the apparatus shown in 
Fig. 39. In the 100-150 c.c. flask K are placed 6-8 gm. of 





* Z. anal. Chem., 44, 504 (1905). 

+ C. R. Sanger, iin, Chem. J., 18, 481 (1891); Z. anal. Chem., 38, 187 
and 377; G. Lockemann, Z. angew. Chem., 1905, 429 and 491. 

+ This method was given in the former editiuas of this book. 

§ Inaug. Dissert. Ziirich, 1907. 

|| Proce. Amer. Acad. Arts and Sciences, No. 8, 1907. 





COLORIMETRIC DETERMINATION OF ARSENIC. 209 


granulated zinc coated with copper*-and about 20 c.c. of sul- 
phuric acid free from arsenic (1 vol. conc. acid+7 vols. water). 
At the end of ten minutes all the air should be expelled from the 
apparatus. The outlet D+ is now covered with a piece of mercuric 
chloride paper and kept in place by a small piece of ground glass. 
According to whether little or much arsenic is present, al or a 


=P 


xo qe nnn 1 








_ part of the solution in T is allowed to flow into the flask K. At 
the end of twenty minutes the experiment is finished. By com- 
paring the color of the spot produced on the mercuric chloride 





* Cf. Vol. I. 
| For quantities of arsenic under 0.02 mg., the upper diameter of the 


tube D should be 8 mm. and for larger quantities it should be 16 mm. The 
upper edge of the tube is ground perfectly flat. 


210 GRA*’ METRIC ANALYSIS. 


paper, with the standard spots, the quantity of arsenic present 
is determined. 

The disks of mercuric chloride paper are prepared by dipping 
pieces of pure filter paper into a saturated solution of mercuric 
chloride and drying them in an oven at a temperature of 60°—70°. 

The standards are prepared by carrying out a series of expe:- 
iments with known quantities of arsenic. The spots thus obtained 
soon lose their color when expgsed to moist air, but when dry 
can be kept in the dark for several days. An older standard 
is not reliable, but can be used to estimate the. approximate 
quantity of arsenic and then, by making two or three 
standards with known quantities, the exact amount of arsenic 
can be determined. The standard solution of arsenious acid 
used in preparing the scale should contain 20 mg. of As,O, in a 
liter. Then , 

0.05 em.=0.001 mg. As,O,, 
0.1 c.c. =0.002 etc. 


For smaller amounts of arsenic the above solution is diluted with 
nine times as much water and thus made one-tenth as strong. — 


(6) Method of C. R. Sanger. 


Three grams of uniformly granulated, pure zinc is placed in the 
30 c.c. evolution flask (Fig. 40) -which is fitted with a stopper 
holding a thistle tube and a gas delivery tube. The enlargement 
of the horizontal tube contains a wad of cotton and at the outer 
end is a piece of thick filter-paper which has been dipped into 
mercuric chloride solution and dried. Through the thistle tube 
is poured 15 c.c. of arsenic-free, dilute hydrochloric acid* (1:6), 
and the hydrogen evolution is allowed to proceed for at least ten 
minutes to drive out the air from the apparatus. At the end 
of this time, a measured, or weighed, quantity of the arsenic 
solution to be tested is added to the flask, which is then nearly 
filled with water. After a few minutes, the mercuric chloride 
paper begins to color and at the end of thirty minutes will attain 





* When hydrochloric acid is used it is unnecessary to plate the zine with 
copper. 


ie Sil 
Ears, 
Bbin* 


COLORIMETRIC DETERMINATION OF ARSENIC. 2Ir 


the maximum coloration. By comparison with standards, the 
quantity of arsenic present is estimated. | 

Inasmuch as the color of the standards is strongly tufinenebt 
by moisture, Sanger recommends preserving the strips in a per- 
fectly dry condition. For this purpose a little phosphorus pent- 
oxide is placed in a small test-tube, followed by a little cotton, 
and then the strip of paper-is shoved in with the colored part at 











Of La, 
ee 
Ste , 


Fie. 40. 


the bottom. The upper end of the paper is moistened with a 
drop of Canada balsam, after which the tube is closed and sealed. 
In this way the color can be preserved for several months, although 
the freshness disappears after a few weeks. See the colored 
chart at the end of the book, upper row. 

The color holds a little better if the strips of paper, after being 
colored, are moistened with strong hydrochloric acid and then 
dried. Some 6N-HCI is placed in a small test tube, heated to 
at least 60°, the strips of paper dipped in the acid and allowed 
to remain there two minutes, washed thoroughly in running water, 
dried and sealed in tubes as described above. After the 
drying, the color is alittle duller. See the colored chart, middle 
row. 


212 GRAVIMETRIC ANALYSIS. 


If, however, the strips of colored paper are treated with normal 
ammonja solution, the spot which was originally red turns black. 
After drying, such strips are kept in small test-tubes over calcium 
chloride. These standards are much more permanent than when 
prepared as above. See the colored chart, bottom row. 

Remark.—To make the strips tniform as regards the length 
of the spot and the color, the following conditions must be 
fulfilled : 

1. The evolution flask must be kept the same size and the 
delivery tubing must be of uniform bore. 

2. The same quantity of zinc of the same size must be used in 
all the tests. 

3. The volume and concentration of the acid must remain 
the same. 

4. The wad of cotton must not get too a After 10 or 
12 experiments it should be renewed. 

5. H,S, SbH, and PH, must not be present, as they give a 
colored spot with the HgCl, paper. 


(c) Electrolytic Determination of Arsenic. * 


Instead of producing the arseniuretted hydrogen by means of 

zinc and acid, it may be formed with the aid of cathodic hydrogen. 
Thorpe passes the arsine through a heated tube and produces an 
arsenic mirror, but Hefti t allows the gas to react with mercuric 
chloride paper. In both cases the apparatus devised by Thorpe 
is used and is shown in Fig. 41. 
_~ As cathode a perforated cone of thin lead foil is used. This 
is suspended from the platinum wire that has been fused into the 
ground-glass stopper of the cathode compartment. The anode 
consists of platinum foil, two or three centimeters wide, which 
is 8 wrapped around the porous cell. 





* Cf. Bloxam., Z. anal. Chem., 1, 483 (1862); T. E. Thorpe, Proc. Chem. 
Soc., 19, 183 (1903); W. Thomson, Manch. Memoirs, 48, No. 17 (1904); 
Ss. R. Tootmann, Chem. Zentr., 1904, I, 1295; H. J. S. Sand and E. Hackford, 
Chem: Zentr., 1904, II, 259. 

¢ Inaug. Dihiiert.; Zurich, 1907. 


ELECTROLYTIC DETERMINATION OF ARSENIC. 213 


Procedure.—Pure, dilute sulphuric acid (1:7) is poured into 
the earthenware cell and into the glass outer vessel, EZ; the level 
of the acid should be about 2 or 3 cm. from the bottom in the 
former, and about 0.5 cm. higher in the latter. For the colori- 
metric determination, the arsenic solution is poured directly 
into the acid of the inner cell. It must be present as arsenious 
acid, and, if this is not the case, it must be reduced with sulphurous 
acid and the excess of the latter expelled by heating. For the 


ELITE 
if 





| 
° rus 
yf . 
| 
NG NA [WS U 
UM aay 
| UMITMMTTINGE: AITIESESULLEE LENNY, im 
i : Sz 


Fia. 41. 


production of mirrors, the air must all be expelled by hydrogen 
before the arsenic solution is added. The tube C is filled with 
crystallized calcium chloride. The outlet at D is covered with a 
disk of mercuric chloride paper (see Method a) and then the 
circuit is closed. The potential should be about 7 volts and the 
current about 2 to 3 amperes. The analysis is finished at the 
end of twenty minutes and the quantity of arsenic estimated by 
comparing the spot with a standard scale. (See Method a.) 
If the apparatus is connected with a horizontal delivery tube, 
Sanger’s method can be used. (See Method 0.) 

Remark.—As regards the influence of the cathode material, 
Thorpe recommends bright platinum foil and’ Hefti uses lead. 


214 GRAVIMETRIC ANALYSIS. 


Polished platinum does not hold arsenic back, but platinum with a 
rough surface does, and since bright platinum becomes dull with 
use, it is easily possible for low results to be obtained. Exper- 
iments performed by Hefti in the author’s laboratory showed 
that zinc alloyed with a trace of copper or platinum, bright 
platinum foil and lead did not hold*back arsenic when used as the 
cathode; on the other hand, zine in the presence of chloroplatinic 
acid and platinum foil with spongy platinum held back a con- 
siderable quantity of arsenic. 

To determine arsenic in a mineral water, 100 c.c., or more if 
necessary, are evaporated to a small volume in a procelain dish. 
The resulting solution is acidified with sulphuric acid, reduced 
with sulphurous acid, the excess of the latter expelled, and the 
analysis continued by one of the above three methods. 


Determination of Larger Quantities of Arsenic as Arsine. 
Method of F. Hefti. 


In the electrolysis of larger quantities of arsenic it was not 
possible, in the past, to recover all the arsenic in the form of 
arseniuretted hydrogen; some arsenic was deposited upon the 
cathode in the form of the element arsenic and was not trans- 
formed into arsine by the further action of the electric current. 
The quantity of arsenic deposited as metal depends upon the 
potential of the electric current at the electrodes,the temperature, 
and the concentration of the arsenic solution. At high potentials, 
low temperature, and low concentration of the solution, the 
quantity of arsenic deposited becomes zero and the yield of 
arseniuretted hydrogen is then quantitative. The estimation of 
the latter is best accomplished iodimetrically. If arseniuretted 
hydrogen is passed through a solution of iodine in potassium 
iodide, it is immediately oxidized to arsenic acid in the cold. 


AsH,-++4H,0+41,=8HI+ H,As0,. 


If the excess of iodine is titrated with sodium thiosulphate 
(see Iodimetric Methods) it is possible to determine the quantity 
of iodine that has reacted with the arsine. If 7 ¢.c. of 0.1N 
iodine were used at the start, and ¢ c.c. of 0.1N thiosulphate 


DETERMINATION OF ARSENIC AS ARSINE. 215 


solution were used for the titration, then the quantity of arsenic 
or arsenic trioxide, present is 


(T —t) X0.000937 gm. arsenic, 
(T —t) X 0.001237 gm. As,O,. 


The apparatus necessary is shown in Fig. 42. The decom- 
position cell is also shown in Fig. 43 and consists of a wide 
U-tube capable of holding 120 c.c. of solution. The tube is 


Ly 


> 


Z) 




















Fia. 42. 


made in two halves, the edges of the bottom being ground so 
that they fit tightly together. Between these edges is placed 
a piece of thin parchment paper, the extending edges of which 
are folded over one side of the tube. A piece of rubber 
tubing holds the two halves of the U-tube together and also 
the parchment paper in place. This tubing is wired tightly in 
place, taking care that the edges of the parchment paper are 
also covered by the wire. In one arm of the tube (the anode 
compartment) which remains open during the whole experi- 
ment, is suspended a platinum plate electrode as anode, and 
the other arm (the cathode compartment) is tightly stoppered 
with a three-holed rubber stopper. Through one hole passes 
a glass tube containing mercury; at the bottom of this tube 


216 GRAVIMETRIC ANALYSIS. 


a platinum wire is sealed in and from this a plate electrode of 
lead foil is suspended to serve as cathode. The wire from the 
negative pole of the battery dips into the mercury. Through 
the second hole in the stopper is passed a gas delivery tube 
leading to the absorption vessel A. The third hole in the stopper 
carries a tube that leads to the Erlenmeyer flask E which, in 
turn, is connected with the empty flask F, and the latter with 
the rubber tubing shown in Fig. 42. This tubing leads to the 
hood. Such an arrangement provides for the regulation of the 
















: 


HAAEULIAUOUGUGAEUUOAEAGAAAEUEAAA UAHA 











MTT 
HNNEOSLFUOUUONGNONOUEOUAOAUAHOOEUUUEUUUOAEENUEL 

















Gasssttddirabsslts, 

























































































Fig, 43. 


pressure in the cathode space. If the pressure there exceeds 
that of the anode space, a part of the arsenic solution will pass 
into the anode compartment and will be lost in the analysis. 
If suction is applied at the extreme end of the absorption 
apparatus, so that bubble after bubble of air passes through 
the Erlenmeyer, then it is very easy to overcome the pressure in the 
absorption vessel without having diminished the pressure in the 
cathode compartment enough to tear the parchment membrane. 
Procedure.—The arsenic solution to be tested must contain 
all the arsenic in the trivalent condition. © 
In the first place, the anode compartment is filled to within 
3 cm. of the top with 10 per cent. sulphuric acid, the arsenic 
solution is placed in the cathode compartment and this is filled 
to within 3.5 cm. of the top (in other words, the level in the 


DETERMINATION OF ARSENiC AS ARSINE. 207 


cathode compartment is about 0.5 cm. lower than on the other 
side of the U-tube); the concentration of the arsenic solution 
in the U-tube, after this dilution with acid, should not ex- 
ceed 80 mgm. As,O, in 50 c.c. of solution. The U-tube is 
placed in ice-water and the gas delivery tube is connected with 
two ten-bulb absorption tubes, of which only one is shown in 
the drawing. Into the first absorption tube is now placed an 
accurately measured volume of tenth-normal iodine solution, 
and into the second tube, which is not shown in the drawing. 
10 c.c. of sodium shiewulphate solution, and about 40 c.c. of 
water. The purpose of the sodium {hiowulphnte solution is tc 
catch any iodine that may escape from the first absorption 
tube. While the apparatus absorption vessels are being filled, 
the arsenic solution should be in the ice-water, and its tem- 
perature should be about 0° when the analysis is ready to 
begin. Gentle suction is started at the end of the second 
absorption tube, the electric circuit is closed* and the suction 
is regulated so that bubble after bubble of air slowly streams 
through the pressure regulator and into the cathode compart- 
ment throughout the whole duration of the electrolysis. More- 
over, care is taken that enough ice remains in the cooling bath. 
When all the conditions are maintained satisfactorily, the liquid - 
in the cell should remain perfectly clear, or at the worst be 
colored only by a slight brownish turbidity, which eventually 
disappears. If a black turbidity is formed that settles to the 
bottom of the U-tube, something has gone wrong and it is 
useless to continue the experiment. In a normal experiment, 
the evolution of the arsine is finished in an hour, when not 
more than 50 mgm. of: As,O, are present. The current is then 
stopped, the contents of the two absorption tubes (first the 
iodine and then the thiosulphate solution) are poured into a 
beaker containing 5 ¢.c. of a saturated solution of pure NaHCO, 
and the excess of iodine is titrated with 0.1N sodium thiosul- 
phate solution using starch solution as indicator. If on mixing 
the contents of the two absorption bulbs the solution is decolorized, 
the titration is finished with 0.1N iodine. 





* A current of 2 to 3 amperes and.7 volts is used. 


218 GRAVIMETRIC ANALYSIS. 


This method can be carried out very easily and gives accurate 
. results in the presence of iron, so that it is suitable for a rapid 
determination of the arsenic present in iron minerals. 


Determination of Arsenic in Mispickel. 


One gram of the finely powdered mineral is fused in a nickel 
crucible with 6 gm. of sodium cagbonate and 1 gm. of potas- 
sium nitrate. The resulting melt is extracted with hot water 
and the residue (Fe,0,, NiO) washed with hot sodium carbonate 
solution. To the filtered solution 200 c.c. of water saturated 
with SO, are added to reduce the arsenic, the solution is boiled 
to expel the excess of SO,, allowed to cool, diluted to 500 e.c. 
with sulphuric acid so that the entire solution contains 10 to 
12 per cent. of H,SO,, and the arsenic is then determined as 
outlined above, using one-tenth of the solution. 

Instead of extracting the melt with water, it may be treated 
with dilute sulphuric acid, whereby all the iron goes into solution. 
After this solution has been reduced with sulphurous acid, the 
analysis of an aliquot part gives the same result as when the 
first procedure is followed. 

Hefti found 42.67 per cent. As by the former process and 
42.73 per cent. by the latter. The mineral analyzed was sup~ 
posed to contain 42.72 per cent. arsenic. 


ANTIMONY, Sb. At. Wt. 120.2. 
Forms: Sb2S3, Sb2O4, and Sb. 


1. Determination as Trisulphide, Sb2S3. 
Method of F. Henz. * 


The best method for the determination of antimony is, in the - 
author’s opinion, the following: 

Hydrogen sulphide is passed for twenty minutes into the cold 
solution of an antimonite or antimonate, then, without stopping 
the current of hydrogen sulphide, the solution is slowly heated to 





* F. Henz, Z. anorg. Chem., 37, 18 (1903). 


DETERMINATION OF ANTIMONY AS TRISULPHIDE. 219 


boiling and the gas passed through it for fifteen minutes more, after 
which the now dense precipitate is allowed to settle and filtered 
through a Gooch crucible which has been heated at 280-300° and 
weighed. The precipitate is washed four or five times by decanta- 
tion with 50-75 c.c. of hot, very dilute acetic acid into which 
hydrogen sulphide has been passed, and washed on the filter with 
the same wash liquid until all chloride is removed. At first the 
filtrate runs through perfectly clear, but after all the mineral acid 
has been removed, the filtrate shows a slightly orange tint, owing 
to an unweighable amount of the antimony sulphide passing 
through in colloidal solution. As soon as this point is reached 
the washing is stopped. 

F,. Henz then proceeds as follows: 

The crucible, after the precipitate has been dried as much as 
possible by suction, is placed in the tube R, Fig. 44, which is 
fitted to a drying oven (about 18 em. long and 10 em. high; 
covered with asbestos paper). The tube R is then closed with a 
rubber stopper that holds a glass delivery tube, and RF is pushed 
into the drying closet until the end of the stopper is reached. To 
protect the rubber stopper during the subsequent heating, its 
inner surface is provided with a Rose crucible cover, which is 
held in place by wrapping the tube a with a strip of asbestos 
paper. 

The air is now expelled from the tube by a stream of dry, 
air-free carbon dioxide * and heated for two hours at 100°-130°. 





7 In order to obtain accurate results it is neceesary to have the carbon 
dioxide perfectly free from air. This may be prepared by the use of the 
Kipp generator as modified by Henz (Chem. Ztg., 1902, 386) see Fig. 45. 

This differs from the ordinary form of the Kipp apparatus only as regards 
the siphon tube a; but herein lies a distinct advantage. The apparatus is 
charged as follows: First of all, pieces of pure marble are placed in 
the middle compartment, the stop-cock is opened, and water is poured 
through the upper compartment, until it begins to run out through the 
stop-cock, which is then closed. By this means all the air has been expelled 
from the lower parts of the apparatus and it only remains to introduce the 
hydrochloric acid. To accomplish this, the water is allowed to run out 
through the siphon while hydrochloric acid (1:4) is poured in at the top 
of the generator. As soon as carbon dioxide begins to be evolved, the tube a 
is closed and the apparatus is ready for use. When the acid has become 


220 GRAVIMETRIC ANALYSIS. 


Inasmuch as the tube R extends so far into the drying oven, there 
is no danger of water condensing in the tube, but it is all expelled 
as vapor at b. 

The precipitate is now dry and the air completely expelled 
from the heating tube. 

The tube R is now withdrawn a little from the oven, about 


















































Fia. 44, Fig. 45. 


5 cm., as shown in the drawing, and the temperature is raised to 
280-300° and kept there for two hours. 

Hereby some sulphur is volatilized and collects in the tube R 
outside the oven. The antimony pentasulphide is also com- 
pletely changed into the black modification of the trisulphide by 
this heating.* The crucible is allowed to cool in the stream of 





too weak, it is removed through the siphon while a fresh supply is poured 
in at the top; there is no need of taking the apparatus apart during this 
operation. It is obvious that the same apparatus can be employed to | 
advantage for generating hydrogen or hydrogen sulphide. 

* According to Paul (Z. anal. Chem., 31, 540 (1892)), the transformation 
of antimony pentasulphide can be accomplished in his drying oven (shown 


DETERMINATION -OF ANTIMONY AS TRISULPHIDE. 221 


carbon dioxide, transferred to the balance case,* and after standing 
half an hour is weighed. The black antimony trisulphide is not 
at all hygroscopic. <A further heating in the current of carbon 
dioxide will rarely show any change in weight. 


(b) Method of Vortmann and Metzel. 


When antimony is precipitated by hydrogen sulphide from a 
hot solution which is strongly acid with hydrochloric acid, the 
sulphide eventually becomes grayish black in color, is crystalline, 
and can be filtered easily and washed with water without the 
slightest tendency to pass into the hydrosol condition. 

The solution, in an Erlenmeyer flask, is treated with concen- 
trated hydrochloric acid, adding 24 ¢.c. of the concentrated acid to 
each 100 c.c. of the neutral solution. It is heated to boiling, and 
the hot solution subjected to the action of hydrogen sulphide gas. 
The Erlenmeyer flask containing the solution is placed in a dish of 
boiling water and the water in the latter is kept boiling during the 
precipitation. It is advisable to introduce the hydrogen sulphide 
gas quite rapidly at first, but towards the end a slow stream is 
sufficient. The antimony sulphide as it comes down is yellow at 
first, but as the precipitation proceeds, it becomes redder; grad- 
ually it becomes heavier and denser, assumes a crystalline form 
and becomes darker, and finally black in color. The transforma- 
tion into the crystalline form is hastened by shaking the flask: 
At first, while the precipitate is of a yellowish color, there is no 
need of shaking the flask but later on it is very desirable to do so. 
The shaking, however, should not be too violent, as otherwise 
some of the precipitate is likely to adhere to the upper portions of 
the flask and escape the transformation. The duration of the — 





in Fig. 20 of this book) by heating to a temperature of 230°. This is per- 
fectly true, but the transformation takes place more readily at a temperature 
of 280°. It is more difficult to replace the air completely with carbon dioxide 
in Paul’s drying oven and often some white antimony oxide is noticeable in 
the crucible. 

* A piece of writing-paper should be rolled up and placed in the tube R, 
so that the crucible does not come in contact with any of the sulphur sub- 
limate, on withd-awing it. The crucible is removed with the paper. 

+ Z. anal. Chem., 44, 526 (1905). 


222 GRAVIMETRIC ANALYSIS. 


whole process amounts to from 30 to 35 minutes. Finally a 
heavy, dense, crystalline precipitate of antimony trisulphide 
is obtained which settles well and permits a rapid filtration. The 
solution is diluted with an equal volume of water, which is allowed 
to flow around the walls of the flask in order to wash down any 
adhering sulphide. The dilution almost always causes the forma- 
tion of a slight yellow turbidity. The reason for this is that a 
little of the antimony is held in solution by the strong acid and 
as the solution is diluted this is caused to precipitate by the 
dissolved hydrogen sulphide. The flask is, therefore, once more 
shaken, placed in the vessel of boiling water and more hydrogen 
sulphide is introduced. In two or three minutes the solution 
above the precipitate will become clear. It is filtered through 
a Gooch crucible, washed with water to remove the acid, then 
with alcohol, and placed in the drying oven. 


2. Determination as Tetroxide, Sb20, (Bunsen). 


This method is based upon the fact that antimony pentoxide, 
when ignited at a definite temperature, changes into SbeOx. 
Bunsen,* who first proposed the method, later abandoned it 
because his assistant succeeded in volatilizing more than 0.1 gm. 
of the precipitate by heating it over the blast lamp.t Brunck,t 
Rossing § and Henz || have shown, however, that under certain 
conditions accurate results can be obtained, although they did 
not specify the exact temperature at which the precipitate 
should be ignited. If the pentoxide is ignited in a large porcelain 
crucible over the blast lamp, it is possible to change the antimony 
pentoxide quantitatively into the tetroxide; if, however, a small, 
thin-walled, procelain crucible is used, the tetroxide loses oxygen 
and is transformed into the volatile trioxide, whereby low results 
are obtained. It is, therefore, purely accidental if exact results 
are obtained by such a procedure. In 1897, Baubigny4 dis- ~ 

* Ann. Chem. u. Pharm., 106, 3 (1858). 
t lbid., 192, 316 (1878). 

{ Z. anal. Chem., 34, 171 (1895). 

§ Ibid., 41, 9 (1902). 

|| Loe. cit. 

{] Compt. rend., 124, 499 (1897) 





DETERMINATION OF ANTIMONY AS TETROXIDE. 223 


covered that antimony pentoxide is converted quantitatively 
into the tetroxide at a temperature of 750°-800° and begins to 
form the volatile trioxide at a little above 950°. The author’s 
assistant, Dr. E. G. Beckett,* has confirmed the work of Bau- 
bigny. At 750°-800° the transformation is complete and at 1000° 
it is possible to volatilize 0.35 gm. of the precipitate in about 
thirty minutes. If these facts are borne in mind, it is possible 
to get accurate results, although even then the method is less 
satisfactory than the determination as trisulphide. 
Procedure.—In the majority of cases it is desired to determine 
the amount of antimony present in a mixture of its tri- and penta- 
sulphides, or in a mixture of one or the other of the two compounds 
with sulphur. It is best to proceed as follows: The sulphide 
of antimony, precipitated from hot solution, is washed first with 
hot water, then with alcohol, afterwards with a mixture of 
alcohol and carbon disulphide (in order to remove the sulphur) ,f 
again with alcohol and finally with ether, afterwards drying the 
precipitate. The bulk of the precipitate is separated from the 
filter and placed upon a watch-glass and the filter is placed in a 
small porcelain dish and boiled with a little of a freshly prepared 
solution of ammonium sulphide, stirring constantly with a glass 
rod. The resulting solution is poured through a small filter into 
a 30 c.c. porcelain crucible, and the filter is treated repeatedly - 
with ammonium sulphide until it is no longer colored brownish 
red at the edge of the paper, where it begins to dry; the extrac- 
tion of the antimony sulphide is then complete. The solution 
in the crucible is evaporated to dryness and the main part of 
the precipitate is added. To oxidize the antimony sulphide, 
Beckett places the crucible, with a dish of fuming nitric acid 
beside it, under a bell-jar and allows it to stand over night. The 
vapors of fuming acid slowly oxidize the precipitate in the crucible 
and in the morning it is possible to complete the oxidation by 
means of nitric acid (sp.gr. 1.4) without the reaction being too 
violent. The crucible is then heated on the water-bath until 
the precipitate becomes white and the greater part of the acid is 





* Inaug. Dissert. Zurich, 1909. 
+ Thiele, Ann. d. Chem. und Pharm., 268, 372. 


224 GRAVIMETRIC ANALYSIS. 


expelled. A little water is added and, with stirring, enough 
concentrated ammonia to give an alkaline reaction. The con- 
tents of the crucible are now evaporated to dryness on the water- 
bath, carefully heated in an air-bath (Fig. 11, p. 27) until no 
more fumes of sulphuric acid are evolved, and then for half an 
hour at 800° in an electric oven. After cooling in a desiccator, 
the crucible is transferred quickly to a glass-stoppered weighing 
‘beaker, allowed to stand twenty Minutes in the balance case, 
and then weighed.* The ignition and weighing are repeated 
until a constant weight is obtained. 


3. Determination of Antimony as Metal. 


Antimony may be deposited from acid solutions by means of 
the electric current; the metal, however, does not adhere well to 
the electrode, so that this method cannot be used for its quantita- 
tive determination. On the other hand, the following method 
is suitable; it was first proposed by Parrodi and Mascazzini,t+ 
then modified by Luckow,{ and afterwards improved by Classen 
and Reiss.§ According to the experience in the author’s laboratory, 
it is not so accurate as the trisulphide method. 

Tf a solution of sodium or ammonium sulphoantimonite, or 
antimonate, containing not more than 0.3 gm. Sb in a volume of 
about 140 c.c. is subjected to electrolysis with a current of 1-1.5 
amperes at 70° for 90 minutes, the antimony will be deposited 
upon a platinum dish, which has been gently sand-blasted, as steel- 
gray, metallic antimony, and the deposit adheres so firmly that 
it can be dried and weighed without loss. The chief condition for 
the success of this operation is the absence of polysulphides. In 
case these substances are present the antimony is incompletely 
deposited and in some cases not at all, or the deposited antimony 
may pass into solution, on account of being oxidized to sodium 





* Sb,O, is hygroscopic and must be weighed in a weighing beaker, as 
Finkener, Dexter, and Beckett have all found. 

+ Z. anal. Chem., 18, 587 (1879). 

t Ibid., 19, 13 (1880). 

8 Berichte, 14, 1629 (1881); 17, 2474 (1884); 18, 408 (1885); 27, 2074 


(1894). 


DETERMINATION OF ANTIMONY AS METAL, _ . 225. 


antimonite by means of the sodium polysulphide which is formed 
at the anode during the electrolysis: 


| 28b +3NacSo= 2NagSbSs. 


It is necessary, therefore, to prevent the formation of poly- 
sulphides during the electrolysis. For this reason Lecrenier * 
added sodium sulphide to the bath, whereby the polysulphide is 
transformed into thiosulphate: 


NaS + Na2SOz = Na2Se03 + NaoS. 


Ost and Klapproth f carry out the electrolysis with the aid of | 
a diaphragm to keep the polysulphide away from the cathode. 
It is better, however, to make use of potassium cyanide for this 


purpose. { 
NagSe + KCN = Nags + KOCNS. 


Procedure.—In most cases the antimony is first isolated as 
the sulphide, which is either precipitated by hydrogen sulphide 
from acid solution or obtained by acidifying an alkaline solution 
of the thio-salt. The filtered and washed precipitate, cor- 
responding to not over 0.2 gm. Sb, is dissolved on the filter by 
“pouring pure sodium sulphide solution (sp.gr. 1.14) over it.§ 





* A. Lecrenier, Chem. Ztg., 18, 1219 (1889). 

+ Z. angew. Chem., 1900, 828. 

{ Cf. A. Fischer, Ber., 36, 2048 (1903); Z. anorg. Chem., 42, 363 (1904); 
Hollard, Bull. Soc. Chem., 28 [3] 292 (1900); F. Henz, Z. anorg. Chem., 37, 
31 (1903). ; 

§ A. Inhelder prepares the solution of sodium sulphide as follows: 666 gms. 
of purest sodium hydroxide (prepared from sodium) are dissolved in 2 liters 
of water and the solution divided into halves. One-half is placed in a long- 
necked flask of such a size that the solution just reaches the neck of the flask. 
~The flask is closed with a two-holed rubber stopper and a rapid current of 
well-washed hydrogen sulphide is introduced through a glass tube 1 cm. 
wide, keeping out the air as much as possible. When the solution ceases 
‘to increase in volume (1000 c.c. of NaOH solution should give 1218 c.c. of 

sodium NaSH solution). When this is accomplished, the other half of the 
original sodium hydroxide solution is added. The solution of Na,S thus 
prepared is colored a pale yellow, and after standing some time, or sooner 


226 GRAVIMETRIC ANALYSIS. 


The solution is caught in a weighed platinum dish with unpolished 
nner surface, or in a beaker if a platinum gauze electrode is to be 
used. After washing the filter with the sodium sulphide solu- 
tion, the total volume of the liquid in the platinum dish should 
not be over 80 c.c.; if less than this, enough more sodium sul- 
phide solution is added. The solution is diluted with 60 c.c. of 
water and 2-3 gms. of the purest potassium cyanide are added 
and the liquid is stirred with the anéde until all the cyanide has 
dissolved and the solution is well mixed. It is heated to 60°-70° 
and electrolyzed with a current of 1-1.5 amperes and electrode 
potential of 2-3 volts. After 1.5 to 2 hours all the antimony 
will be upon the cathode in the form of a firmly-adhering, light- 
gray deposit.* Now, without breaking the circuit, the electrolyte -~ 
is siphoned off, while water is added until the current ceases to 
pass through the liquid (the voltmeter connected as ammeter 
points to the zero reading). The cathode is removed, washed 
thoroughly with water, then with absolute alcohol, dried at about 
80°, cooled in a desiccator, and weighed. 

The results obtained by this method are invariably too high, 
as F. Henz ¢ showed in the author’s laboratory, the error amount- 





on shaking, tetragonal crystals of Na,S+9H,O are deposited. The solution 
keeps indefinitely in a well-stoppered flask; a slight precipitate of black 
metal sulphide (FeS, NiS, Ag,S) is formed after some time. 

If the sodium sulphide solution is prepared from caustic soda purified by 
alcohol, the final solution is colosed a deep yellow or: brown and it contains 
more foreign sulphides, partly in suspension and partly in colloidal solution. 
If the suspended sulphides are removed by filtration, a further precipitate 
will appear on standing. 

For the antimony determination, the saturated sodium sulphide solution, 
which has a specific gravity of 1.22, is diluted until the specific gravity is 1.14. 
According to A. Classen, a satisfactory solution can be prepared by dissolving 
the purest grade of commercial Na,S. Classen formerly recommended that 
the solution be prepared by boiling a solution of sodium hydroxide which 
had been saturated with hydrogen sulphide. Such a solution after an hour’s 
boiling, while introducing a stream of hydrogen, contained 69 per cent. 
NaSH and 31 per cent. NaS (F. Wegelin). . 

* To make sure that the deposition is complete, the liquid may be trans- 
ferred quickly to a second dish and electrolyzed for half an hour longer, 
It is seldom that there will be any gain in weight shown by this dish. 

t Z. anorg. Chem., 37, 31 (1903). 


DETERMINATION OF ANTIMONY AS METAL. 227 


ing to about 1.5 to 2 per cent. of the total antimony present. If, 
~ however, the antimony deposit is dissolved and precipitated as 
the trisulphide, the weight of the latter corresponds to the 
theoretical value, showing that the deposit contained all the anti- 
mony. If, as A. Fischer * recommends, the deposit is dissolved 
ia alkali polysulphide, and again electrolyzed with addition of 
potassium cyanide, the same weight of antimony is obtained as 
at first, but the antimony is not pure. 

The error in the analysis is so constant that values not far 
- from the truth will be obtained by subtracting 1.6 per cent. of 
the weight of antimony deposited electrolytically. 

This error of the electrolytic antimony determination was 
first detected by Henz, but has been confirmed by a number of 
other investigators, including O. M. M. Dormaar,t F. Forster 
and O. Wolff,t and recently by A. Inhelder.§ 

According to Dormaar, Forster and Wolff, the high values are 
due to the presence of a little sulphur and more oxygen. Férster 
and Wolff assert that the metal contains from 1 to 1.5 per cent. 
of oxygen. The error is greater in proportion to the quantity 
of free sodium hydroxide present in the electrolyte. According 
to the work of Scheen,|| which was suggested by Classen, the 
error is due to enclosed mother-liquor and is greater in propor- 
tion as the electrode surface is rough. Scheen, therefore, rec- 
ommends a bright electrode surface, or one that is dulled but 
slightly. 

A. Inhelder | has carefully repeated the experiments of 
Scheen, using new, polished dishes, but has not been able to 
confirm Scheen’s conclusions, D. Karl Mayr also obtained 
high values no matter whether the electrode surface was bright 
or dull. 

Cleaning the Electrodes. Ost ** recommends heating with a 





* Berichte, 36, 2348 (1903). 

+ Z. anorg. Chem., 58, 349 (1907). 
t Z. Elektrochem., 138, 205 (1907). 
§ Inaug. Dissert. Ziirich, 1910. 

|| Z. Elektrochem., 14, 257 (1908). 
“ Inaug. Dissert. Ziirich, 1910. 

** Z. angew. Chem., 1901, 827. 


ina : GRAVIMETRIC ANALYSIS. 


mixture of equal parts concentrated nitric acid and a saturated 
solution of tartaric acid. The antimony will also dissolve 
readily by heating with a solution of alkali polysulphide. 


TIN, Sn. At. Wt. 119.0. 
Forms: SnOz, Sn. 
1. Determination of Tin Dioxide, SnOz.. 


Two cases are to be distinguished: 
(a) The Tin is Present as Metal (in an Alloy). 
(b) The Tin is Present in Solution. 


(a) The Tin ts Present in an Alloy. 


Method I. 


In case the tin is present in an alloy it may be treated accord- 
ing to the method of Busse* as follows: 

About 0.5 gm. of the alloy, in the form of borings, is treated in 
a beaker with 6 c.c. of nitric acid (sp. gr. 1.5), 3 ¢.c. of water are 
slowly added and the beaker is then quickly covered with a watch- 
glass. As the water is mixed with the acid a violent reaction takes 
place. When the evolution of nitric oxide (brown vapors on coming 
in contact with the air) has ceased, the solution is heated to boiling 
and diluted with 50 c.c. of boiling water; the precipitate is allowed 
to settle completely, then filtered, washed, and dried. After burning 
the filter, moistening the ash with nitric acid and drying on the 
water-bath, the precipitate is ignited, at first gently and finally 
strongly, over the Teclu burner or blast-lamp. It is weighed as 
SnO,. 

The tin dioxide thus obtained is never pure; it always contains 
small amounts of other oxides and must be purified as follows: 
After weighing, the precipitate is mixed with six times as much of a 
mixture consisting of equal parts calcined sodium carbonate and — 
pure sulphur, and this mixture is heated in a covered crucible over 
a small flame until the excess of sulphur is almost entirely removed. 
This point is easily recognized by there being no longer any odor 





* Zeit. f. anal. Chem., 17, 53. 


DETERMINATION OF TIN AS TIN DIOXIDE. 229 


of SO, and no blue flame of burning sulphur evident between the 
cover and the erucible. After cooling the melt is treated with a 
little hot water, whereby the tin goes into solution * as sodium 
sulphostannate (cf. Vol. I, p. 222), together. with some copper and 
iron. The deep-brown liquid, therefore, is treated with sodium 
sulphite solution until it becomes only slightly yellow in color, after 
which any iron or copper, etc., will be quantitatively precipitated 
as sulphides. The latter are filtered off and washed, first with water 
to which a little sodium sulphide has been added and finally with 
hydrogen sulphide water. As a rule the amount of insoluble sul- 
phide formed by this treatment is so small that after drying it can 
be ignited in the air and changed to oxide without introducing any 
appreciable error. If this weight is subtracted from the original 
amount of impure stannic oxide, the weight of pure stannic oxide 
will be obtained. If, however, the amount of impurity present 
with the residue of metastannic acid should be large, the different 
metals must be separated according to one of the methods for the 
separation of the sulpho-bases and the weight of each oxide deter- 
mined separately and the sum of their weights subtracted from the 
original weight of the tin dioxide. Instead of determining the 
amount of impurity present with the tin dioxide, the filtrate from 
the insoluble sulphides can be acidified with acetic acid and the tin 
precipitated as yellow stannic sulphide, which, after it has com- 
pletely settled, is filtered off and changed by careful ignition into 
tin dioxide, as described on p. 233, and weighed as such. 


Method II. 


The alloy is dissolved in nitric acid, the insoluble metastannic 
acid is filtered off and washed as in the first method. Instead of 





* Frequently a single fusion with sodium carbonate and sulphur is insuf- 
ficient; this is recognized by obtaining a sandy residue insoluble in water. 
In this case the residue is filtered, washed, dried, and the fusion repeated 
until all the tin is brought into solution. 

+ The sodium sulphite changes the sodium polysulphide to monosulphide, 
in which copper and iron sulphides are insoluble. 


230 GRAVIMETRIC ANALYSIS. 


drying and igniting the precipitate, however, it is washed into a 
porcelain evaporating dish, evaporated on the water-bath almost 
to dryness, and then treated with 1 ¢.c. of pure sodium hydroxide 
solution and 10-15 c.c. of concentrated sodium sulphide solution 
(see foot-note, p. 225). The evaporating dish is covered with 
a watch-glass, and the dish with its contents is heated for about | 
45 minutes on the water-bath, whereby all the tingshould pass into 
solution, and the other metals remain undissolved as sulphides; 
they are removed by filtration. 

The filter, upon which the metastannic acid was filtered, still 
retains some of the precipitate. It is, therefore, laid in a second 
small evaporating dish, covered with about 1 c.c. of sodium 
sulphide solution and heated on the water-bath. After half an 
hour, the tin should all be dissolved. The solution is poured 
through a small filter and the latter is washed with a little hot 
water. 

The two filters are dried, ignited in a platinum spiral, the ash 
treated with concentrated hydrochloric acid and the resulting 
solution is added to that obtained by the solution of the alloy in 
nitric acid. 

For the determination of the tin, the two solutions containing 
sodium thiostannate are combined, acidified with acetic acid and 
boiled to expel the hydrogen sulphide. The precipitated stannic 
sulphide is filtered off, washed once with water to remove the 
most of the alkali salts, then transferred back to the original 
beaker and dissolved in 10 c.c. of 50 per cent. caustic potash,. 
and 1 gm. tartaric acid, these quantities sufficing for 0.1 to 0.2 
gm. of tin. (The last traces of precipitate adhering to the filter 
are dissolved in a very little sodium sulphide solution.) To. 
the solution, pure 30 per cent. hydrogen peroxide (Perhydrol, 
Merck) is added until the yellow liquid becomes perfectly colorless, 
then a cubic centimeter in excess. The solution is boiled for 
about ten minutes to make sure that the oxidation is complete, 
and that the excess of peroxide is decomposed. As soon as no 
more bubbles of oxygen are evolved, the solution is allowed to 
cool somewhat and 15 g. of oxalic acid dissolved in a little hot 
water are cautiously added. The warm solution is electrolyzed 
directly as described on page 234. 


DETERMINATION OF TIN AS TIN DIOXIDE. 231 


The precipitated stannic sulphide, as obtained above by 
acidifying the sodium thiostannate solution, may be ignited in a 
porcelain crucible and weighed as SnOz. The results are usually 
a little high and the method is not as accurate as the electro- 
lytic determination. Cf. page 233; £. 


Method ITI. 


The translator prefers to use a more dilute nitric acid for dis- 
solving the alloy than was recommended by Busse. It is possible 
to obtain, in this way, residues of metastannic acid which are 
fully as pure and the work is not as unpleasant as when the more 
concentrated acid is employed. 

Procedure.—0.5 gm. of borings are dissolved in a small beaker 
with 15 ¢.c. off nitric acid, sp. gr. 1.2. The solution is evaporated 
just to dryness on the water-bath, and the beaker removed as 
soon as this stage is reached. A mixture is prepared of 20 c.e. 
nitric acid sp. gr. 1.2, and 40 c.c. water and this is divided into 
three portions. The residue is treated successively with each 
portion of the above mixture, each time heating to boiling and 
decanting off the solution through a hardened filter paper. The 
washing is completed by boiling and decanting with a 1 per cent. 
solution of ammonium nitrate. As much of the precipitate as 
possible is allowed to remain in the original beaker, and the 
total volume of the filtrate should not exceed 150 ¢c.c. The first 
portions of the filtrate are carefully examined for metastannic 
acid, refiltered if necessary, and each successive portion removed 
from below the funnel before new wash water is added. 

The residue of slightly impure metastannic acid is treated 
as described under either of the above methods. In case Method 
II is chosen, however, it is advisable to treat the filters containing 
the residue from the sodium sulphide treatment with 15 c.c. of 
hot dilute nitric acid (7 c.c. HNQOs, sp. gr. 1.2 and 8 c.c. water) 
instead of burning and treating with hydrochloric acid as directed 
above. The resulting solution of the impurities that were 
originally present in the metastannic acid, is filtered and added 
to the original nitric acid solution. The filters are dried, ignited 


232 GRAVIMETRIC ANALYSIS. 


and the ash weighed as SnOo. The amount thus found is 
added to the result obtained from the sodium thiostannate 
solution. 

Remark.—Sometimes a little metastannic acid is left undis- 
solved by the treatment with alkaline sulphide. It is not safe, 
therefore, to discard the filters. 


(b) Tin ts Present in Solution. 
(a) The Solution Contains Tin only. . 


If the solution contains only tin in the form of stannic salt 
(chloride or bromide), a few drops of methyl orange are added 
and then ammonia until the pink color of the indicator is 
changed to yellow. Ammonium nitrate (obtained by the neutral- 
ization of 20 c.c. of concentrated ammonia with nitric acid) is added 
and the solution is diluted to a volume of 300 c.c., heated to boiling, 
filtered after the precipitate has settled, washed with hot water 
containing ammonium nitrate,* dried, ignited in a porcelain 
crucible, and weighed as SnOv. 

Remark.—lf the solution contains non-volatile organic acids, 
this method cannot be used for the determination of tin. In this 
case the tin must be first precipitated as sulphide by means of 
hydrogen sulphide (cf. p. 233). If the tin is not in solution as 
stannic salt, but as stannous salt, the solution must be first oxidized 
by the addition of bromine water until a permanent yellow color is 
obtained, after which the solution is neutralized with ammonia and 
treated as above described. 

According to J. Lowenthal, tin may be precipitated from slightly 
acid stannic chloride or bromide solutions in the presence of ammo- 
nium nitrate. Methyl orange is added to the solution and then 
ammonia until a yellow solution is obtained;{ dilute nitric acid is 
now added, drop by drop, until the solution just becomes pink again, 
more ammonium nitrate solution is added (20 ¢.c. of concentrated 
ammonia exactly neutralized with nitric acid), the solution is 





* The ammonium nitrate prevents the formation of soluble, amorphous 
stannic acid; it “salts out” the precipitate (cf. Vol. I, p. 71). 

+ The excess of acid cannot be removed by evaporation on account of the 
volatility of stannic chloride. 


DETERMINATION OF TIN AS TIN DIOXIDE. 233 


diluted to 300 c.c., boiled for some time, filtered, washed with water 
containing ammonium nitrate, dried, ignited, and weighed as SnQ,. 
This method is employed when the sotution contains small amounts 
of alkaline earths; they remain in solution. Sodium sulphate can 
be used instead of ammonium nitrate to “salt out’’ the tin precipi- 
tate, but although the tin will be quantitatively precipitated, some 
sodium sulphate will be also thrown down, so that high results will 
be obtained. 3 


(0) The Solution Contains, besides Tin, Metals of the Preceding 
Groups or Organic Substances. 


In this case, independent of whether the tin is present in the 
form of stannic or stannous salts, hydrogen sulphide is conducted 
into the dilute solution until it is saturated with the gas; the solu- 
tion is allowed to stand until the odor of hydrogen sulphide has 
almost disappeared and then filtered. The precipitate is washed 
with a solution of ammonium nitrate (or ammonium acetate), 
dried, transferred as completely as possible to a porcelain crucible, 
and the ash of the filter added. The tin sulphide is at first gently 
heated in a covered crucible to avoid loss by decrepitation, and after- 
wards in an open crucible until the odor of sulphur dioxide is no 
longer perceptible. The temperature now is raised gradually until 
finally the full heat of a good Teclu burner is obtained or the cruci- 
ble is heated over the blast-lamp. As tin dioxide holds fast to 
some sulphuric acid with great tenacity, after cooling the crucible 
somewhat a piece of ammonium carbonate the size of a pea is added, 
the crucible covered and again heated, after which it is weighed as 
$nO,. The heating with the addition of ammonium carbonate is 
repeated until a constant weight is obtained. 

Remark.—¥. Henz * in testing this method always obtained 
results which were a little too high. This is due to the fact that 
it is difficult to wash the stannic sulphide precipitate free from 
sodium salts. _The author recommends, therefore, dissolving 
the well-washed stannic sulphide precipitate in a little sodium 
sulphide, transforming this solution into potassium stannioxalate, 
and determining the tin by electrolysis, according to page 234. 

* Z. anorg. Chem., 37, 39 (1903). = 





234 GRAVIMETRIC ANALYSIS. 


2. Determination of Tin as Metal. 


The electrolytic deposition of tin from a solution of the 
ammonium double oxalate gives excellent results.* It is necessary, 
however, that some free oxalic acid is always present while the 
solution is undergoing electrolysis. During the process, ammo- 
nium oxalate is changed by anodic oxidation into ammonium 
carbonate and carbon dioxide, 


(NH4)2C204+ 0 = (NH4)2CO3+COzg, 


and the solution will smell of ammonia as a result of the hydrolysis 
of ammonium carbonate. When this point is reached no more 
tin is deposited. The ammonia often precipitates some stannic 
acid, which escapes the electrolysis. It is necessary, therefore, 
to avoid letting the bath become ammoniacal, and this is best ac- 
complished by adding a little solid oxalic acid from time to time. 

Procedure.—In the course of an analysis it is usually necessary 
to precipitate the tin from a solution of alkali thiostannate. This 
is best accomplished as follows: The thio-salt is decomposed 
by acidifying with acetic acid, the deposited sulphide dissolved 
in caustic potash solution, the solution oxidized with hydrogen 
peroxide, and finally acidified with oxalic acid, all exactly as 
described on page 230. The final solution, about 150 e.c. in 
volume, is heated to 60°-70° and electrolyzed with a current of 
1 ampere and 3-4 volts potential at the electrodes; from time 
to time a little solid oxalic acid is added. At the end of about 
six hours all the tin will have been deposited upon a gauze 
electrode. The deposit is washed with water, exactly as pre- 
scribed for nickel on page 136, then with water, dried by holding 
above a flame, cooled in a desiccator, and weighed. The results 
are excellent. 

Remark.—lIf ammonium oxalate is used in place of the potas- 
sium oxalate, the electrolysis requires more time (eight to ten 





* Cf. Classen’s ‘‘Quant. Anal. by Electrolysis’? and his “ Ausgewahlte 
Methoden der analytischen Chemie.” 


SEPARATION OF ARSENIC, ANTIMONY, AND TIN. 235 


hours). By the addition of hydroxylamine the duration of the 
electrolysis is shortened (Engel). 

F. Henz proposed to prevent the bath becoming ammoniacal 
by adding sulphuric acid during the course of the electrolysis, 
but subsequent experiments have shown that it is better to pro- 
ceed as described above. If too much sulphuric acid is added, 
all the tin is not precipitated. The chief conditions are the 
presence of enough oxalate and a slightly acid solution. 


Separation of Arsenic, Antimony, and Tin from the Members of 
the Ammonium Sulphide Group. 


The separation is effected by passing hydrogen sulphide into 
the acid solution of the above metals whereby arsenic, antimony, 
and tin are precipitated as sulphides, while the remaining metals 
remain in solution.. 

From an alloy, or the solid sulpho-salts of the above metals, 
arsenic, antimony, and tin may be readily volatilized by heating 
in a stream of chlorine; the chlorides of these three metals are 
readily volatile, while those of the remaining metals are only 
difficultly so. 


Separation of Arsenic, Antimony, and Tin from Mercury, Lead, 
Copper, Cadmium, and Bismuth. 


if the metals are all in solution, they are precipitated by means 
of hydrogen sulphide and the precipitated sulphides after being 
filtered and washed are treated with alkali sulphide solution. 
If mercury is present, ammonium polysulphide should he used, 
but in the absence of this metal sodium polysulphide works better 
(cf. Vol. 1.) 

If the metals of this group are in the form of an alloy (arsenic 
and mercury are seldom met with to any extent), the antimony 
and tin are separated from the remaining metals on treating 
the alloy with nitric acid. The tin is left behind as meta- 
stannic acid, insoluble in dilute nitric acid, with the antimony 
as nearly insoluble antimonic acid. In the presence of tin, all 
phosphorus and arsenic are thrown down in the insoluble residue 
as phosphate and arseniate of metastannic acid. The small 


236 ; GRAVIMETRIC ANALYSIS 


amount of the latter (and the remaining metals of this group) are 

KF precipitated by hydrogen sulphide and separated 

Pe from the copper group by means of alkaline 

sulphide solution. | 

The separation of tin from the remaining 

metals of the group can be illustrated by a 
practical example. 
> 


Analysis of Bronzes. 
Method I. 


A bronze is an alloy of tin and copper in vary- 
ing proportions. It almost always contains 
besides these metals, more or less lead, aluminium, 
iron, Manganese, zinc, and phosphorus. 

Procedure.—About 0.5-1 gm. of the alloy in 
the form of borings* is placed in a beaker, 
treated with 6 c.c. of nitric acid, sp. gr. 1.5,f and 
3 c.c. of water are added, after which the 
beaker is immediately covered with a watch- 
glass. When the reaction begins to diminish, 
the liquid is heated to boiling, until no more 
brown fumes are evolved, when 50 e.c. of 
boiling water are added; the precipitate (con- 
taining all the tin, the phosphoric acid, and always 
small amounts of copper oxide) is allowed to settle completely, 











Fic. 46. 





* The borings are usually somewhat oily, in which case they should be 
washed with ether that has been distilled over potash, dried at about £0°C., 
and weighed after cooling in a desiccator. The washing with ether is best 
accomplished in a Soxhlet’s fat-extraction apparatus, as shown in Fig. 46. 
The borings are placed in the extraction-tube, which is filled with ether nearly 
‘up to the bend b of the siphon-arm. The tube is then connected with the 
condenser K. After this from 20 to 30 c.c. of ether are added to the flask 

_and this is heated gently on the water-bath. The ether vapors. pass through 
the wide side-arm to the condenser K, where they are condensed and drop 
upon the borings. As soon as the ether in the tube has reached the height 
b, it is siphoned back into the flask, where it is again distilled. All the oil 
will ke removed from the borings in from half an hourtoanhour. - 

+ See pages 228 and 231, <F 


ANALYSIS OF BRONZES. 237 


is filtered, washed with hot water, dried, ignited in a porcelain 
crucible, and weighed. In this way the weight of the SnO,+ P,O,+ 
foreign oxide is obtained. In order to obtain the weight of foreign 
oxide (chiefly copper oxide) the precipitate is fused with a mix- 
ture of sodium carbonate and sulphur as described on p. 228, 
The sulphides, remaining after the solution of the melt in hot 
water, are filtered off, converted into oxides by ignition in the 
air, and weighed. By subtracting this weight from that pre: 
viously obtained, the weight of SnO,+ P.O, is obtained. In order 
to obtain the weight of the SnO, a separate portion is analyzed 
according to the method of Oettel as described below for phos- 
phoric acid, and the amount of phosphoric anhydride subtracted 
from the weight of SnO,+ P,O;,. 

The oxides obtained by the ignition of the insoluble sulphides 
are dissolved in a little nitric acid (in case Fe,O, is present a 
little hydrochloric acid is also necessary) and the solution of the 
nitrates is added to the first filtrate from the impure metastannic 
acid. To this solution an excess of dilute sulphuric acid is added, 
and it is evaporated on the water-bath as far as possible. and then 
heated over a free flame until dense, white fumes of sulphuric 
acid are evolved. After cooling, 50 c.c. of water and 20 c.c. of 
alcohol are added, the precipitate of lead sulphate is filtered off . 
and its weight determined as described on p. 174. The filtrate 
from the lead sulphate is heated to remove the alcohol and the 
copper precipitated by means of hydrogen sulphide and weighed 
as Cu.S according to p. 183. In the filtrate from the coppcr sul- 
phide the iron, aluminium, and zinc (also manganese) will be 
found. It is evaporated to a small volume in order to expel the 
hydrogen sulphide, oxidized by the addition of a few drops of 
concentrated. nitric acid, and the iron and aluminium sepzrated 
from the zine by means of a double precipitation with ammonia,* 
whereby the iron and aluminium are left behind as hydroxides 





* If considerable zinc is present, the above separation is inexact. . In this 
case the filtrate from the copper sulphide is treated with sodium acetate, 
heated to 60°, saturated with hydrogen sulphide, and the iron and:aluminium 
determined in the filtrate, the zine in the precipitate. -If manganese is 
present in the alloy, it.should be separated from iron and aluminium as 
described on vp. 149 to 155. 


238 GRAVIMETRIC ANALYSIS. 


and are separated and determined according to p. 107. The 
zine is precipitated from the filtrate after acidifying with acetic 
acid, by passing hydrogen sulphide into the boiling solution. The 
precipitated zine sulphide is filtered off, dissolved in hydrochloric 
acid, evaporated to drynéss in a weighed platinum dish, and trans- 
formed to oxide by heating with mercuric oxide by Volhard’s 
method (cf. p. 142). 

For the phosphorus determination ®ettel * recommends the fol- 
lowing procedure: From 2-5 gms. of the substance are dissolved, 
as before, in nitric acid; and the impure metastannic acid with all 
the phosphorus is filtered off, dried, and transferred as com- 
pletely as possible to a porcelain crucible. The ash of the filter 
is added, and the contents of the crucible ignited. After cooling, 
the substance is mixed with three times as much solid potassium 
cyanide, the crucible covered, and the contents fused; the stannic 
oxide is reduced to metal, 


SnO,+2KCN =2KCNO+Sn, 


while the phosphoric acid is converted into potassium phos- 
phate. 

By skilfully rotating the crucible during the fusion, it is possi- 
ble to unite the small particles of molten tin into a larger button 
whereby the subsequent filtration is greatly facilitated. After 
cooling, the melt is treated with water and filtered. The filtrate 
is cautiously treated with hydrochloric acid under a good hood 
and boiled to remove the hydrocyanic acid. It is then saturated 
with hydrogen sulphide in order to remove traces of copper and 
tin which almost always remain in the solution. The filtrate is 
freed from hydrogen sulphide by boiling, made ammoniacal, and 
the phosphoric acid precipitated as magnesium ammonium phos- 
phate by the addition of magnesia mixture. After standing for 
twelve hours, the latter is filtered off, washed with 2} per cent. 
ammonia water, dried, and changed by ignition to magnesium 
pyrophosphate, in which form it is weighed. 





* Chemiker-Zeitung (1896), p. 19. 


DETERMINATION OF PHOSPHORUS. 239 


Ordinary bronzes may be analyzed very nicely in the fol- 
lowing manner: The alloy is treated with nitric acid as described 
above, the metastannic acid removed by filtration and the 
filtrate electrolyzed, using a dull platinum dish as cathode, and 
a plate as anode, both of which are weighed. The electrolysis 
is carried out with a current of 1 to 1.2 amperes at about 60° and 
at the end of two and one-half to three hours the electrodes are 
washed without breaking the circuit. On the anode will be found 
all the lead as PbO, and on the cathode will be found the copper. 
The siphoned solution contains the iron, aluminium and zine, 
which are determined as above. The phosphorus is determined 
in a special sample. 

Remark.—The method just outlined will give exact results 
only when the metastannic acid is purified and the recovered 
solution of copper and lead nitrates added to the main solution. 
In the electrolysis, the chief dangers to be feared are having 
the solution so acid that the copper is not all precipitated, or 
so dilute that a spongy deposit is obtained. 


Method II. 


An excellent method for the analysis of phosphor bronze con- 
sists in dissolving the alloy as described under Tin, Method IIL, p. 
231, and determining the tin as there described. The copper and 
lead are determined in the nitric acid solution by electrolysis with a 
current of 0.2 ampere, the copper being deposited on the cathode 
and the lead as peroxide on the anode. The electrolysis is usually 
finished in twelve hours, but it is well to clean the electrodes after 
weighing the deposits and then to test the solution with the current 
for an hour or so longer to see whether any lead or copper remains 
in the solution. Often alittle more copper will be found, especially 
if the solution was a little too acid. During the electrolysis the 
concentration of the acid gradually diminishes so that eventually 
all the copper will be thrown down. The iron, aluminium, and 
zine remain in solution, and are determined as above outlined. 

For the phosphorus determination,* one gm. of the borings 
is weighed into a small beaker and dissolved in 20 c.c. of aqua 





* Cf. Dudley and Pease, Eng. and R. R. Journ., March, 1894. 


240 GRAVIMETRIC ANALYSIS. 


regia, made by mixing equal volumes of the concentrated acids 
just previous to use. The beaker is covered with a watch-glass, 
and, after solution is complete, the contents heated nearly to boil- 
ing for fifteen minutes. After cooling, 25 c.c. of water are added, 
and then just sufficient ammonia (sp. gr. 0.90) to redissolve. 
the copper hydroxide* and to produce a deep blue colored 
solution; thereupon 50 c.c. of colorless ammonium sulphide are — 
introduced. This should be enough"to precipitate the sulphides, 
and the supernatant liquid should show no blue color. If it 
does, more ammonium sulphide must be added. The solution: 
is digested at a temperature near the boiling-point for fifteen 
minutes, the precipitated sulphides of copper and lead allowed to 
settle, and then filtered into a 300 c.c. Erlenmeyer flask, decanting 
the clear liquid carefully from the precipitate, and finally throwing , 
the precipitate upon the filter. When the filter has drained the 
filter and precipitate is returned to the beaker, 50 c.c. of ammonium 
sulphide wash water (one part colorless ammonium sulphide to 
three parts of water) are added, and the mixture is heated, and 
stirred occasionally, for ten minutes; it is then poured upon 
another filter, washed with 50 c.c. of ammonium sulphide wash 
water and allowed to drain completely, The total volume should 
not be over 250 c.c., but it is not necessary to evaporate in case 
this volume is slightly exceeded. To the filtrate 10 c.c. of mag- 
nesia mixture are added and the solution shaken. The flask is 
placed in ice water and allowed to stand with occasional shaking 
for two hours. The precipitate of magnesium ammonium phos- 
phate is filtered upon a small filter and washed with ammonia 
water (one part 0.96 sp. gr. ammonia to three parts water) until 
nearly free from sulphide. 10 c¢.c. of hydrochloric acid (one part 
HCl, sp. gr. 1.20, to four parts water) are placed in the flask, 
taking care that all of the precipitate adhering to the walls of the 
flask is dissolved, and then poured through the filter, allowing the 
solution to run into a No. 1 beaker. The flask and filter are 
washed with 10 c.c. more of the same acid. 3 c.c. of magnesia 
mixture are added to the filtrate, which is heated to boiling, 
removed from the flame, and then treated with ammonia (sp. gr. 
0.90= 10 per cent. NH3) until the latter is present in large excess. 





* A precipitate of lead ‘or tin hydroxide remains insoluble. 


SEPARATION OF ARSENIC FROM ANTIMONY. 24t 


The solution is allowed to stand in ice water for two hours, and is 
stirred occasionally. The precipitate is then filtered and washed 
with 2} per cent. ammonia water until free from chlorides, and 
ignited with the usual precautions, weighing as MgoP207. 


SEPARATION OF THE SULPHO-ACIDS FROM ONE ANOTHER. 


1. Arsenic from Antimony. | 
(a) Method of Bunsen.* 


Principle.—If a slightly acid solution of an alkali arsenate 
and antimonate is treated with hydrogen sulphide in the cold 
and the excess of the latter immediately removed by conduct- 
ing air through the solution, the antimony is quantitatively 
precipitated as pentasulphide, while the arsenic remains in solu- 
tion. 3 

Procedure.—Assume the arsenic and antimony to be present 
‘in the solution as arsenious and antimonous acids. Both ele- 
ments are precipitated by hydrogen sulphide, filtered, and washed 
with water. The greater part of the precipitate is transferred 
by mears of a spatula to a 200-c.c. porcelain casserole, and the 
precipitate remaining on the filter is dissolved into the casserole 
by dropping a solution of hot dilute pure caustic potash upon it. 
From 3-5 gms. of pure solid caustic alkali are added, and the 
precipitate dissolves to a clear solution.t 

The casserole is now covered with a perforated watch-glass. 
It is placed upon the water-bath, and chlorine is conducted into 
the solution until all the alkali is decomposed; this takes from 
one-half to three-quarters of an hour. By this operation the 
arsenite and antimonite are oxidized to arsenate and antimonate 
and a small amount of potassium chlorate is formed. Concen- 
‘trated hydrochloric acid is now added to the warm solution drop by 
drop from a pipette until all the chlorate is decomposed and no 
more chlorine is evolved. The watch-glass is removed, the solu- 
tion is evaporated to half its volume, and then an equal amount of 
concentrated hydrochloric: acid is added and the solution again 


* Ann. d. Chem. und Pharm., 192, 305. 
{ If alkaline earths were the only metals present besides the arsenic and 
antimony, the first precipitation with hydrogen sulphide would be omitted. 





242 GRAVIMETRIC ANALYS S. 


evaporated to half its volume. The contents of the casserole 
are washed by means of dilute hydrochloric acid into a large beaker, 
diluted with water to a volume of 600 ¢.c. and for every decigram 
or less of the antimony 100 c.c. of freshly prepared hydrogen 
sulphide water are added. An orange precipitate of antimony 
pentasulphide is formed at the end of a short time. A strong 
current of air (filtered through a wad of cotton) is then passed 
through the solution without dela} until the excess of hydrogen 
sulphide is completely removed; this usually requires about 
twenty minutes. In order to avoid loss during this operation 
a large beaker should be used to contain the solution and it should 
be covered with a perforated watch-glass. The precipitate of 
antimony pentasulphide is likely to contain traces of arsenic 
pentasulphide so that it is dissolved once more in caustic 
potash and the above operation repeated. The precipitate now 
obtained will be pure antimony pentasulphide. It is filtered 
through a Gooch crucible, dried at 280° C. in a stream of ear- 
bon dioxide as described under antimony, and weighed as 
SbeS3.* 

For the arsenic determination, the combined filtrates are con- 
centrated somewhat by evaporation, a few drops of chlorine 
water are added and hydrogen sulphide is passed into the warm 
solution (being kept on. the water-bath) for from six to eight 
hours, after which it is allowed to cool in a rapid stream of 
. hydrogen sulphide. After allowing the precipitate to settle for 
twenty-four hours, it is filtered through a Gooch crucible, washed 
with water, then three times with alcohol, four times with a mix- 
ture of pure carbon bisulphide and alcohol (cf. p. 180), and 
finally three times with pure alcohol. After drying at 110° C., 
the precipitate is weighed as As,S,. 

Remark.—lf the solution contains no very large excess of 
hydrogen sulphide, the precipitate will always contain trisulphide, 
‘go that it is safer to dissolve it in ammoniacal hydrogen per- 





* Bunsen weighed the antimony as pentasulphide after washing with car- 
bon bisulphide. As, however, antimony pentasulphide is likely to be changed 
to the trisulphide on treating with carbon bisulphide, the above procedure 
is better. According to Braun, Sb,S, is reduced to Sb,S, on long-continued 
treatment with CS,. 


SEPARATION OF ARSENIC FROM ANTIMONY. 243 


oxide * and then to precipitate the arsenic with magnesia mix- 
ture as Magnesium ammonium arsenate, as described on p. 206, 
weighing it as Mg2As2O7. 

Remark.—The method gives very accurate results, but con- 
sumes considerable time. 


(b)- Method of Fred. Neher.t 


This, in the author’s estimation, the best method for the separa- 
tion of arsenic and antimony, depends upon the fact that arsenic 
is precipitated from a solution strongly acid with hydrochloric 
acid by a rapid stream of hydrogen sulphide, while antimony is 
not. 

Procedure.—Starting with a precipitate consisting of the tri- 
sulphides of arsenic and antimony, this is dissolved in caustic 
potash solution and oxidized exactly as described under the 
previous method. When free from chlorate, the acid solution 
is washed into an Erlenmeyer flask and cooled by surround- 
ing the flask with ice. In another flask some concentrated 
hydrochloric acid (sp. gr. 1.2) is likewise cooled. When both 
solutions are at 0° C., the arsenic antimony solution is diluted 
with twice its volume of the strong hydrochloric acid. Into 
this cold solution a rapid stream of hydrogen sulphide is passed 


- for one and one-half hours. The flask is stoppered up and allowed 


to stand one to two hours. The As,§, is filtered through a Gooch 
crucible and washed with hydrochloric acid (1 vol. water, 2 vols. con- 
centrated hydrochloric acid) until 1 ¢.c. of the filtrate after being 
considerably diluted with water and tested with hydrogen sul- 
phide shows no precipitation. It is then washed with water, and 





* For this purpose as much of the precipitate as possible is placed in a 
beaker, the portion adhering to the filter is dissolved by hot ammonia into 
the same beaker, and this is warmed until the precipitate has entirely dis- 
solved. After this, for every 0.1 gm. of As,S;, 30-50 c.c. of pure 3 per cent. 
H,0, are added, the solution heated for some time on the water-bath and 
then boiled ten minutes. 

{ Z. anal. Chem., 32, 45. 


244 GRAVIMETRIC ANALYSIS. 


finally with hot alcohol. After drying at 110° C., the precipitate 
is weighed as AsoSs. * 

The filtrate from the arsenic sulphide is diluted largely with 
water and saturated with hydrogen sulphide. The SboSs is filtered 
‘through a Gooch crucible, dried at 280° C. in a current of carbon 
dioxide and weighed. 


(c) The Tartaric*Acid Method. 


Principle.—The separation is based upon the fact that if 
magnesia mixture is added to a solution of an alkali arsenate 
and antimonate containing tartaric acid, only arsenic will be pre- 
cipitated. 

Procedure.—The sulphides are oxidized as described under 
(a) by solution in aqueous caustic potash and introduction of 
chlorine. The solution thus obtained is made acid, treated with 
tartaric acid and an excess of ammonia added. This should 
not cause any turbidity. If a precipitate is formed, it shows that 
an insufficient amount of tartaric acid is present. In this case 
the clear solution is decanted off, the precipitate is dissolved by 
warming with tartaric acid, and the two solutions are mixed. 
To the clear, ammoniacal solution, magnesia mixture is added 
slowly with constant stirring (cf. p. 206. foot-note). After stand- 
ing twelve hours, the precipitate of magnesium ammonium 
arsenate is filtered off (it usually contains a little basic mag- 
nesium tartrate), washed a few times with 24 per cent. ammonia, 
uissolved in hydrochloric acid, and reprecipitated by the addi- 
tion of an excess of ammonia. After standing for twelve hours 
mere, the precipitate is filtered, washed with 24 per cent. 
ammonia, and weighed as magnesium pyroarsenate as described 
on p. 206, 


* If the solution was not cold, some arsenic trisulphide will be found 
in the precipitate. The results are scarcely affected, however, when the 
‘precipitate is merely washed with water and alcohol, because the free sulphur 
is weighed with the sulphide of arsenic. If, however, the precipitate is 
washed with CS,, it is evident that the results will be too low. For the 
highest degree of accuracy, it is advisable to dissolve the precipitated sulphide 
in ammoniacal hydrogen peroxide, or in fuming nitric acid, and to deter- 
mine the arsenic as Mg,As,O, as described on page 206. Sag 





SEPARATION OF ARSENIC FROM ANTIMONY. 245 


Remark.—Arsenic can also be separated from tin according 
to the above method, except that more tartaric acid is necessary 
to prevent the precipitation of the tin than is the case when an- 
timony alone is present (cf. p. 255). 


(d) Method of E. Fischer* 


Principle.—This separation depends upon the ready vola- 
tility of arsenic trichloride in a current of hot hydrochloric acid 
gas, under which conditions antimony chloride is not volatile. If 
the arsenic is present as arsenic acid, which is usually the case, 
the distillation must take place in the presence of some reducing 
agent.t 

Procedure.—The apparatus shown in Fig. 47 is used for this 
determination. In the course of analysis, the arsenic and anti- 
mony, as a rule, are obtained first in the form of the sulphides, and 
these are dissolved, as described under (a), in caustic potash 
solution and oxidized by chlorine. Instead of using chlorine, 
the alkaline solution may be boiled with hydrogen peroxide or 
potassium percarbonate. If the latter method is used for the 
oxidation, the boiling must be continued until there is no further 
evolution of oxygen. 

The oxidized solution is transferred, by means of a long- 
stemmed funnel, to the 500-c.c. distilling flask, A, in which has 
been placed 1.5 gms. of potassium bromide;t the solution is 
diluted in the flask with fuming hydrochloric acid to a volume of 
about 200 c.c. The receiver, V, consists of a large flask of from 

* Z. anal. Chem., 21, 266. The process as described is the modification 
of M. Rohmer, Ber., 84, 33 and 1565 (1901). 

{ Fischer used a ferrous salt, O. Piloty and A. Stock used hydrogen 
. sulphide (Ber., 30, 1649), and Friedheim and Michaelis used methyl alcohol 
(Ber., 28, 1414). 

{ Instead of the potassium bromide, hydrogen bromide may be used 
which has previously been prepared by treating 1 gm. of bromine with 
sulphurous acid. It is not permissible to introduce the bromine into the 
flask, A, in order to convert it to hydrogen bromide by introducing sulphur 
dioxide gas into the flask, because it is then possible for bromine vapors 
to get into the receiver by means of the air which is first expelled from 
the apparatus, and the bromine would oxidize the volatilized AsCl,, and 
thus interfere with the subsequent determination of the arsenic by pre- 
eipitation as the trisulphide, or by titration. 





246 GRAVIMETRIC ANALYSIS. 


1.5—2 liters capacity; it is kept surrounded by a current of cold 
water coming from the condenser and contains at the start, 800 c.e. 
of cold distilled water. Then, with the apparatus all connected 
as shown in the drawing, the distilling flask is heated and its 
contents partially distilled in a current of hydrogen chloride,* 
meanwhile constantly passing a little sulphur dioxide into the 
flask, until at the end of about forty-five minutes, the volume of 
2: 

















a CFF Na (Am)Cl 


Fic. 47. 


liquid in A is reduced to about 40 c.c. The flame is then removed 
and the T-tube between the two evolution flasks removed in order 
to prevent liquid from backing up into the wash bottles. The 
adapter tube which connects the condenser with the receiver is 
rinsed off and the receiver removed. 

A new receiver is now placed at the end of the apparatus and 
a seccond distillation is made in order to make sure that all of 
the arsenic has been volatilized.| Then, for the determination of 
the arsenic, the contents of the two receivers are each diluted to a 
volume of about 1250 c.c., and the excess of sulphurous acid is 
removed by heating to boiling and passing a stream of carbon 
dioxide through the liquid as is shown in Fig. 48. When the 
sulphur dioxide has all been expelled (as can be shown by insert- 
ing a stopper with delivery tube into the flask so that the escaping 
vapors can be led into a dilute sulphuric acid solution of deci- 
normal permanganate which will be decolorized by sulphur 
dioxide), the solution is allowed to cool and the arsenic determined 
as trisulphide according to the directions on p. 205 and weighed 





* If there is any tendency to suck back, a little more sulphur dioxide 


should ke introduced. } 
+ Rohmer found that as much as 0.15 gm. arsenic was volatilized com- 


: pletely by one distillation. 


DETERMINATION OF ARSENIC BY TITRATION. 247 


as AsoS3 after treatment with CS2 (pp. 170, 223), or it may be 
titrated with iodine. 


Determination of Arsenic by Titration. 


The solution is treated with a few drops of phenolphthalein 
and solid potassium hydroxide is introduced until a permanent 
pink color is imparted to the liquid. Th» solution is then decolor- 
ized by the addition of a few drops of hydrochloric acid; 5 gms. 
of sodium bicarbonate are added, and the solution titrated with 

-decinormal iodine solution as described on p. 688.* 

The antimony is determined by treating the contents of the 

distilling flask with 2 or 3 gms. of tartaric acid, washing the 




















ni 
TATA LRHEH 





itil 











Fia. 48. 


solution into an Erlenmeyer flask, expelling the sulphur dioxide 

as above,t and determining the antimony gravimetrically by 

precipitating as the trisulphide according to the directions on 

p. 218, or it is estimated volumetrically by titration with iodine 
as Jeaibed on p. 688. 





* A blank determination should be made with all the reagents that are 
to be used, and the iodine solution must be standardized in a solution as 
dilute as that in which the analysis is made. 

+ The escaping gas will not decolorize a solution of 2-3 c.c. dilute sulphuric 
acid and one drop of 0.01N. KMnO,, when all the SO, is expelled. 


248 GRAVIMETRIC ANALYSIS. 


Determination of Arsenic in Commercial Sulphuric Acid. 


About 30 ¢.c. of concentrated hydrochloric acid and a little 
potassium bromide, or hydrogen bromide, are placed in the dis- 
tilling flask A (Fig. 47), whereupon 50 to 100 gms. of the acid to 
be tested (the weight is determined by difference) is introduced 
through a funnel that is fastenedeby means of rubber tubing to 
the upper end of the delivery tube which enters the flask;* the 
funnel is rinsed with concentrated hydrochloric acid, and the 
distillation begun. 

When the contents of the distilling flask have been concen- — 
trated so that concentrated sulphuric acid remains, the acid is 
kept hot by means of a small flame until all of the arsenic has 
been expelled. On account of the high temperature, 1 gm. of 
arsenic will be driven over in about fifteen minutes. The analysis 
is finished as described above. 


Separation of Antimony from Tin. 
(a) F. W. Clarke’st Method. 


Of all the present known methods for the separation of anti- 
mony from tin this is probably the most -accurate. It depends 
upon the fact that antimony is completely precipitated from a 
solution containing oxalic acid, while stannic salts are not. Stan- 
nous sulphide, however, is decomposed by oxalic acid, forming an 
insoluble crystalline stannous oxalate, so that the tin must be in 
the stannic form. 





* When the concentrated sulphuric acid runs into the flask, it often happens 
that distillation begins to take place and some of the arsenic would be lost 
if the flask, A, were left open. 

7 Chem. News, Vol. 21, p.124. Cf. also Réssing, Zeitschr. fiir anal. Chem., 
XLI, 1. F. Henz, Z. anorg. Chem., 37, 18 (1903). Vortmann and Metzl, 
Z. anal, Chem:, 44, 525 (1905). 


SEPARATION OF ANTIMONY FROM TIN. 249 


Procedure.—In the majority of cases it is a question of sepa- 
rating antimony from tin after these metals have been separated 
from the members of the copper group by means of alkaline poly- 
sulphide; i.e., the tin and the antimony are in the form of their 
soluble sulpho-salts. 

The solution of the sulpho-salts, containing not more than 
0.3 gm. of the two metals, is placed in a 500-c.c. Jena beaker and 
treated with a solution of 6 gms. of the purest caustic potash 
(one-third the sum of the weights of tartaric and oxalic acids to 
be added) and 3 gms. of tartaric acid (ten times the maximum 
weight of the two metals). To this mixture 30 per cent. hydro- 
gen peroxide is added slowly until the yellow solution is com- 
pletely decolorized; then 1 c.c. in excess is added and the 
solution is boiled for a few minutes to change any thiosulphate 
to sulphate and to decompose the greater part of the excess 
peroxide. As soon as the evolution of oxygen ceases, the solu- 
tion is cooled somewhat, the beaker covered with a watch-glass, 
and a hot solution of 15 gms. pure recrystallized oxalic acid 
is cautiously added (5 gms. for 0.1 gm. of the mixed metals). 
This causes the evolution of considerable gas (CO,+0,). Now, 
in order to completely remove the excess of hydrogen peroxide, 
the solution is boiled vigorously for ten minutes. The volume 
of the liquid should amount to from 80 to 100 e.c. After this 
a rapid stream of hydrogen sulphide is conducted into the boiling 
solution, and for some time there will be no precipitation, but 
only a white turbidity formed. At the end of five or ten minutes 
the solution becomes orange-colored and the antimony begins 
to precipitate, and from this point the time is taken. At the 
end of fifteen minutes the solution is diluted with hot water to 
a volume of 250 c.c., at the end of another fifteen minutes the 
flame is removed, and ten minutes later the current of hydrogen 
sulphide is stopped. The precipitated antimony pentasulphide 
is filtered: off through a Gooch crucible which, before weighing 
and after drying, has been heated in a stream of carbon dioxide 
at 300° C. for at least one hour. The precipitate is washed 
twice by decantation with 1 per cent. oxalic acid and twice 
with very dilute acetic acid before bringing it in the crucible. 


250 GRAVIMETRIC ANALYSIS. 


Both of these Sank liquids should be boiling hot and saturated 
with hydrogen sulphide. 

The crucible is heated at 280°-300° in a current of carbon 
dioxide (free from air) to constant weight and its contents 
weighed as Sb,5,. 

To determine the tin, the filtrate is evaporated to a volume 
of about 150 c.c., the excess of oxalic acid nearly neutralized 
with ammonia seat the tin deposited electrolytically as described 
on p. 234. 

According to Vortmann and Metzl,* antimony may be sepa- 
rated from tin by passing hydrogen sulphide into a solution 
containing hydrochloric and phosphoric acids of the proper 
concentration. 


(b) Method of H. Rose. 


Principle.—This method is based upon the insolubility of 
sodium metantimonate and the solubility of sodium stannate 
in dilute alcohol. 

Procedure.—Both metals are assumed to be present in the 
form of an alloy. The alloy is treated with nitric acid, whereby 
the antimony and tin are left in the form of their oxides (cf. pp. 
228, 231, and 236). The residue is filtered off, washed with am- 
monium nitrate water, dried, transferred as completely as possible 
to a large silver crucible and the ash of the filter added, after 
which the precipitate is gently ignited. From ten to twelve 
times as much solid sodium hydroxide and a little sodium nitrate, 
or better, sodium peroxide, are added and the silver crucible 
is placed within a larger porcelain one in order to protect it 
from the flame: the contents are fused and kept liquid for 
twenty minutes. After cooling, the crucible is placed in a 
large porcelain dish and its contents treated with hot water 
until the melt has disintegrated, leaving the insoluble part in 
the form of a fine meal. One-third of the solution’s volume 
of alcohol (sp. gr. 0.833) is now added, the mixture is well stirred 
and filtered after standing twelve hours. The residue remaining 





* Z. anal. Chem., 44, 533 (1905). 


SEPARATION OF ANTIMONY FROM TIN. 251 
hz 


on the sides of the dish is washed onto the filter with dilute 
alcohol (1 vol. aleohol+2 vols. water). The sodium metanti- 
monate is washed first with a mixture of 1 vol. aleohol+2 vols. 
water, then with 1 vol. alcohol+1 vol. water, and finally with 
3 vols. alcohol+1 vol. water,* until the filtrate when acidified 
with hydrochloric acid and tested with hydrogen sulphide no 

longer gives a yellow coloration (tin sulphide). 

If considerable tin and little antimony were originally present, 
a single fusion of the oxides with caustic soda does not afford a 
complete separation, as the residue of sodium pyroantimonate 
always contains some tin. It is, therefore, dried, separated from 
the filter and placed in a silver crucible. The filter is treated 
repeatedly in a porcelain crucible with fuming nitric acid until 
the paper is completely. destroyed and the excess of acid is then 
removed by heating in an air-bath. The contents of the porce- 
lain crucible are subsequently dissolved in a little caustic soda 
solution and washed into the silver crucible; the water is then 
removed by heating the silver crucible at first on the water- 
bath and finally in an air-bath. Ten grams of solid caustic soda 
are now added, the mixture fused, and the melt treated in the 
same way as before. 

The second residue of sodium metantimonate is free from 
tin. It is dissolved from off the filter by a mixture of hydro- 
chloric and tartaric acids,t in which it is readily soluble. From 
this solution the antimony is precipitated by hydrogen sulphide 
and determined as described on p. 218. For the tin determina- 
tion, the alcoholic filtrate is gently heated to remove the alcohol, 
acidified slightly with hydrochloric acid, and the tin precipitated 
as sulphide by hydrogen sulphide and determined according to 
P. 233, 8. ; | 

Remark.—If the oxide residue which was first fused with 
sodium hydroxide and nitre consisted solely of tin and antimony 
- oxides, this method gives very good results. As a rule, however, 





_ * A few drops of sodium carbonate solution should be added to all the 
alcoholic wash liquids. 
+ A mixture consisting of equal volumes dilute hydrochloric acid (1:4) 
and 5-10 per cent. tartaric acid is used. 


252 GRAVIMETRIC ANALYSIS. 


most antimony and tin alloys contain lead and other metals whose 
oxides remain to small extent with the tin and antimony on 
treatment of the alloy with nitric acid, so that the sodium metan- 
timonate is subsequently rendered impure by the presence of these 
metals. The antimony determination therefore gives too high re- 
sults. In this case the method of W. Hampe* should be used. 

The alloy is dissolved in aqua regia (as described below in 
the analysis of bearing metal) ang the tin and antimony sepa- 
rated from the remaining metals by means of colorless sodium 
sulphide. From the solution of the sulpho-salts the tin and anti- 
mony are precipitated by making barely acid with dilute sulphuric 
acid; the precipitate is washed and dissolved in a little warm 
sodium sulphide. After cooling, sodium peroxide is added te 
the concentrated solution in small amounts until the liquid 
becomes colorless, and when treated with more sodium peroxide 
a distinct evolution of oxygen takes place. By this treatment 
sodium antimonate is formed; this separates out to some extent, 
while the tin remains in solution. In order to completely pre- 
cipitate the antimony from the solution, one-third as much alcohol 
(sp. gr. 0.833) is added, after which the precipitate is filtered off 
and treated as above described. 


Analysis of Bearing Metal. 


This alloy contains tin, antimony, lead and a little copper 
and usually small amounts of iron, bismuth and zinc. 

One gram of borings are placed in a 400-c.c. beaker and 
dissolved in 15 c¢.c. concentrated hydrochloric acid, to which 
3 c.c. of concentrated nitric acid are immediately added. The 
alloy will usually dissolve in the cold after standing a while; 
when rich in lead, however, it will be necessary to heat for some 
time on the water-bath. As soon as all has dissolved, the solu- 
tion (it should be yellow, or greenish yellow if much copper is 





* Chem. Ztg., 18, p. 1900. 


ANALYSIS OF BEARING METAL. 253 


present) is diluted with 15 times as much alcohol, added in 
small portions with constant stirring.* 

' After standing for twelve hours, and stirring frequently, the 
precipitated lead chloride is filtered into a weighed Gooch 
crucible, washed with absolute alcohol, dried at 150° and weighed. 
In the filtrate will be found a few milligrams of lead in the 
presence of antimony, tin, copper, bismuth, iron and zinc. 
The alcoholic filtrate is poured into a large, deep porcelain dish t¢ 
and the alcohol is evaporated off at as low a temperature as 
possible. It is necessary to avoid evaporating the solution to 
dryness as in that case some SnCl, will be volatilized. When 
the alcohol is all gone, 0.1 gm. of potassium chlorate is added, 
the solution is evaporated to a small volume and then there 
is added one gm. of tartaric acid and enough caustic potash 
to make the solution barely alkaline. It is now treated, as 
recommended by Finkener, with freshly prepared hydrogen 
sulphide water until no further precipitation takes place. In 
this way all the Cu, Bi, Fe, Zn and the last of the Pb are pre- 
cipitated as sulphides (precipitate a) while all the Sn and Sb 
remain in solution (solution b).§ 





*This stirring is indispensable because lead chloride separates out: 
very slowly from a supersaturated alcoholic solution containing other 
chlorides. The complete precipitation is best recognized by the fact that 
no mark is left upon the sides of the beaker when the stirring rod is rubbed 
against it. 

+ Alloys low in lead are not treated with alcohol in this way. In such 
cases it is best to decompose the alloy with chlorine gas, as described in the 
analysis of tetrahedrite on page 359. 

t In evaporating off the alcohol there is a tendency for the solution to 
creep over the edges of the dish so that it is advisable to employ a deep 
dish and to evaporate the liquid in small portions. 

§ The separation is complete only when all the tin is in the quadrivalent 
condition. In driving off the alcohol there is always some stannous 
chloride formed which must be subsequently oxidized by means of KCIO, 
and HCl. 


254 GRAVIMETRIC ANALYSIS. 


Treatment of Precipitate a. 


The precipitate is filtered off, washed with hydrogen sulphide 
water, dissolved in nitric acid (sp. gr. 1.2) and evaporated — 
with hydrochloric acid to remove the nitric acid, and the solution 
of chlorides diluted so that its acidity corresponds to 1 part 
HCl (sp. gr. 1.12) to 25 parts water. The Cu, Pb, and Bi are 
precipitated as sulphides by hydfogen sulphide, filtered and 
washed with water containing H,S. The filtrate contains the 
iron and zine (Filtrate c). 

The precipitate is dissolved in nitric acid, evaporated with 
the addition of 4 or 5 drops of concentrated sulphuric acid, 
and the last of the lead determined as sulphate according to 
p. 174. From this filtrate the bismuth is precipitated with an— 
excess of ammonia and determined as BigO3 according to p. 179. 

In the ammoniacal filtrate from the bismuth precipitation, 
the copper is determined electrolytically, after acidifying with 
sulphuric acid, according to p. 187, or as cuprous sulphide, 
according to p. 188. 

To determine the iron and zinc, the Filtrate c is oxidized by 
boiling with a few drops of concentrated HNOg3 and the iron 
precipitated by an excess of ammonia and weighed as FesQs, 
p. 87. The zine is determined in this last filtrate by acidifying 
with acetic acid, precipitating as sulphide and weighing as such, 
according to p. 143. 


Treatment of Solution b. 


To determine the antimony and tin, the alkaline solution 
is diluted to exactly 250 c.c. in a measuring flask, and after 
thoroughly mixing, 100 c.c. is withdrawn ina pipette, trans- 
ferred to a 400-c.c. beaker, acidified with acetic acid, and boiled 
to expel the hydrogen sulphide. Then 3 gms. of tartaric acid 
and 6 gms. of purest potassium hydroxide are added, whereby 
any precipitated sulphide is redissolved. At this point some 
30 per cent. hydrogen peroxide is allowed to run slowly into the 
solution, until the yellow color disappears, then 2 or 3 ¢.c. in 
excess are added and the solution boiled a few minutes. Then, 


SEPARATION OF ARSENIC FROM TIN. 255 


for each 0.1 gm. of metal present (Sb+Sn), 5 gms. of pure oxalic 
acid are added, the solution boiled ten minutes and the antimony 
separated from the tin as described on p. 248. From the filtrate 
the tin is determined electrolytically. For this purpose the 
oxalic acid solution is evaporated to a volume of about 
200 c.c. and electrolyzed with a gauze electrode. At the end 
of six hours the deposition is complete. The electrodes are 
washed as described on p. 136, dried and weighed. 

Remark.—Rossing’s method,* which was recommended in 
the earlier editions of this book, is not altogether satisfactory. 
Usually the lead results are too high and the tin too low, on 
account of the lead sulphide precipitate being contaminated 
with tin. 


Separation of Arsenic from Tin. 


(a) Method of Fred. N cher. 


The moist sulphides are dissolved in freshly-prepared ammo- 
nium sulphide, evaporated in an Erlenmeyer flask nearly to dry- 
ness and oxidized with hydrochloric acid and potassium chlorate. 
From this solution the arsenic is precipitated as sulphide under the 
conditions described on p. 243. In the filtrate from the arsenic 
pentasulphide all of the tin is found and can be precipitated as sul- 
phide after diluting largely with water and passing in hydrogen sul- 
phide.. Itis finally changed to the oxide as described on p. 233, f. 


(b) Method of W. Hampe.t 


The precipitated sulphides are dissolved as soon as possible in 
freshly-prepared ammonium sulphide, the solution is evaporated 
almost to dryness and oxidized with hydrochloric acid and potassium 
chlorate in a flask connected with a return-flow condenser.$ Tar- 
taric acid and ammonia are then added and the arsenic precipi-- 
tated with magnesia mixture as magnesium ammonium arsenate, as 


Giiwiniins 





* Z, anal. Chem., 41, 1 (1902). 
t Ibid. (1893), $2, p. 45. 
_t Chem. Ztg. (1894), 18, p. 1900. 
§ So that no arsenic trichloride will be lost by volatilization. 


256 | GRAVIMETRIC ANALYSIS. 


described on p. 206, After standing twelve hours, the precipitate 
is filtered off, washed with 24 per cent. ammonia, and, in order 
to remove a little magnesia, the precipitate is dissolved in hydro- 
chloric acid and reprecipitated by the addition of ammonia. After 
standing another ‘twelve hours, the precipitate is filtered off and 
again washed with 24 per cent. ammonia. 

This precipitate can be converted into magnesium pyroar- 
senate and weighed in this form ag described on p. 207. This 
transformation is somewhat tiresome, however, so that Hampe 
prefers to dissolve the precipitate in hydrochloric acid once more, 
to precipitate the arsenic by means of hydrogen sulphide, and 
then to determine the magnesium in the evaporated filtrate as 
magnesium pyrophosphate according to p. 66 or p. 67. From 
the weight of the latter the amount of arsenic can be computed 
as follows: | 

Mg,P,.0,:2As=p:2 
ne 2As 
Mg,P,0,? 


or 
x=0.6734-p gm. arsenic. 


Separation of Antimony from Arsenic and Tin. 


(a) Method of Rose. 


If the metals are present in solution, they are precipitated as 
sulphides with hydrogen sulphide, heated with fuming nitric acid 
in a large covered beaker until the sulphur is completely oxidized, 
washed into a porcelain dish, and the excess of acid removed by 
evaporation on the water-bath. The almost-dry residue is treated 
with concentrated sodium hydroxide solution and the contents 
of the dish are transferred to asilver crucible, after which a little 
solid sodium hydroxide is added and the contents of the crucible 
dried in an air-bath. It is then fused * and kept liquid for about 
twenty minutes ‘by heating over a Teclu burner. After cooling, 
the melt is disintegrated with water, one-third as much alcohol 
(sp. gr. 0.833) is added in order to completely precipitate the 
sodium metantimonate, and after standing twelve hours the 





* The ‘silver crucible is placed in a larger porcelain one so as to avoid 
contact with the flame. 


GOLD IS PRESENT IN SOLUTION. 257 


precipitate is filtered and subjected to the treatment described on 
p. 251. The filtrate containing all the arsenic and tin is acidified 
with hydrochloric acid, whereby stannic arsenate is precipitated. 
Without filtering, hydrogen sulphide is conducted into the liquid, 
the precipitated sulphides of tin and arsenic are filtered off, oxi- 
dized with hydrochloric acid and potassium chlorate, and the 
arsenic separated from the tin as described on p. 255. 


(b) Method of Hampe. 


The moist sulphides are oxidized as described on p. 255, b, and 
the arsenic determined in the same way. 

In the combined filtrates from the magnesium ammonium 
arsenate the -antimony and tin are precipitated by hydrogen 
sulphide, after making the solution acid. These are separated 
either according to the method of Clarke (p. Be or that of Rose 
(p. 250). 


SUPPLEMENT TO THE HYDROGEN SULPHIDE GROUP, 


GOLD, PLATINUM, SELENIUM, TELLURIUM, VANADIUM, 
MOLYBDENUM, TUNGSTEN. 


GOLD, Au. At. Wt. 197.2. 


Gold is always determined as the metal itself. We have three 
cases to distinguish: 
1. The gold is present in solution. 
2. The gold is alloyed with copper and silver. 
3. The gold is present in an ore. 


1. Gold is Present in Solution. 


In almost ’all cases gold is deposited as metallic gold from 
its solutions and weighed after filtering and washing. 

For the deposition of gold the following reducing agents are 
to be considered: ferrous sulphate, oxalic acid, formaldehyde, 
and hydrogen peroxide. If the gold is to be precipitated by means 
of either ferrous sulphate or oxalic acid, there must be no free 
nitric acid present in the solution. If some is present, it must 
be removed by repeated evaporation with concentrated hydro- 
chloric acid and the solution then diluted with water. To this 


258 GRAVIMETRIC ANALYSIS. 


dilute solution a large excess of clear ferrous sulphate solution 
is added, the beaker is covered and its contents are heated for 
several hours on the water-bath. The precipitate is then filtered 
off, washed first with water containing hydrochloric acid until the 
iron is completely removed, and then with pure water. The pre- 
cipitate is dried, transferred as completely as possible to a porce- 
Jain crucible, the ash of the filter added, and the gold is 
ignited and weighed. In this way*gold can be separated from 
almost all metals, even platinum, but not from silver. If silver 
is present, which is of course never the case in a dilute hydrochloric 
acid .solution, it is first removed by the addition of hydrochloric 
acid, the precipitated silver chloride filtered off, and the filtrate 
treated as above described. | 

_ For the precipitation of gold by means of oxalic acid, the 
slightly acid solution is diluted with water, oxalic acid or ammo-_ 
nium oxalate is added with a little sulphuric acid, and the covered 
beaker is allowed to stand forty-eight hours in a warm place. 

_ The yellow scales of the deposited gold are filtered off and 

washed, as above described, with hydrochloric acid and then with 
water. It is then ignited and weighed. 


Deposition of Gold by Means of Hydrogen Peroxide (L. Vanino and 
L. Seeman).* 


If a gold solution is treated with potassium or sodium hy- 
droxide solution and then with formaldehyde, or, better still, 
hydrogen peroxide, the gold is soon precipitated quantitatively, 
even in. the cold. By boiling, the finely-divided gold collects 
together and assumes a reddish-brown color. The reaction takes 
place according to the following equation: 


2AuCl, + 3H,0,-+ 6KOH = 6KCl + 6H,0 +30, + Au,. 


If the gold is deposited by this method from very dilute solu-: 
tions it is obtained in such a finecly-divided condition that it 
passes through the filter. If, however, the solution is boiled 
until the excess of hydrogen peroxide is completely destroyed, 
and it is then acidified with hydrochloric acid, the gold can 
be readily filtered. Gold can be separated from platinum by 
this method. 





* Berichte (1099), 32, p. 1963. 


GOLD ALLOYED WITH COPPER AND SILVER. 259 


2. The Gold is Alloyed with Copper and Silver. 


When gold is present in alloys it is most rapidly and most 
accurately determined in the dry way. The principle of the 
method is very simple. 

If a gold-silver alloy is melted in the air with lead upon a 
‘“cupel” (a very porous vessel made of bone-ash)* the lead and 
copper are oxidized, the oxides fuse and are absorbed by the 
cupel, while all the gold and silver are left behind in the form of 
a metallic button, whose weight is obtained. The silver is after- 
wards separated from the gold by the action of nitric acid which 
dissolves the silver but leaves the gold behind. If the weight 
of the gold that is left undissolved is deducted from the weight 
of the gold-silver button the weight of the silver is obtained. 

In order to obtain accurate results a number of precautions 
must be taken. By the cupellation of the alloy some noble 
metal is always lost and the amount lost increases in proportion 
to the amount of lead used and the higher the temperature. 
Furthermore, small amounts of the noble metal are absorbed by 
the cupel and this amount is greater the smaller the amount of 
lead used. This second loss amounts to much less than the former 
one occasioned by the use of too much lead. Consequently, in 
every gold cupellation an unn:cessary excess of lzad must be avoided. 

Experience has shown that the richer a gold-silver alloy is 
in base metal the more lead is necessary for the cupelletion. 
Furthermore, in the separation of gold from silvcr by means of 
nitric acid it is necessary to remember that the separation is only 
quantitative when the alloy consists of three or more parts of 
silver to one part of gold. If less than three parts of silver are 
originally present for one part of gold, it is necessary to add pure 
silver until this proportion is reached. This operation is known 
as quartation or inquartation. The separation of the silver from 
the gold by means of nitric acid is spoken of as parting. Ifa 
gold-silver alloy, in the form of foil, which consists of three parts 
of silver to one of gold, is treated with nitric acid, the latter metal 
remains behind as a brownish scale; if more silver is present, 
it is left as a fine powder, unless the acid is made extremely dilute. 





* According to R. Grund, Oesterr. Z. Berg-Hiittenw., 57, 681, magnesite 
is better than bone-ash. 


260 GRAVIMETRIC ANALYSIS. 


From what has been said, it is clear that accurate results can 
be obtained only when the correct amount of lead is present in 
the alloy that is cupelled, and when the gold and silver are present 
in the proper proportion; i.e., it is necessary to know the approxi- 





Fig. 49. 


mate composition of the alloy before an accurate determination 
can be made. This is determined by 


The Preliminary Assay. 


For this purpose the muffle shown in Fig. 49 is heated to a 
cherry-red heat, a cupel weighing from 6 to 7 gms.* is placed in 
the back part of it, the muffle door is closed, and the cupel heated 
until it has acquired the same color as the muffle. After this 
5 gms. of lead are placed upon the cupel, the muffle is closed until 





* A good cupel will absorb its own weight of litharge. During the cupel- 
lation about one-tenth of the litharge formed 
is lost by volatilization, so that the weight of 
litharge absorbed by the cupel is practically 
that of the original lead button. Fig. 50 repre- 
sents a cupel, together with its cross-section. 





THE PRELIMINARY ASSAY. 261 


the lead is melted and then 0.25 gm. of the accurately-weighed 
alloy is enveloped in a small piece of lead-foil, placed in the 
molten lead (with the help of a pair of tongs), and the muffle closed 
until the alloy has melted and shows a bright upper surface. With 
the help of an iron hook the cupel is now carefully advanced to 
about the middle of the muffle and the door should be left open 
so that there is a ready access of air into the muffle. 

After about twenty minutes the lead will be all absorbed, 
which is shown by the “blick.” * The hot cupel is then removed 
from the muffle and after cooling, the color of the button is ob- 
served. 

(a) If the button is greenish yellow or darker, it contains less 
than three parts of silver to one part of gold, in which case from 
four to six parts of “fine silver” are added (the proper amount 
can be usually told by the practised eye) and the button is cupelled 
in a new cupel with 1 gm. of lead. The button now obtained is 
treated with nitric acid and the residual gold weighed. 

(b) If the button 7s pure white, then three or more parts of 
silver are present to one part of gold. In this case it is imme- 
diately ‘“‘parted’’ and the residual gold weighed. 

After the approximate amount of gold present has been ascer- 
tained,t the analysis proper is made, using the amount of lead as 
indicated in the following table: 


LEAD TABLE, 


Amount of Gold Amount of Lead Necessary for the 
Present in the Alloy. Cupellation of 0.25 gm. of Alloy. 
SOOO Tnoudsnd tha Oe ese So cis ok 0.25 gm. 

900 o iets wibleis few cause 2.50 gms. 
800 Oey eikip bib ee ie ue Bees Es & £00; fF 
700 Sr Garp wate oc Web w arcade Gesan ts 5.50. “* 
600 onthe hig SCAR NAN aE Se eS Oe G00 * 
500 ee ys Sl oe We eee 6.B °'t 
400 or less or ea ey el cg ete aks ee 1 a 





* The blick is the brightening of the metal which appears when the outer 
layer of lead oxide that is constantly becoming thinner finally bursts and the 
bright noble metal shines through. Just before the blick there is a distinct 
iridescence, so that the point can never be mistaken. 

+ In assay laboratories the approximate gold contents of the alloy is deter- 
mined by its streak. A fine-grained piece of silicate is blackened with char- 
coal. The alloy to be tested is rubbed’upon it and the color produced com- 


262 GRAVIMETRIC ANALYSIS. 


The Final Assay. 


For the definite determination of the gold and silver, two 
portions weighing exactly 0.25 gm. are taken; the one to serve 
for the silver determination and the other for the gold. The 
former is cupelled with the correct amount of lead and the weight 
of the gold-silver button is determined. 

If the original alloy was very gvhite, it contains more than 
500 thousandths fine of silver. 

If the alloy was greenish yellow, it contains 550-750 thou- 
sandths of noble metal, and silver is present to a considerable 
extent. 

If, however, the alloy was a beautiful yellow or reddish yellow, 
it contains more than 700 thousandths of noble metal and the 
gold predominates. : 

If, therefore, the alloy was white, once again as much pure silver 
is weighed out as the amount of gold found to be present by the 
preliminary assay (inquartated with one part of silver), and this 
mixture is cupelled with the same amount of lead as the first portion. 

If the original alloy was greenish yellow, it is inquartated * 
with two parts of silver; if it was distinctly yellow or reddish 
yellow it is inquartated with 24 parts of silver. 


Treatment of the Quartered Gold-Silver Button. 


The gold-silver button is removed from the cupel with the — 
“button tongs,” cleaned with a stiff brush (“button brush’’), 
and hammered upon an anvil to a round disk about 1 mm. thick 
(Fig. 51, a). This is heated upon a fresh cupel and quickly cooled 
by placing it upon a piece of brass foil and rolling it between two 
steel rollers to a long strip (Fig. 51, b); it is again heated and 
rolled + up as shown in Fig. 51, c. This little roll is placed in a 





pared with that obtained from alloys containing known amounts of gold. 
Afterwards these streaks are tested with dilute aqua regia; alloys containing 
the same amounts of gold are attacked equally readily. 

* Cf. p. 259. 

+ By hammering the gold-silver ie. the metal becomes so brittle that 
it cannot be converted to a smooth-margined roll, and on the subsequent 
treatment with nitric acid, little pieces would pechabiy drop off. By again 
heating the metal and then quickly cooling, it regains its original softness. 


DETERMINATION OF GOLD IN ORES. 263 


little flask (Fig. 52,7), covered with 30-40 c.c. of nitric acid (sp. 
gr. 1.188) free from chloride, heated to boiling and kept so for 
ten minutes. The acid is then poured off and replaced by the 





Fig. 51. Fia. 52. 


same amount of stronger acid (sp. gr. 1.295) and the above treat- 
ment repeated. After this acid is poured off, the butten is 
washed by decanting three times with distilled water. The flask 
is filled with water, covered with an annealing cup (or lack- 
ing this an ordinary porcelain crucible may be used), and is 
then quickly inverted (Fig. 51, IJ), when the gold will pass 
into the cup. The flask is removed by first raising its mouth 
to the level of the water in the crucible and then sliding it 
off at right angles and skilfully turning the flask right side up. 
The water is poured off from the gold and the crucible is placed 
in the back part of the muffle for a short time, whereby the gold 
is dried and is changed from its former brown and soft condition 
into a harder, beautiful yellow substance. After cooling, it is 
weighed. By subtracting the weight of the gold from the weight 
of the gold and silver together, the amount of silver is obtained. 


Determination of Gold in Ores. 


Principle.—The very finely ground and sifted ore is mixed in 
a No. 9 French crucible with lead oxide, charcoal, and some suit- 
able slag-forming material. The charcoal reduces a part of the 
lead oxide to metal which alloys with the noble metal and 
sinks to the bottom in the form of a button, while the foreign sub- 


264 GRAVIMETRIC ANALYSIS. 


stances should pass into the slag. After cooling, the crucible is 
broken, the slag is hammered off, the lead button cupelled and 
the silver-gold button parted in the same way as before. The 
noble metal should be extracted with as little lead as possible, 
for with an unnecessarily large amount of lead some gold is lost 
during the cupellation. 

The amount of lead reduced from the litharge depends largely 
upon the nature of the ore. Sulphide ores act strongly reducing, 
as is shown by the following equations: 


PbS+2PbO=S0,+3Pb, 
Fe8,-++ 5PbO=280, + FeO+5Pb. 


In such cases less charcoal (or in some cases none at all) should 
be added than would be otherwise necessary to produce the right 
amount of lead, or in case considerable sulphide is present, it is 
sometimes necessary to neutralize its action by the addition of 
oxidizing agents. 

Reducing ores are recognized by their color: they are gray, 
bluish-black, or yellow (pyrite, ete.). Reddish-brown ores (Fe,O,) 
usually act oxidizing: 


Fe,0,+C=CO+2Fe0, 


in which case more charcoal must be added to the charge. 

The best results are obtained when the lead button weighs 
about 18 gms. when obtained from 30 gms. of ore.* In order 
that such a button may be obtained, it is usually necessary to 
make a preliminary assay of the ore. But above all, it is neces- 
sary that the purity of the reagents used should be tested. 


Testing the Reagents. 
The ordinary reagents necessary for a gold assay are: 
1. Litharge (PbO). | 


Litharge, the most important reagent, is a basic flux, for it 
forms with the silicic acid of the ore a readily fusible silicate; 





* This amount is usually sufficient; with very rich gold ores 10-15 gms. 
is enough, while with very poor ores as much as 120 gms. may be used to 
advantage. Cf. Ricketts and Miller, Notes on Assaying, New York, 1897. 


REAGENTS FOR GOLD-SILVER. ASSAY. 265 


at the same time, however, it is a desulphurizing agent, as is shown 
by the above reaction. 

The litharge used must be dry and free from minium, for 
the latter oxidizes silver, carrying it into the slag, so that low 
results would be obtained in the silver determination. The — 
litharge should be free from silver (which is almost never the case), 
or its silver contents must be known; this is determined once for 
all by the following experiment: 


RR eee a aad k's swine ¢ 120 gms. 
Sodium bicarbonate (NaHCO,) ........... 60 “ 
Avvo (erage: ROT On) oe es aS a 


are mixed thoroughly upon a sheet of glazed paper and the mix- 
ture placed in a No. 9 French crucible and covered with a layer 
of finely-powdered, dry common salt. The covered crucible is 
placed in a glowing coke-oven, 

As soon as the contents of the crucible have reached the state 
of quiet fusion, the crucible is removed from the fire, its walls are 
gently tapped by the tongs, and it is lightly tapped upon its 
bottom in order to knock down any small particles of lead adher- 
ing to the sides and to make all of the free metal collect together 
on the bottom in the form of a button. 

After cooling the crucible is broken, the slag removed from 
the lead button by hammering it upon an anvil, and it is cupelled 
upon a cupel weighing only a few grams more than the button 
itself. The resulting silver button is weighed. The amount of 
silver obtained must be deducted whenever the corresponding 
amount of litharge is used in an assay. 


2. Sodium Bicarbonate (NaHCO,). 
3. Anhydrous Borax (Na,B,0,). 


2 and 3 require no testing. 


4. Charcoal. 
The reducing power is determined as follows: 
NN oS OP Sa, sais dees Rik 280s K4ee + 60 gms, 
SAMI ORPOORALG «ooo ics cece es sess 15..." 


CRARMOR 36 oss ik sdaaes ces etianes ties even «2 bos 


266 GRAVIMETRIC ANALYSIS. 


are mixed, as in the testing of litharge, in a French crucible No. 9 
with a cover of ordinary common salt and fused. After cooling, 
the weight of the lead button obtained is determined and this ex- 
presses in terms of lead the reducing power of the charcoal. 

1 gm. of charcoal should reduce about 30 gms. lead. 


5. Nitre (KNO,) 


Serves as an oxidizing agent. Its gxidizing power expressed in 
terms of lead is dete:mined: 


Nites... ..<0.5 cs wend oe Rida ee 3 gms 
Latharge . ..¢ so iin nesue once pee ee ee Fe: Ba Ge 
Chavenal 255 osc ane Parr ree a 1 gm. 
Sodium Dicarbomate iio 655445 veins ea ee 15 gms. 


are mixed and fused as before and the weight of the lead button de- 
termined. If under (4) it was found that 1 gm. charcoal would 
reduce P gm. lead, and if p gm. of lead were obtained in this ex- 
periment, then the difference P—p shows the amount of lead that 
was oxidized by 3 gms. nitre, or the oxidizing power of the nitre. 

1 gm, nitre oxidizes about 4 gms. lead. 


6. Common Salt. 


Ordinary table salt is heated in a large Hessian crucible until 
it melts, and the contents of the crucible are poured into a shal- 
low iron mould with a raised edge. The solidified crust is finely 
powdered and preserved in a stoppered flask. 

After the reagents have all been tested the next step is the - 


Preliminary Assay. 


Five grams of the finely-powdered and sifted ore are weighed 
out and mixed with: 


Litharge s .c.33 cae mie epee ey wey re ee oc 80 gms. 
Sodium biearbonste &352525% ¢ Fosiew ee sek <i 
Botak i4 6: 564.0% cso ees Eee eae Sees Ba: 


placed in a crucible and covered with a layer of common salt. After 
fusing, cooling, and hammering off the slag, the lead button ob- 
tained is weighed. 

Since in an ordinary assay we start with 30 gms. of ore, the 


GOLD-SILVER ASSAY. 267 


weight of the lead button now obtained multiplied by 6 will give 
the weight of the button from the real assay. We will distinguish 
- four cases: 


(1) The lead button weighs 3 gms. 


Consequently the button obtained from 30 gms. of ore would 
weigh 18 gms. In this case the ore is assayed with the following 
proportions of flux: 


Gee ee cot oe os, Ga ee 30 gms, 
Rane 5g os kts Ss bce aS 80“ 
ogi Diearoonate: 222.6 Sec eee ee es 263% 
MI a as oe bee se i 3 se age 


(2) The lead button weighs less than 3 gms. 


Evidently the ore acts reducingly, but not enough so to yield a 
button weighing 18 gms. when 30 gms. of ore are used; it is, 
therefore, necessary to add charcoal to the flux. 

_ Example.—Let us assume that the lead button obtained by the 
preliminary assay weighed 1 gm., then the button obtained from 
30 gms. of ore would weigh 6 gms. . In order to obtain a button 
weighing 18 gms. it is necessary to add enough charcoal to supply 
12 gms. of lead. If 1 gm. of charcoal was found to reduce 30 gms. 
of lead, then it is necessary to add 12+30 gms.=0.4 gms. of 
charcoal. 


(3) The lead button weighs more than 3 gms. 


In this case the ore has a strong reducing power, and to obtain 
the lead button of the right weight it is necessary to add some nitre. 

Example.—Suppose the button to weigh 6 gms.; this would 
mean a 36-gm. button when 30 gms. of ore were used; i.c. 18 gms. 
too much lead would be produced. We must add, therefore, 
- enough nitre to oxidize this 18 gms. of lead. If the oxidizing power 
of 1 gm. of nitre was found to be 4 gis. of lead, then 18+4=4.5 
gms. of nitre must be added to the flux. 

Remark.—Ores which have a very strong reducing power would 
frequently require the addition of enough nitre to cause the con- 
tents of the crucible to boil over. In such a case, about 40-50 ems. 
are placed in a “‘roasting-dish’’ and roasted in a muffle, and from 
this roasted ore the portions are taken for the preliminary and 


268 GRAVIMETRIC ANALYSIS. 


final assays. The results, however, must be expressed in terms of 
the unroasted ore. 


(4) There is no lead button formed. 


The ore is either neutral or possesses an oxidizing action. The 
assay is repeated, using 1 gm. of charcoal, and from the results 
' now obtained the final assay is based. 


Final Assay. 

For the final assay from 30-120 gms. of ore * are taken (accord- 
ing to the amount of gold present) and the corresponding amount 
of sodium bicarbonate is added. The amount of litharge also varies 
with the amount of ore, and in some cases as much as 240 gms. are 
necessary, although as arule 80 gms. are sufficient. Otherwise the 
procedure is exactly the same as in the preliminary assay. The lead 
button is cupelled and the weighed silver-gold button is parted as 
described on p. 263. : 

PLATINUM, Pt. At. Wt. 195.0. 

Platinum is best determined as metallic platinum. 

The following three cases will be considered: 

1. The platinum is present in a hydrochloric acid solution 
either alone or together with other metals, but other platmum 
metals are absent. 

2. The platinum is present alloyed with gold and silver. 

3. The platinum is alloyed with small amounts of the plati- 
num metals together with small amounts of base metals. 


1. The Platinum is Present in Hydrochloric Acid Solution Either 
Alone or Together with Other Metals. 


The platinum is either precipitated from the solution as ammo- 
nium chloroplatinate, (NH,),PtCl,, which is decomposed by ignition 
and the residual platinum weighed; or the platinum is precipi- 
tated as metal by the addition of reducing agents to the solution; 
or finally the platinum is precipitated as sulphide by conducting 
hydrogen sulphide into the hot solution and changed to platinum 
by ignition. The two former methods serve to separate platinum 





* Usually ‘assay tons” are used as units in weighing out the ore, and 
~ weights are calibrated in terms of this unit instead of the gram. An 
“assay ton” contains the same number of milligrams that there are ounces: 
troy to a ton, so that by weighing the button obtained in milligrams, it is, 
at once known how many ounces per ton the ore carries. 


PLATINUM, — 269 


from most other metals, while the latter serves to separate plati- 
num only from the metals of the alkali, alkaline earth, and ammo- 
_ nium sulphide groups, and not from members of the hydrogen sul- 
phide group. 


(a) Precipitation of Platinum as Ammonium Chloroplatinate. 


The solution, concentrated as much as possible, is nearly neu- 
tralized with ammonia, an excess of ammonium chloride and consid- 
erable alcohol are added, and the mixture allowed to stand twelve 
hous under a glass bell-jar. It is then filtered through an asbestos 
filte> tube 10-15 em. long, washed with 80 per cent. alcohol until 
a few drops of the filtrate leave no residue on being evaporated 
to dryness on a platinum foil. The precipitate is dried by con- 
ducting.a stream of air warmed to about 90° C. through the tube. 
After cooling the tube is weighed, a plug of ignited asbestos * is 
introduced, and the tube is again weighed; in this way the weight 
of the asbestos plug is found. A stream of dry hydrogen is now 
conducted through the tube, and the latter is heated at as low a 
temperature as possible until no more hydrochloric acid is evolved 
and all the ammonium chloride has been driven off, after which the 
tube is cooled in a desiccator and weighed. 

Instead of filtering the precipitate upon asbestos an un- 
weighed paper-filter may be used. The moist precipitate is 
placed together with the filter in a large porcelain crucible 
so that the apex of the filter-paper points upward, and the 
covered crucible is then ignited. This ignition must be performed 
with great ‘care, as otherwise there can be a considerable loss dur- 
ing the process. At first the precipitate is dried by gently warm- 
ing the covered crucible, and when the odor of alcohol has disap- 
peared, the temperature is raised very slowly until the crucible is at 
a strong red heat. During the whole operation there must be no 
_ visible escape of vapors from the crucible. The decomposition is 
complete when there is no longer a penetrating odor arising from 
the covered crucible. When this point is reached, the cover (whose 
under side will be covered with carbon) is removed for the first 
time and leaned against the crucible and the contents of the latter 





* Ammonium chloroplatinate decrepitates during the heating. . To pre- 
vent loss of substance it is heated between two asbestos plugs. 


270 GRAVIMETRIC ANALYSIS. 


are ignited with free access of air until the carbon from the filter- 
paperis completely burned. Often a slight deposit of platinum * 
will be found in the upper part of the crucible and upon the cover, 
so that the latter must always be weighed with the crucible. 

Remark.—If it seems likely that the precipitate of ammonium 
chloroplatinate is contaminated with other substances (e.g. so- 
dium chloride, etc.) the precipitate can be dissolved in water after 
it has been washed with alcohol and* dried. The platinum may 
then be determined, as described on p. 50, by precipitating with 
mercury, washing with dilute hydrochloric acid and then with 
water, and finally weighing. 

The results obtained by this method are satisfactory but some- 
what lower than the true values; the following process is more 
accurate: 


(b) Precipitation of Platinum by Reducing Agents. 

The solution is freed from any excess of acid by evaporation, 
placed in an Erlenmeyer flask into the neck of which is ground 
to fit a return-flow condenser. The solution is neutralized with am- 
monia, an excess of formic acid and a little ammonium acetate are 
added, and the contents of the flask after being diluted to a volume 
of 200 c.c. are heated to about 80° C. on the water-bath until the 
evolution of carbon dioxide has nearly ceased. The flask is now 
connected with the return-flow condenser, and its contents boiled for 
twenty-four hours. The precipitated metal is filtered off, washed 
with dilute hyd:ochloric acid, then with water, dried, ignited, and 
weighed. 


2. The Platinum is Alloyed with Gold and Silver. 


An alloy is seldom found which contains only the above three 
noble metals; usually copper is also present. The first step, then, 
is to separate che noble metals from the others by cupellation with 





* By means of the dry distillation of the filter, carbon monoxide is formed, 
and by the decomposition of the ammonium chioroplatinate chlorine is set 
free. These two gases act upon the metallic platinum and form volatile com- 
pounds (PtCl,.CO, PtCl,.2CO, and 2PtCl,.38CO), which, however, are later 
decomposed by the aqueous vapor. This causes the deposit of platinum in 
the upper part of the crucible. In order to avoid loss, a large crucible should 
be used. 


- PLATINUM. 271 


lead as described on p. 259, after which the hammered and rolled 
button is treated with pure concentrated sulphuric acid. [Nitric 
acid cannot be used, for some platinum would be dissolved with 
the silver.] After boiling for ten minutes, the silver will be com- 
pletely dissolved, provided at least two parts of silver are present 
for each part of platinum, which is usually the case. If more 
platinum is probably present than corresponds to the above ratio, 
pure silver should be added, and the mixture cupelled once more 
with 1 gm. of lead. 

After the alloy has been boiled for ten minutes with sulphuric 
acid it is allowed to cool, filtered, and the treatment with sulphuric 
acid repeated once again. The metal remaining behind (in the 
form of a roll or as a powder) is washed three times by decantation 
with water, ignited, and weighed as described under gold. This 
gives the weight of the gold and the platinum together, and by sub- 
tracting this amount from the original weight of the noble metals 
obtained after cupellation, the weight of the silver is obtained. 


Separation of Gold from Platinum. 


Principle.—If an alloy of gold and platinum is treated with 
nitric acid, neither metal is attacked. If, however, the alloy con- 
tains three parts of silver to one of gold and platinum taken to- 
gether, and the alloy is treated at first with acid of sp. gr. 1.16 and 
then with acid of sp. gr. 1.28, the platinum gradually goes into 
solution with the silver. 

Procedure.—The gold-platinum alloy is cupelled-with three times 
its weight of pure silver and 1 gm. of lead, the resulting button is 
hammered and rolled, after which it is treated with nitric acid (of 
the strength stated above), and the residual metal weighed. It is 
again cupelled with three parts of pure silver, and the same proc- 
ess repeated. This is continued until a constant, weight is finally 
obtained for the residual gold; the third operation usually accom- 
plishes this. ! 

Instead of effecting the separation of the gold from the platinum 
in this way, the two metals may be dissolved in aqua regia, and 
the gold precipitated by means of ferrous sulphate, as described on 
p. 257. This is a good method. 


279 GRAVIMETRIC ANALYSIS. 


According to Vanino, and Seemann,* the separation is effected 
much more quickly by precipitating the gold from an alkaline 
solution by means of hydrogen peroxide. In order to determine 
the platinum, it is precipitated from the boiling acid filtrate by 
hydrogen sulphide and weighed as metal after ignition in a porce- 
lain crucible. 


Analysis of Commercial Platinum, according to Deville and Stas. 

Five grams of the platinum alloy + are heated for five hours 
at a temperature of about 1000° C. with ten times as much lead 
in a crucible of purified gas-carbon; this crucible is embedded in 
one of clay which is filled with charcoal. After cooling, the lead 
button is treated with very dilute nitric acid until there is no 
longer any gas evolved. 

In this way a solution, A, is obtained, containing about 98.4 
per cent. of the lead used, all the palladium and copper, and small 
amounts of platinum, rhodium, and iron, and a residue, B, consist- 
ing of a black metallic powder, which is filtered off, and will contain 
the remainder of the platinum and rhodium with all of the ine 
and ruthenium. 


1. Treatment of the Nitric Acid Solution A. 


The lead is precipitated by the addition of slightly more than the 
theoretical amount of sulphuric acid, and filtered. If the lead sul- 
phate is pure white, it is washed with dilute sulphuric acid. If it 
is not absolutely white, it is washed with a solution of ammonium 
carbonate until it becomes so; small amounts of lead are dissolved 
by this operation. This last wash liquid, therefore, is concen- 
trated, to precipitate the lead carbonate, filtered, and after making 
acid with hydrochloric acid, added to the main filtrate. 

The solution is evaporated to about 100 c.c., and when cold is 
poured into a saturated solution of ammonium chloride. The 
mixture is heated to boiling and allowed to cool again. The am- 
monium chloroplatinate is filtered off and washed with a saturated 
solution of ammonium chloride; in this way, the greater part of 
the platinum is obtained. 





* Berichte 1899, p. 1971. 
+ All commercial platinum contains other platinum metals, especially 
iridium. 


PLATINUM. 273 


The filtrate from the platinum precipitate is boiled with formic 
acid and ammonium acetate as described on p. 270, b. In this case 
the remainder of the platinum, the palladium, and the rhodium will 
be precipitated. These metals are filtered off, and the copper and 
iron are determined in the filtrate in the usual way. The formic 
acid precipitate (consisting of a black metallic powder) is dried and 
fused with potassium bisulphate in a porcelain crucible. The melt 
is treated with water, the solution decanted from the unattacked 
platinum and washed alternately with ammonium carbonate and 
nitric acid (to remove traces of lead sulphate), then with dilute 
hydrofluoric acid, and finally with water; it is then dried and 
weighed. The filtrate from the platinum contains palladium and 
rhodium. The former is precipitated by the addition of mercurie 
cyanide, and boiling until the odor of hydrocyanic acid has disap- 
peared. The voluminous, yellowish-white precipitate of palladous 
cyanide is washed first by decantation, then upon the filter, dried, and 
ignited at first cautiously and then strongly over the blast until the 
paracyanide is completely destroyed; finally heating in a current 
of hydrogen (as in the case of copper sulphide, p. 183) in order to 
reduce any palladium that has been oxidized by the previous treat- 
ment. As soon as the flame is removed, the supply of hydrogen is 
at once cut off in order to prevent its being absorbed by the metal. 
The palladium is weighed after cooling. 

The rhodium is precipitated from the filtrate by means of formic 
acid, as before, and the deposited metal is dried, ignited in a stream 
of hydrogen, allowed to coo] in the gas, and then weighed. 


2. Treatment of the Residue B. 


The washed residue is warmed with dilute aqua regia (in this 
case 2 vol. nitric acid, 8 vol. hydrochloric acid, and 90 vol. water), 
and in this way solution C is obtained, which contains the rest of 
the lead, platinum, and rhodium, and residue D, consisting of 
~ lamelle of iridium and ruthenium. 


3. Treatment of the Solution C. 


After evaporating to asmall volume, the lead is removed by sul- 
phuric acid, the solution again evaporated, taken up in hydro- 
chloric acid, and the platinum present is precipitated by pouring 


274 GRAVIMETRIC ANALYSIS. 


into a cold saturated solution of ammonium chloride exactly as 
described under 1, p. 272. The platinum precipitate contains a 
little rhodium, and after washing it with a saturated solution of 
ammonium chloride, it is placed at one side for the time being. 

The filtrate, together with the wash water, is evaporated until 
more platinum and rhodium separate out on cooling, and this pre- 
cipitate is filtered off and washed as before. 

Both filters, together with the pgecipitates, are now placed in a 
small, weighed porcelain dish, dried, and reduced at as low a tem- 
perature as possible, in a stream of illuminating gas, and heated 
somewhat in a mufile so as to remove the carbon from the filter. 
The metal thus obtained (platinum-+ rhodium) is weighed. For the 
separation of the rhodium from the platinum, the spongy metal is 
heated in the same dish with potassium bisulphate, gradually rais- 
ing the temperature until a dull-red heat is obtained. After cool- — 
ing, the melt is extracted with water, the unattacked platinum (it 
may still contain small amounts of rhodium) is filtered off, washed, 
and again fused with potassium bisulphate. This operation is 
repeated until the rhodium is completely extracted, which is known 
by the melt showing no yellow color after ten minutes. 

The platinum is washed, ignited, and weighed as described 
under 1. 

The combined filtrates from the platinum contain rhodium and 
still a little platinum. Ammonia, acetic and formic acids, there- 
fore, are added once more, and the solution boiled for a long time. 
The precipitated metal is filtered off, ignited, weighed, afterward 
fused at a distinct red heat with potassium bisulphate, and the 
cold melt extracted with water. If a residue remains after this 
treatment, it is filtered off, weighed, and treated with dilute aqua 
regia. If it dissolves, it is platinum; if it does not, it is rhodium. 

The filtrate from the ammonium chloroplatinate, which con- 
tained some rhodium, is diluted, formic acid and ammonium ace- 
tate are added, and it is gently boiled for two or three days in an 
Erlenmeyer flask connected with a return-flow condenser. The 
liquid evaporates somewhat in spite of the condenser, and the evap- 
orated part is replaced from time to time with a dilute solution of 
ammonium formate. In this way small amounts of platinum and 
rhodium are precipitated, which are filtered off and separated by 


PLATINUM. 275 


fusion with potassium bisulphate as before. In the filtrate there 
are likely to be present traces of platinum, rhodium, and iron. 

The iron is first removed by the addition of chlorine water and 
afterward ammonia; the ferric hydroxide is filtered off, ignited, and 
weighed. In order to remove the last traces of platinum and rho- 
dium, this last filtrate is evaporated to dryness, the residue heated 
with nitric acid in order to remove the ammonium chloride com- 
pletely, and then boiled for a long time with formic acid and am- 
monium acetate. The traces of metal thus obtained are washed 
with hydrofluoric acid and added to the main portion of platinum 
and rhodium. | 


4. Treatment of the Residue D. 


The undissolved, gray lamellz consisting of iridium, ruthenium, 
and small amounts of iron obtained by the action of dilute aqua 
regia, are filtered off, dried, ignited in an atmosphere of hydrogen 
or illuminating gas, and weighed. 

The weighed metal is then fused in a pure gold crucible with 
potassium nitrate and carbonate. Tor this purpose, a previously 
melted mixture of 3 gms. potassium nitrate and 10 gms. potassium 
carbonate is placed in the crucible, the metal added, and the mix- 
ture heated for two hours at a dull-red temperature. In this way 
the ruthenium is changed completely into water-soluble potassium 
ruthenate, K,RuQ,, and the iridium is oxidized to Ir,0,; the latter 
forms, to some extent, a soluble compound with the alkali. 

The melt is treated with water, and the solution, together with 
the suspended Ir203,* is poured into a stoppered cylinder, the pre- 
cipitate allowed to settle, and the clear liquid decanted off into a 
retort. 

The residue remaining in the cylinder is covered repeatedly with 
a dilute solution of sodium hypochlorite and sodium carbonate, 
until the yellow color is completely removed. The decanted liquid 
is added to the main solution in the retort. This solution con- 
tains all the ruthenium and a part of the iridium. It is saturated 
with chlorine in the cold, distilled, and the distillate received in a 
mixture of alcohol (distilled over potassium) and pure hydro- 
chloric acid. 


* Cf. W.Palmaer, Z. anorg. Chem., 10, 332 (1896). 





276 GRAVIMETRIC ANALYSIS. 


After the distillation is complete, the alcoholic distillate is evap- 
orated to dryness and the ruthenium chloride thus obtained is 
reduced to metal by heating in a stream of hydrogen. After 
weighing, the purity of the ruthenium is tested. It must dissolve 
completely’ in a concentrated solution of sodium hypochlorite. 

The liquid remaining in the retort is evaporated to a small vol- 
ume, the insoluble residue remaining in the cylinder (that was 
washed with sodium hypochlorite and,sodium carbonate) is added, 
and the mixture boiled with caustic soda solution, with the addi- 
tion of a little alcohol, until all of the iridium is precipitated. 

The dark-blue precipitate, consisting of iridium oxide and small 
amounts of ferric hydroxide, is filtered off, washed, and strongly 
ignited. The ferric oxide contained in it is then extracted with 
hydrochloric acid containing some ammonium iodide, and the re- 
sidual iridium oxide is washed successively with water, chlorine 
water, and hydrofluoric acid in order to remove gold that came 
from the crucible and silicic acid from the caustic soda. It is then 
ignited in hydrogen and the iridium weighed. 

The iron present in the hydrochloric acid extract is precipitated. 
as ferric hydroxide, ignited, and weighed. Its purity is tested by 
heating in a stream of hydrogen and hydrochloric acid, to see if it 
can be completely changed to ferrous chloride and volatilized as 
such. | 7 
F. Mylius and F. Forster* have recommended that platinum 
be tested for small amounts of impurity by taking three separate 
portions each weighing 10 gms. The first portion is tested for 
‘palladium, iridium, and ruthenium according to the lead pro- 
cedure just described of Deville and Stas. The second portion 
serves for the iron determination; the metal is dissolved in aqua 
regia, the platinum metals precipitated by formic acid, and the 
- jron determined in the filtrate. In the third portion, rhodium, 
silver, copper, and lead are determined by volatilizing the plati- 
num as PtCl,CO at 238° C. (temperature of boiling quinolin) in 
a stream of carbon monoxide and chlorine, and determining the 
above substances in the residue. 

Remark.—The determination of the iron in a separate portion is 
to be recommended, for in the lead procedure some iron is always 
obtained from the carbon crucible. 


See 





* Berichte 1892. 9 364, 


‘SELENIUM. 277 


SELENIUM, Se. At. Wt. 79.2. 


Selenium is usually determined as the element itself. 
Three cases are to be considered: 


I. The selenium is present as alkali selenite or as selenious acid. 
II. The selenium is present as alkali selenate or as selenic acid. 
III. The selenium is present as potassium selenocyanide. 


I. The selenium is present as selenite or as free seleinous acid.— 
‘The solution is acidified with hydrochloric acid, saturated with 
sulphur dioxide gas, boiled, filtered through a Gooch crucible, and 
washed first with water, then with alcohol. The residue is dried 
at 105° C. and weighed. 

Remark.—The precipitation of selenium by sulphur dioxide is 
always quantitative whether the solution is concentrated or dilute, 
whether it contains much or little free acid. This latter fact is of 
importance in the separation of selenium from tellurium, for the 
latter element is not precipitated by sulphur dioxide when consid- 
erable hydrochloric acid is present (cf. p. 279). 

Phosphorous acid does not precipitate selenium from cold, 
dilute, strongly acid solutions; this fact is made use of in the 
separation of selenium from mercury (cf. p. 281). 

Il. The selenium is present as alkali selenate or as free selenir 
acid.—As selenium in the form of selenic acid is not precipitated 
. by sulphur dioxide, phosphoric acid, or hydrogen sulphide, it 
must be first reduced to selenous acid by long-continued boiling 
with hydrochloric acid (cf. Vol. 1); the above procedure is then 
followed. 

Ill. The selenium ts ‘scat as potassium selenocyanide.—The 
solution, concentrated as much as possible; is treated with hydro- 
chloric acid, boiled, allowed to settle, and the precipitate filtered 
through a Gooch crucible, dried at 105° C., and weighed. 

Remark.—¥rom very dilute solutions. of potassium selenocy- 
anide, selenium separates out only very slowly according to this 
method; it is therefore advisable to concentrate the solution as 
much as possible, but when this cannot be done, the boiling with 
hydrochloric acid should be continued for some time and the liquid 
allowed to stand before filtering. 


278 GRAVIMETRIC ANALYSIS. 


In practice, selenium is obtained usually in none of the above 
forms, but as impure selenium (selenium sponge) or as selenide, 
and by the treatment of these substances one or the other of the 
above selenium compounds is obtained. 

If the selenium or selenide is acted upon by concentrated nitric 

acid,* or aqua regia, all of it is dissolved in the form of selenous acid 
(not selenic acid). After evaporating the solution several times 
with hydrochloric acid in order to gemove the excess of nitric 
acid, the selenium is precipitated by sulphur dioxide as described 
under 1. 
: If the finely powdered selenium or selenide is intimately mixed 
- with two parts sodium carbonate and one part potassium ni- 
trate, placed in a nickel crucible, covered with a layer of 
sodium carbonate and potassium nitrate and heated gradually 
until it fuses, all the selenium forms alkali selenate and on ex- 
tracting the melt with water it goes into solution; in this way it 
is separated from most of the remaining oxides. The solution, 
however, often contains small amounts of lead. In order to re- 
move the latter, the filtrate is treated with hydrogen sulphide, 
and again filtered; the solution is freed from hydrogen sulphide 
by boiling. strongly acidified with hydrochloric acid, boiled until 
no more chlorine is evolved and the selenium is precipitated by - 
sulphur dioxide according to II. 

Remarks.—The mixture must be heated very slowly, as other- 
wise some selenium is likely to be lost by volatilization. 

Selenium and very many selenium compounds may be satisfac- 
torily determined as follows: The dry, finely powdered sponge is 
fused at as low a temperature as possible in a current of hydrogen + 
with twelve times as much potassium cyanide. After the mass 
has fused quietly for about fifteen minutes it is allowed to cool in 
hydrogen. It is then extracted with water, the solution is heated 
to boiling, and analyzed according to III. 





* Mercury cyanide is unacted upon by nitric acid, but is dissolved by 
aqua regia. é 
+ A Rose crucible (Fig. 37, p. 185) is used, or a round-bottomed flask 
with a long neck made of difficultly fusible glass, from which the air 1s replaced 
by hydrogen. In the latter case the delivery-tube must be so wide that the 
neck of the flask is nearly filled with it. 


SELENIUM AND TELLURIUM. 279 


It is necessary to boil the solution of potassium selenocyanide 
before acidifying it, for small amounts of potassium selenide 
(K2Se) are almost always present, and on acidifying with hydro- 
_chloric acid this is decomposed with evolution of hdyrogen selenide. 
On boiling, the potassium selenide is changed to potassium 
selenocyanide according to the equation: 


K.Se+ KCN + H20+0=2KOH+ KCNSe. 


TELLURIUM, Te. At. Wt. 127.5. 


Tellurium is usually determined as the element itself, 

If sulphur dioxide is conducted into a hydrochloric acid solu- 
tion containing tellurous acid, black tellurium is quantitatively 
precipitated, provided the solution does not contain too much acid. 
If tellurous acid is dissolved in 200 ¢c.c. of hydrochloric acid, sp. gr. 
1.175, no tellurium will be precipitated on passing sulphur dioxide 
into the cold solution. If, however, the solution is diluted with 
an equal volume of water and sulphur dioxide is passed into the 
boiling solution, all the tellurium will be precipitated. The pre- 
cipitate is filtered off, washed with water until free from chlorides, 
then with alcohol, dried at 105° C. and weighed. The oxidation of 
the tellurium during the drying is so slight that it can be disre- 
garded.* 


Separation of Selenium and Tellurium from the Metals of 
Groups III, IV, and V. 


By conducting sulphur dioxide into the solution fairly acid with 
hydrochloric acid, the selenium and tellurium will be quantita- 
tively precipitated while the other metals remain in solution. 





* The presence of nitric acid prevents the complete precipitation of the 
tellurium by means of sulphur dioxide and similarly the presence of sulphuric 
acid is harmful. To remove nitric acid, sodium chloride is added and the 
solution evaporated to dryness repeatedly with hydrochloric acid. According 
to Brauner the addition of sodium chloride is absolutely necessary, as other- 
wise an appreciable amount of tellurium will be volatilized as chloride. 
A. Gutbier (Ber. 34, 2724 (1901) ) reports that all these difficulties are over- 
come by precipitating tellurium from a hot solution by means of hydrazine 
hydrate or hydrazine hydrochloride, but not the sulphate. See also P. 
Jannasch and M. Miller, Ber. 31, 2393 (1898). 


280 GRAVIMETRIC ANALYSIS. 


Separation of Selenium and Tellurium from the Metals of Group II. 
(a) From Copper, Bismuth, and Cadmium. 


Sulphur dioxide is passed into the boiling solution, acid with 
hydrochloric acid, whereby all of the selenium and tellurium and 
usually some of the bismuth are precipitated. The precipitate 
after being washed is dissolved in nitric acid, the solution evapo- 
rated to dryness, taken up in concentrated hydrochloric acid, 
diluted with a little water and precipitated with hydrogen sul- 
phide. The precipitate, consisting of the three sulphides, is washed 
and then treated with sodium sulphide solution whereby selenium 
and tellurium pass into solution while the bismuth remains behind 
as its brown sulphide and is filtered off. 

The solution containing the selenium and tellurium i is acidified 
with nitric acid, carefully evaporated to dryness and the residue 
boiled with 200 c.c. of hydrochloric acid, sp. gr. 1.175, until there 
is no longer any evolution of. chlorine: The deposited sulphur is 
then filtered off through a Gooch crucible, and the filtrate satu- 
rated with sulphur dioxide gas; all the selenium is in this way pre- 
cipitated. The latter is filtered off through a Gooch crucible and 
washed successively with a mixture of 90 vol. HCl (sp. gr. 1.175) 
and 10 vol. water, dilute hydrochloric acid, and finally absolute 
alcohol. The precipitate is dried at 105° C. and weighed. The 
filtrate is diluted with an equal volume of water and the tellurium 
precipitated by passing sulphur dioxide into the boiling solution. 
This precipitate is washed with water until free from chlorides, 
then with absolute alcohol, after which it is ads at 105° C. and 
weighed. 

Remark.—The above method is suitable for the separation of 
selenium and tellurium from small amounts of bismuth, but does 
not effect the separation of selenium (and tellurium) from copper. 
In this case, more or less copper selenide is formed according to 
the conditions, and this compound is not decomposed quantita- 
tively by sodium sulphide.* In this case, the method of B. 
Brauner and B. Kuzmat may be used. 


* Cf. E. Keller, J. Am. Chem. Soc., 19, 771. 
t Berichte, 1907, 3362. 





SELENIUM AND TELLURIUM. 281 


The tellurium and selenium are precipitated in a pressure 
flask, by means of SO:, the precipitate, which is contaminated 
with copper, antimony and bismuth, is filtered (using a Gooch 
crucible) washed, dissolved in nitric acid, the solution evaporated 
to dryness and the residue taken up in caustic potash solution 
(1:5). The alkaline solution is placed in an Erlenmeyer flask 
upon a water-bath, and little by little 4-6 gm. of ammonium 
persulphate are added, whereby the potassium tellurite is oxidized 
to potassium tellurate and the selenite to selenate. When all 
the persulphate has been introduced, the solution is heated to 
boiling to decompose the excess of persulphate, then acidified 
with sulphuric acid and allowed to cool. Now, 100 c.c. of H,S- 
water are added, the excess of the H,S expelled by passing CO, 
through the solution, and the precipitated CuS (BiS,, Sb.S,) 
filtered off, and treated as described on p. 235. The filtrate is 
boiled with hydrochloric acid to reduce the telluric acid to 
tellurous acid, and the solution is reduced by means of SO, and 
analyzed as described above. | 

The first filtrate from the impure Te and Se will contain the 
greater part of the Cu, Bi, etc. 


(b) From Antimony, Tin and Arsenic. 


If considerable antimony is present, tartaric acid is added to 
the solution, and the selenium and tellurium are then precipitated 
by boiling with sulphur dioxide. — 

According to Muthmann and Schréder* this method of 
separating tellurium from antimony is not quantitative; some 
antimony is always precipitated with the tellurium. A. Gutbier,t 
however, finds that a perfect separation can be accomplished by 
means of hydrazine hydrochloride (not the sulphate). 


(c) From Mercury. 


The mercury selenide, or telluride, is dissolved in aqua regia, chlo- 
rine water is added, and the solution is diluted largely with water. 
Phosphorous acid is added,{ and after twenty-four hours standing, 

* Z. anorg. Chem., 14, 433 (1897). 

+ Z. anorg. Chem., 32, 263 (1902). 

t Selenous and tellurous acids are not precipitated by phosphorous acid 
from dilute hydrochloric acid solution, but are precipitated from hot con- 
centrated solutions. 





282 GRAVIMETRIC ANALYO5IS. 


the mercury is precipitated completely as mercurous chloride, and 
is determined as such according to p. 170. 

The filtrate containing selenium and tellurium is concentrated, 
taken up in water, and the selenium separated from the tellurium 
according to the method of Keller (see below.) 


(d) From Gold and Silver. 


The separation of selenium and teflurium from silver offers no 
difficulty, inasmuch as the latter can be precipitated by hydro- 
chloric acid and determined as the chloride. 

The gold is precipitated as described on p. 257 by oxalic acid 
and the selenium and tellurium in the filtrate by means of sulphur 
dioxide. The three metals may also be precipitated together by 
sulphur dioxide, weighed, and the selenium and tellurium after- 
ward volatilized by roasting, leaving the gold behind. © 

Tellurium may be separated from gold by precipitating the lat- 
ter with ferrous sulphate. In the case of selenium, however, it is 
also precipitated quantitatively by ferrous sulphate from solutions 
strongly acid with hydrochloric acid. 


Separation of Selenium from Tellurium. 
A. Method of E. Keller.* 


Keller’s method is based upon the fact that tellurous acid is not 
precipitated from solutions strongly acid with hydrochloric acid 
while selenium is precipitated quantitatively. 

Procedure.—The mixture of the two elements precipitated 
by sulphur dioxide is dissolved in nitric acid and carefully — 
evaporated to dryness. The dry mass is treated with 200 c.c. of 
hydrochloric acid (sp. gr. 1.175), boiled to remove the nitric acid 
and saturated with sulphur dioxide. The precipitated selenium 
is filtered through a Gooch crucible, washed first with a mixture 
of 90 vol. HCl (sp. gr. 1.175) and 10 vol. water, then with dilute 
hydrochloric acid, then with water until free from chloride, finally 
with absolute alcohol. The selenium is then dried at 105° C. and 
weighed. The filtrate is diluted with an equal volume of water, 





* Jour. Amer. Chem. Soc., 19, 771. 


SEPARATION OF SELENIUM FROM TELLURIUM. 283 


heated to boiling, and the tellurium precipitated by sulphur diox- 
ide and treated in exactly the same way as the selenium. 

Accerding to Keller, this method gives thoroughly satisfactory 
results, as long as the amount of tellurium present does not exceed 
5 gms. Even then the separation can be effected by increasing the 
amount of acid to 450 c.c. 


B. The Potassium Cyanide Method. 


The precipitate of selenium and tellurium produced by sulphur 
dioxide is fused with twelve times as much of pure (98 per cent.) 
potassium cyanide, in an atmosphere of hydrogen, as described on 
p. 278. The tellurium is almost wholly changed to potassium 
telluride, K,Te (a small amount of potassium tellurocyanide is 
probably formed), while the selenium is changed for the most part 
into potassium selenocyanide, and to a slight extent into potassium 
selenide. 

The brown melt is dissolved in water, and a slow current of air is 
conducted through the solution whereby the K,Te is rast 
decomposed according to the equation 


K,Te+ H,O+ 0=2KOH-+ Te. 


After standing twelve hours the tellurium is filtered off through a 
Gooch crucible, washed with water, then with absolute alcohol, 
dried at 105° and weighed. 

The colorless filtrate is heated to boiling * in order to change any 
potassium selenide into the double cyanide; it is then acidified 
with hydrochloric acid under a good hood (hydrocyanic acid!), 
filtered, and the selenium determined according to p. 277. 

Remark.—This method gives slightly low results for tellurium 
and high values for selenium. This is due to the fact that a little 
potassium tellurocyanide is formed by the fusion and this com- 
‘pound is not decomposed by the current of air, but is subsequently 
precipitated with the selenium on acidifying the solution. 





#0t. p. 279: 


284 GRAVIMETRIC ANALYSIS. 


Determination of Selenium and Tellurium in Crude Copper. 


Many copper ores contain selenium and tellurium, so that the 
crude copper obtained from such ores always contains these ele- 
ments. The amount present may be determined, according to 
Keller,* as follows: According to the amounts of selenium and 
tellurium present, from 5 to 100 gm. of the copper are taken for 
the analysis The sample is dissolved in nitric acid and an excess 
of ammonia is added whereby the phosphorus, arsenic, antimony, 
tin, bismuth, selenium, and tellurium are precipitated together with 
the ferric hydroxide, while the copper is held in solution by the 
excess of ammonia. The precipitate is filtered off and washed with 
dilute ammonia-water until the copper is completely removed. The 
precipitate is dissolved in hydrochloric acid and this solution satu- 
rated with hydrogen sulphide in the cold, whereby selenium and 
tellurium together with arsenic, antimony, tin, and bismuth are 
thrown down as sulphides and are separated by filtration from the 
iron and phosphorus. The precipitate thus obtained is treated 
with sodium sulphide and filtered. The filtrate containing all the 
selenium and tellurium in the presence of arsenic, antimony, and 
tin as sulpho salts is acidified with nitric acid and carefully evapo- 
rated to dryness. The residue is dissolved in 200 c.c. of hydro- 
chloric acid (sp. gr. 1.175) and treated as described on p. 282, A. 


MOLYBDENUM, Mo. At. Wt. 96.0. 
Form: Molybdenum Trioxide, MoO,. 


If the molybdenum is present as ammonium molybdate, a 
weighed portion is heated in a spacious porcelain or platinum 
crucible, at first carefully and later to a dull red heat; this leaves 
the molybdenum trioxide behind in the form of a dense powder, 
appearing yellow when hot and almost white when cold. 

There is no danger of losing any of the molybdenum by volatili- 
zation, provided the dull red heat is not exceeded. 

If the molybdenum is present as alkali molybdate, it is changed 
to mercurous molybdate or to its sulphide, and then analyzed as 
described below. : 








——= 


* Jour. Amer. Chem. Soc., 22, 241. 


PRECIPITATION OF MOL YBDENUM. 285 


Precipitation of Molybdenum as Mercurous Molybdate. 


In the course of analysis it is frequently necessary to determine 
molybdenum in alkali molybdates obtained by fusion with an alkali 
carbonate. é 

The greater part of the alkali is neutralized with nitric acid, 
and to the slightly alkaline solution a barely acid solution of mer- 
curous nitrate is added until no further precipitation is effected. 
The liquid is then heated to boiling, the black precipitate, consisting 
of mercurous carbonate and mercurous molybdate, is allowed to set- 
tle, is filtered and washed with a dilute solution of mercurous 
nitrate. The precipitate is dried, transferred as completely as pos- 
sible to a watch-glass, and the precipitate remaining on the filter is 
dissolved in hot dilute nitric acid into a large porcelain crucible, 
The solution is then evaporated to dryness, the main portion of the 
precipitate added to the residue, and the whole is heated very care- 
fully over a low flame until the mercury is completely volatilized, 
after which the residual molybdenum trioxide is weighed. 

Remark.—It was formerly customary to add a slight excess of 
mercurous nitrate solution and then to add mercuric oxide to neu- 
tralize the excess of nitric acid (the solution of mercurous nitrate 
contains free nitric acid). According to the above procedure of 
Hillebrand, the addition of mercuric oxide is wholly superfluous, 
for the basic mercurous carbonate suffices to remove the slight 
amount of free nitric acid. 


Precipitation of Molybdenum as Molybdenum Sulphide. 


The precipitation of molybdenum as the sulphide can take place 
in two different ways: either the acid solution may be precipitated 
by hydrogen sulphide gas, or the solution of ammonium sulpho- 
molybdate may be acidified with dilute acid. 


(a) Precipitation of Molybdenum Sulphide from Acid Solutions. 


The molybdenum solution, slightly acid with sulphuric acid,* is 
placed in a small pressure-flask and saturated in the cold with 
hydrogen sulphide. The flask is closed, heated on the water- 





* In many cases, e.g., for the separation of Mo from Ba, Sr, and Ca, it is 
necessary to effect the separation in a hydrochloric acid solution. 


286 GRAVIMETRIC ANALYSIS. 


bath until the precipitate has completely settled, and filtered 
after it has become cold. The precipitate is washed with very 
dilute sulphuric acid and finally with alcohol until the acid has 
been completely removed. The moist filter is placed in a large 
porcelain crucible and dried upon the water-bath. The crucible 
is then covered and very carefully heated over a small flame until 
no more hydrocarbons are expelled. The cover is then removed, 
the carbon burned from the sides of*the crucible at as low a tem- 
perature as possible, and, by raising the temperature gradually, 
the sulphide is changed to oxide. The operation is finished when 
no more sulphur dioxide is formed. After cooling, a little mercuric 
oxide suspended in water is added to the contents of the ervcible, 
the mixture is well stirred, evaporated to dryness on the water- 
bath, the mercuric oxide is removed by gentle ignition, and the resi- 
due of molybdenum trioxide is weighed. The mercuric oxide is 
added in order to remove particles of unburned carbon. 

It is much easier to transform the molybdenum trisulphide into 
the oxide as follows: The sulphide is filtered through a Gooch cru- 
cible, washed with water containing sulphurie acid, and then with 
alcohol, and dried at 100°C. The crucible is placed within a larger 
nickel one, covered with a watch-glass,« and carefully heated over 
a small flame whereby the sulphide is for the most part changed 
to the oxide. As soon as the odor of sulphur dioxide can no longer 
be detected, the watch-glass is removed and the open crucible 
heated until it is brought to a constant weight. The molybdenum 
oxide thus obtained always contains traces of SO,, and consequently 
has a bluish appearance. The results, nevertheless, are excellent. 


(b) Hydrogen Sulphide is passed into the Ammoniacal Molyb- 
denum Solution 


until it assumes a bright-red color, when it is acidified with sul- 
phuric acid and the precipitate treated as described under (a). 
The Separation of Molybdenum from the Alkalies 


ean take place by precipitation as mercurous molybdate or as sul- 
phide, as described above. 








* To avoid loss by decrepitation. 


SEPARATION OF MOLYBDENUM. 287 


Separation of Molybdenum from the Alkaline Earths. 


The substance is fused with sodium carbonate, the melt ex- 
tracted with water and filtered. The solution contains all the 
molybdenum as alkali molybdate, while the alkaline earths remain 
undissolved as carbonates. From the aqueous solution the molyb- 
denum is determined as previously described. 


Separation of Molybdenum from the Metals of the Ammonium 
Sulphide Group. 


The molybdenum is precipitated as sulphide (preferably from a 
sulphuric acid solution) by treatment with hydrogen sulphide under 
pressure (sée p. 286). If the solution contains titanium, it is better 
to first add ammonia and ammonium sulphide, whereby the metals 
of Group III will be precipitated and the molybdenum will remain 
in solution in the form of its sulpho salt. After filtration, the 
molybdenum is precipitated as sulphide by the addition of acid 
(see p. 286, b). 


Separation of Molybdenum from the Metals of Group II. 
(a) From Lead, Copper, Cadmium, and Bismuth. 


The solution is treated with caustic soda and then with sodium 
sulphide, digested some time in a closed flask, and filtered. The 
molybdenum remains in solution as its sulpho salt, while the other | 
metals are precipitated as sulphides. After filtering, the solution 
is acidified with sulphuric acid and heated in a pressure-flask until 
the precipitate has settled and the supernatant liquid appears col- 
orless. After allowing to cool, the molybdenum sulphide is fil- 
tered off and converted to oxide, as described on p. 286. 


(b) From Arsenic. 


_ The solution, which must contain the arsenic as arsenic acid, is 
treated with ammonia, the arsenic precipitated by magnesia mix- 
ture (see p. 206) and filtered off. The filtrate is acidified with sul- 
phurie acid and the molybdenum precipitated as sulphide by means 
of hydrogen sulphide. 


288 GRAVIMETRIC ANALYSIS. 


Separation of Molybdenum from Phosphoric Acid. 


The phosphoric acid is precipitated from the ammoniacal solu- 
tion as magnesium ammonium phosphate (see phosphoric acid) 
and the molybdenum is precipitated as sulphide from the filtrate 
(cf. p. 285, a). Another way is to saturate the ammoniacal 
solution with hydrogen sulphide, acidify with hydrochloric acid, 
and then precipitate the molybdenum" sulphide. In the filtrate 
the phosphoric acid is precipitated as magnesium ammonium 
phosphate under the customary conditions. 


TUNGSTEN, W. At. Wt. 184.0. 


Tungsten is determined as its trioxide, WO3. 

If the tungsten is present as ammonium tungstate, as mercu- 
rous tungstate, or as tungstic acid, it is readily changed by ign‘tion 
in the air to yellow tungsten trioxide. Since the trioxide is not 
volatile there is no loss to be feared during its ignition. It is even 
advisable to heat the crucible finally over a good Teclu burner or 
over the blast-lamp. 

If the tungsten is present as alkali tungstate, the tungstic acid 
may be precipitated as such, 0» by means of mereurous nitrate as 
mercurous tungstate; by ignition the yellow trioxide is obtained 
and weighed. 


Precipitation of Tungstic Acid. 


The aqueous solution of the alkali tungstate is treated with an 
equal volume of concentrated hydrochloric acid, evaporated to 
dryness on the water-bath and heated for an hour in the hot 
closet at 120°. The residue is moistened with a little hydro- 
chloric acid, diluted, boiled, filtered and washed with 6 per cent. 
hydrochloric acid, or with 10 per cent. ammonium nitrate solu- — 
tion. The yellow tungstic acid is ignited and weighed. In this 
way the greater part of the tungstic acid is precipitated, but 
there remains in the filtrate a weighable amount which can be 
recovered by repeated evaporations with hydrochloric or nitric 
acid. 


TUNGSTEN. 289 


Remark.—The reason why the tungstic acid is not removed 
completely by the first treatment with nitric or hydrochloric 
acid is that there is always a small amount of an acid tungstate 


formed: 
‘Na,.WO,+3W0O,=Na,W,0O,,, 


which it is hard to decompose by acids. If, however, the filtrate 
is evaporated to dryness with ammonia, then, according to 
Philipp,* the acid tungstate is converted into ordinary tungstate, 


Na,W,0,,+6NH,OH= Na,WO,+3(NH,),WO,4+3H,0, 


and the latter is decomposed by the nitric acid treatment. 

This is perfectly true, but on evaporating with acid some more 
of the acid tungstate is formed, which can be transformed into 
insoluble tungstic acid only by repeatedly evaporating the filtrates 
to dryness with hydrochloric or nitric acid. 

The washing of the precipitate with acid, or with ammonium 
nitrate solution, is necessary in order to prevent the formation 
of hydrosol, which would result if pure water were used. 


Precipitation of Tungsten as Mercurous Tungstate, according to 
Berzelius.+ 


In the majority of cases it is a question of separating tungstic 
acid from a solution obtained after fusing with sodium carbonate. 
The concentrated solution is treated with a few drops of methyl 
orange, nitric acid is added until the indicator turns pink, the 
solution is boiled to expel all the carbonic acid, allowed to cool, 
and then an excess of mercurous nitrate solution is added. The 
yellow precipitate settles quickly and the supernatant liquid 
should appear clear as water. After standing three or four hours, 
the precipitate is filtered off, washed with water containing mer- 
curous nitrate (5 c.c. saturated mercurous nitrate solution 
diluted with water to 100 c.c.) dried, ignited in a porcelain crucible 
under a good hood, using the flame of a Bunsen burner, and 

weighed as WO,. 
Remark.—The tungstic acid is quantitatively precipitated by 

" * Ber., 15, 501 (1882). 

~ fJahresber., 21, II, 143. Cf. O. v. der Pfordten, Ann. 222, 152 (1883). 





290 GRAVIMETRIC ANALYSIS. 


a single treatment with mercurous nitrate. The process is pref- 
erable to the above, therefore, when nothing else is present which 
precipitates under the same conditions. 


Precipitation of Tungsten as Benzidine Tungstate, according to 
G. v. Knorre.* 


If a neutral solution of sodium tungstate is treated with 
benzidine hydrochloride, a white flecculent precipitate of ben- 
zidine tungstate is formed and the precipitate is insoluble in water 
containing benzidine hydrochloride; when formed in the cold 
it is hard to filter and, on being washed with pure water, tends 
to run through the filter. If the precipitate is formed from a 
hot solution, however, it comes down in a more compact condi- 
tion and after cooling ¢ can be easily filtered and washed without 
loss with water containing benzidine hydrochloride. | | 

The moist precipitate is ignited in a platinum crucible, heated 
to 800° in an electric oven, and the residue of WO, is weighed. 

The precipitation can also take place satisfactorily from a 
cold solution if, before adding the precipitant, a little dilute 
sulphuric acid or alkali sulphate is added to the solution. In this 
case u mixture of crystalline benzidine sulphate and amorphous 
benzidine tungstate is formed which can be filtered after stand- 
ing five minutes. The benzidine sulphate is entirely volatile, 
so that equally good results are obtained by either of the above 
two procedures. 

If the tungsten is present as tungstate after fusing with 
sodium carbonate, the melt is dissolved in water, a little methyl 
orange is added to the clear solution, then hydrochloric acid 
until the pink color is obtained, and finally 10 c.c. of 0.1 N. 
sulphuric acid. Benzidine hydrochloride gives a_ precipitate 
which can be filtered in five minutes. The washing with dilute 
benzidine hydrochloride is continued until the evaporation 
of a few drops of the filtrate on platinum foil leaves no weighable 
residue. The precipitate is ignited wet as described above. 

* Ber., 88, 783 (1905). 

7 Since benzidine tungstate is appreciably soluble in hot water contain- 


int benzidine hydrochloride, it is necessary in all cases to postpone the 
filtration until the solution is cold. 





TUNGSTEN. 291 


5 Preparation of the Benzidine Solution. 


q 


Twenty grams of commercial benzidine are triturated in a 
mortar with water, washed with about 400 ¢.c. of water into a 
beaker, treated with 25 c.c. hydrochloric acid (sp. gr. 1.2), heated 
until solution is completed and a brown liquid is formed, which 
is filtered and diluted to a volume of one liter.* Of this solu- 
tion, 5.6 c.c. are sufficient to precipitate 0.1 gm. of WOs3. 

If-the analysis is carried out with the aid of sulphuric acid, 
it is necessary to add at least one cubic centimeter of the benzi- 
dine solution for 10 c.c. of 0.1 N sulphuric acid added. 


- Preparation of the Wash Liquid. — 


Ten c.c. of the above solution are diluted with distilled water 
to a volume of 300 ec.c. : 

Remark.—The method is excellent. W. Kunz found 0.1028 
gm., 0.1029 gm., 0.1026 gm. and 0.1033 gm. in aliquot parts of 
a solution supposed to contain 0.1029 gm. of WQ,. 


Determination of Tungsten in Tungsten Steel. 
(a) Method of G. v. Knorre.+ 


V. Knorre observed that on dissolving tungsten steel in 
hydrochloric or dilute sulphuric acid, out of contact with the air, 
all the tungsten remains’ behind as metal, contaminated with 
more or less iron. If the finely divided tungsten is filtered off, 
it oxidizes quickly in the air, forming grayish-yellow tungstic 
acid, which on further washing with water gives a turbid filtrate; 
the tungstic acid does not pass through the filter if, instead of 
using water, the washing is accomplished with a dilute solution 
of benzidine hydrochloride. By the treatment of the tungsten 
steel with acids, therefore, the greater part of the iron is separated 
from the tungsten. To complete the separation, the washed 
“precipitate is ignited wet in a platinum crucible, the residue is 
fused with four times as much sodium carbonate, the melt 





* Instead of dissolving the benzidine in hydrochloric acid, 28 gms. of 
benzidine hydrochloride may be dissolved in 1 liter of water to which 6 e.c. 
of hydrochloric acid have been added. 

+ Ber., 38, 783 (1905). 


292 GRAVIMETRIC ANALYSIS. 


extracted with a water, the ferric oxide filtered off, and the tung- 
sten determined as described on p. 289. It is probable that 
aluminium-tungsten alloys can be analyzed in a similar manner. 


(b) L. Wolter’s Method.* 


L. Wolter, who has carefully studied the determination of 
tungsten in tungsten steel, founds that most of the methods 
are unpractical because they require the metal in a very fine 
condition. Now a steel with a high percentage of tungsten 
is so hard that it is practically impossible to get borings or filings 
without the steel tools becoming badly worn; in such cases 
the sample to be analyzed becomes contaminated with foreign 
material and the accuracy of the analysis is affected. For this 
reason if it is required to analyze a steel with a high percentage 
of tungsten, it should suffice to hammer the sample in a steel 
mortar until a coarse powder is obtained. Unfortunately a 
coarse powder dissolves extremely slowly in dilute or concen- 
trated acids and fusion with sodium carbonate and potassium 
nitrate, or with sodium peroxide, has little effect upon large 
particles; it is otherwise with a potassium bisulphate fusion, 
whereby even large particles of tungsten steel are readily attacked. 

Procedure.—From 0.2-0.5 gm. of the coarse material, which 
may be in the form of coarse pieces, is fused in a 40-45 c.c. 
platinum crucible with thirty times as much potassium bisul- 
phate. To prevent too violent effervescence, at first only 
one-third of the bisulphate is added and the well-covered, inclined 
crucible is heated over a small flame until white vapors are evolved. 
The flame is then removed for half a minute because a fairly 
violent reaction is now taking place within the crucible. In 
fact when this point is reached, a strong effervescence can be 
heard. After cooling somewhat, the remainder of the bisulphate 
is added in two separate portions. The reaction now takes 
place more slowly and the temperature is raised until the bottom 
of the crucible begins to redden and dense white vapors are 
evolved. The heating is now carefully continued until finally 





* Chem. Ztg., 1910, 2. 


TUNGSTEN. : 293 


the whole crucible is heated to redness. After about fifteen 
minutes the reaction will be finished. The molten mass in the 
crucible should show merely a gentle evolution of gas and there 
should be no black particles of unattacked steel visible. When 
this point is reached, the mass is allowed to cool, the crucible 
and cover boiled with water, carefully washed, and the water 
added is measured so that the volume of the solution will amount 
to about 60 or 75 c.c. The liquid, which is somewhat turbid with 
tungstic acid, is treated with 20 c.c. of concentrated hydro- 
chloric acid and boiled until the precipitated tungstic acid has 
become a pure yellow. After standing half an hour on the water- 
bath, the solution is filtered through a small filter and the pre- 
cipitate washed with 10 per cent. ammonium nitrate solution. 
The lemon-yellow precipitate is dissolved on the filter in hot, 
dilute ammonia, the solution caught in a weighed platinum 
crucible, and the filter washed with ammonium nitrate. The 
solution of ammonium tungstate in the crucible is evaporated 
to dryness on the water-bath, then covered with a watch-glass, 
and very cautiously heated over a free flame. until no more 
ammonium salts are evolved. After igniting the residue over 
the full flame of a Bunsen burner, it is weighed as WO,. In 
this way the greater part of the tungsten is obtained. The 
filtrate from the tungstic acid precipitate still contains tungsten. 
In order to recover the later, it is treated as described on page 288. 
If the steel contained silicon, the weighed tungstic acid will always 
contain silica; it is covered with a few drops of hydrofluoric 
acid, evaporated to dryness, again treated with hydrofluoric 
acid, evaporated once more and finally ignited over the flame 
of a Bunsen burner. The crucible will now contain no silica, 
but only tungstie acid. 


Separation of Molybdenum from Tungsten. 


(a) The Sulphuric Acid Methad. 


This method, proposed by M. Ruegenberg and E. F. Smith,* 
depends upon the fact that unignited molybdic acid is readily dis- 





* J. Am. Chem. Soc., 22, 772. 


294 GRAVIMETRIC ANALYSIS. 


solved by warming with sulphuric acid (sp. gr. 1.378), while tung-. 
stic acid is not. 

W. Hommel* tested this method in the author’s laboratory, and 
could not obtain good results except by digesting the moist oxides 
with concentrated sulphuric acid, and afterward diluting with three 
times as much water. 

Procedure.—(a) Both acids. are present in a moist, freshly 
precipitated state. ° 

The mixture is covered with concentrated sulphuric acid in a 
porcelain dish and heated over a free flame. By this means, usu- 
ally a small amount of the tungstie acid is oxidized to the blue 
oxide, so that the yellow precipitate of tungstic acid is tinted with 
green. On adding one or two drops of dilute nitric acid, the green 
color disappears and the tungstic acid is of a pure yellow color. 
After digesting for half an hour, the separation is complete. 
After cooling, the liquid is diluted with three times its volume of 
water, filtered, washed with water containing sulphuric acid, then 
two or three times with alcohol, ignited (after burning the filter by 
itself) in a porcelain crucible, and weighed as WQ,. 

The molybdenum is precipitated from the filtrate by passing 
hydrogen sulphide into the sulphuric acid solution in a pressure- 
flask, and the precipitate is treated as described on p. 286. 

If only a little sulphuric acid is used for the separation, the 
filtrate from the tungstic acid can be evaporated in a platinum 
dish, the sulphuric acid driven off for the most part, and the residue 
washed into a weighed platinum crucible with ammonia, and then 
evaporated, ignited, and weighed. In case large amounts of molyb- 
denum are present, however, it is always safer to precipitate the 
molybdenum as sulphide. 


(8) Tungsten and Molybdenum are Present in the Form of thetr 
Ignited Oxides. 


These ignited oxides cannot be separated by treatment with 
sulphuric acid. According to W. Hommel, they can readily be 
brought into solution by heating for half an hour on the water-bath 
with concentrated ammonia in a pressure-flask, shaking frequently. 

After cooling, the contents of the flask, whether dissolved or 





* Inaug, Dissert., Giessen, 1902. 


TUNGSTEN. 205 


not, are washed into a porcelain dish, evaporated to dryness, and 
treated as described under (a). 

It is still better to fuse the ignited oxides with four times as 
much sodium carbonate, and treat the melt as described under (a). 


(b) Sublimation Method.* 


If a mixture of the trioxides of tungsten and molybdenum, 
or of their alkali saits, is heated at 250-270° C. in a current of 
dry hydrochloric acid, the molybdenum is volatilized completely 
as MoO,.2HCl, which collects on the cooler parts of the tube as a 
beautiful, white, woolly sublimate, while the tungsten trioxide re- 
mains behind in the boat. 

Procedure.—The oxides of the two elements, or their sodium 
salts, are weighed into a porcelain boat, and the latter is placed in a 
tube made of difficultly fusible glass, of which one end is bent verti- 
cally downward and is connected with a Péligot tube containing a 
little water. The horizontal arm of the tube is passed through a 
drying-oven (to serve as an air-bath) (see Fig. 19, p. 3&), and is 
connected with apparatus for generating hydrochloric acid gas. 
The hydrochloric acid before reaching the tube is slowly passed 
through a flask containing concentrated hydrochloric acid, and 
then through sulphuric acid. As soon as the temperature has 
- reached about 200° C. the sublimation of the molybdenum begins. 
From time to time the sublimate collecting in the tube is driven 
toward the Péligot tube ¢ by carefully heating with a free flame, so 
that it will be possible to see whether any more molybdenum is being 
volatilized. After heating for an hour and one-half or two hours, 
the operation is usually complete. The boat, now containing tung- 
sten trioxide, or the latter with sodium chloride, is removed, and in 
case only the former is present, it is weighed after drying in a desic- 
cator over caustic potash. In case, however, sodium chloride is 
present (when the tungsten was originally present as sodium tung- 
' state) this is removed by treatment with water, and the filtered 
WO, is weighed. 

* Péchard, Comptes rendus, 114, p. 173, and 46, p. 1101. 

+ By the absorption of the MoO,.2HCI in the water of the Péligot tube, 
the brick-red acid chloride, Mo,0.Cl,, is often formed: 

3[Mo0,.2HCI] + 2HCI= 4H,0 + Mo,0,Cl,. 
This substance is insoluble in hydrochloric acid, but readily soluble in nitrie acid. 





296 GRAVIMETRIC ANALYSIS. 


_ For the determination of the molybdenum, the sublimate in the 
tube is washed out by means of water containing a little nitric acid, 
and finally the nitric acid solution of the entire sublimate is care- 
fully evaporated to dryness in a porcelain dish. The residue is 
dissolved in ammonia, washed into a porcelain crucible, evaporated 
to dryness, and changed to the oxide by gentle ignition. 


(c) The Tartaric Acid Method of H. Rose. 


The alkali salts of the two metals are dissolved in considerable 
tartaric acid, an excess of sulphuric acid is added, and the molyb- 
denum precipitated according to p. 285, by hydrogen sulphide in 
a pressure-flask. The molybdenum sulphide is filtered off and 
changed by roasting in the air to the trioxide. For the determi- 
nation of the tungsten, the tartaric acid is first destroyed by re- 
peated evaporation with nitric acid, and the precipitated tungstic 
acid is finally filtered off and changed by ignition to the trioxide. 

Remark.—This method gives-correct results, but is not so satis- 
factory as the preceding one on account of the time consumed in 
removing the tartaric acid. 


Analysis of Wolframite (Wolfram). 


The monoclinic Wolframite is an isomorphous mixture of 
Ferberite, FeWO,4, and Hiibnerite, MnWO,4, but often contains 
small amounts of silicic, niobic, tantalic, and stannic acids, 
besides calcium and magnesium. 

About 1 gm. of the extremely finely-powdered mineral is fused 
with 4 gms. sodium carbonate in a platinum crucible over a good 
burner for from one-half to three-quarters of an hour. After cool- 
ing, the melt is boiled with water and filtered. The residue con- 
tains iron, manganese, calcium, and magnesium, and sometimes 
small amounts of niobic and tantalic acids. The solution contains 
all the tungstic acid, and silicic acid (stannic acid). 

The tungstic acid is separated, as above described, either by 
evaporating with nitric acid or by precipitating with mercurous 
nitrate. The precipitate is ignited, and weighed as impure WO,. 

The oxide obtained in this way almost always contains silicic 
acid and sometimes stannic acid. To remove the former the resi- 


- TUNGSTEN. 207 


due is heated with hydrofluoric and sulphuric acids, first on the 
water-bath and finally over a free flame, and the residue is weighed. 
The difference shows the amount of silicic acid present. Stannic 
acid is usually present in such small amounts that it is not usually 
determined. 

The separation of tungsten from tin, apes may be effected 
‘according to Rammelsberg) by repeated ignition with pure, dry 
ammonium chloride. The tin is volatilized as stannic chloride, 
while the tungsten remains behind. 

This last operation is conducted as follows: The Feathic obtained 
after the treatment with hydrofluoric acid is mixed with six to eight 
times as much ammonium chloride, the crucible is placed within 
a second larger crucible,* and the latter is covered and ignited 
until the ammonium chloride is completely expelled. This opera- 
tion is repeated three times. The inner crucible is then heated 
with ready access of air until its contents become of a pure-yellow 
color, after which it is cooled and weighed. The ignition with 
ammonium chloride and weighing of the residue is repeated until 
a constant weight is obtained. 

To determine the iron, manganese, calcium and magnesium, 
the insoluble residue from the sodium carbonate fusion is dis- 
solved in hydrochloric acid, the solution evaporated to dry- 
ness, the dry residue moistened with concentrated hydrochloric 
acid which is allowed to act for ten minutes, then diluted with 
water, boiled and any silica filtered off. In the filtrate, the iron 
is separated from the other metals by the basic acid acetate 
process described on p. 152. The manganese is precipitated 
by heating with, bromine (see p. 123) and the calcium and 
magnesium separated as cescribed on p. 77. 

Remark.—F¥or the determination of niobium and tantalum, a 
larger portion of the substance, about 5 gms., must be taken. The 
_ finely-powdered material is treated with hydrochloric acid to which 
about one-fourth of its volume of nitric acid has been added, and 
digested on the water-bath until the residue is colored a pure yel- 
low. The latter is filtered off, washed with water containing acid 





* This is to prevent any stannic oxide from collecting on the outside of 
the crucible; the oxide is formed when tin chloride comes in contact with 
moist air. 


298 GRAVIMETRIC ANALYSIS. 


until the iron reaction can no longer be obtained, when the residue 
is taken up in ammonia and filtered; in this way the tungstic acid 
is removed. The residue is usually dark-colored <nd consists of 
enclosed mineral as well as silicic, stannic, niobic, and possibly 
tantalic acids. It is treated with aqua regia again, water is - 
added, and the filtered residue is once more treated with ammonia. 
The final residue is now free from tungsten; it is ignited, weighed, 
and freed from silica by treatment with sulphuric and hydrofluoric 
acids. The residue of tin dioxide and niobium pentoxide (and 
perhaps tantalum pentoxide) is placed in a porcelain boat and 
ignited in a current of hydrogen. The metallic tin is ex- 
tracted with hydrochloric acid, and the residue, consisting of 
Nb205(Ta205), is weighed. 

Iron alloys rich in tungsten are acted upon only slowly by 
aqua regia. If, however, they are first roasted in the air, they are 
comparatively easily brought into solution.* Deconipositien 
by a potassium bisulphate fusion is still better (see p. 292). 


Analysis of Tungsten Bronzes. 


The analysis of these alkali salts of complex tungsten acids, 
discovered by Wohler in 1824,} offered for a long time considerable 
difficulty on account of the fact that acids do not decompose them 
very readily. 

By fusion with alkalies in the air, or better still in the presence 
of potassium nitrate, the tungsten bronzes can be converted 
without difficulty into normal alkali tungstate, and the tungsten 
determined by one of the methods already described. It is 
obvious that the alkalies cannot be determined in the same 
sample, so that Philippt proceeded as follows: 

The bronze is treated with ammoniacal silver nitrate solution, 
whereby the WOz is oxidized to WOg3 with the precipitation of an 
equivalent amount of silver, whereas the whole of the tungsten 
remains in solution in the form of alkali and ammonium tungstates. 
In the filtrate obtained after filtering off the deposited silver, the 





* Preusser. Zeit. f. anal. Chem., 1889, p. 173. 
ft Pogg. Ann., 2, 350. 
t Berichte, 15, 500 (1882). 


TUNGSTEN. 299 


tungstic acid is precipitated by treatment with nitric acid and 
determined as WO3. After removing the excess of silver, by 
precipitating it as the chloride, the filtrate is evaporated to dryness 
with the addition of sulphuric acid, and the alkali weighed as 
sulphate. | 

Although the above method affords satisfactory results in the 
analysis of bronzes containing comparatively little tungsten, it 
is wholly inadequate in the case of bronzes rich in tungsten. The 
method of Brunner,* which follows, is applicable in all cases. 
It is based upon the fact that the bronzes can be transformed very 
easily, and without loss of alkali, into normal tungstates by heat- 
ing them with ammonium persulphate, or ammonium acid 
sulphate. 

Procedure.—About 0.5 gm. of the finely-powdered bronze is 
treated in a porcelain crucible with 2 gms. of alkali-free ammonium 
sulphate and 2 c.c. of concentrated sulphuric acid + and carefully 
heated over a very small flame. During the heating the contents 
of the crucible are frequently shaken about a little by cautiously 
moving the crucible. The escape of gases from the crucible soon 
ceases and when sulphuric acid vapors begin to be evolved, the 
decomposition of the bronze results. 

In the case of sodium and lithium bronzes, the fused mass 
appears greenish, whereas with a potassium bronze the color is 
yellowish white. After a part of the ammonium sulphate has 
been volatilized, the mass in the crucible is allowed to cool, 
another gram of ammonium sulphate is added and 1 c.c. of con- 
centrated sulphuric acid, whereupon the contents of the crucible 
are once more heated as before until sulphuric acid fumes come off 
thickly; the crucible is then allowed to cool. 

The greenish or yellowish-white fusion is softened by treat- 
ment with water and rinsed into a porcelain dish. After adding 
50 ¢.c. of concentrated nitric acid, the contents of the evaporating 
dish are digested on the water-bath for three or four hours, and 





* Inaug. Dissert, Ziirich, 1903. 

{ If ammonium persulphate is used the addition of sulphuric acid is 
unnecessary. The only objection to the use of the persulphate lies in the 
fact that the commercial salt often contains some potassium persulphate. 


300 - GRAVIMETRIC_ANALYSIS. 


then, after diluting with water, the residue of pure yellow tungstic 
acid is filtered off. 

In order to recover the small amount of tungstic acid remain- 
ing in the filtrate, it is evaporated as far as possible on the water- 
bath, allowed to cool, diluted with a little water, carefully 
treated with an excess of ammonia, again evaporated on the — 
water-bath and treated as described on p. 288. 

The final filtrate from the fungstic acid determination is 
evaporated to dryness, the ammonium salts expelled by ignition 
and the residue of alkali sulphate weighed (cf. pp. 41 and 42). 


Separation of Tungsten from Tin. Augenot’s Method.* 


One gram of the finely powdered mineral is intimately mixed 
in an iron crucible with 8 gms. of sodium peroxide, and the 
mixture is carefully fused over the Bunsen burner for about 
fifteen minutes. After cooling, the melt is softened with water 
and transferred into a 250-c.c. flask (if lead is present, carbon 
dioxide is conducted into the solution for a few minutes), the 
solution is diluted to the mark, well mixed and filtered through a 
dry filter, rejecting the first few c.c. of the filtrate. 

For the determination of the tungstic acid, Augenot proceeds 
according to H. Borntrager.t 100 c.c. of the filtrate are allowed 
to flow into a mixture of 15 c.c. concentrated nitric acid and 45 c.c. 
of concentrated hydrochloric acid, evaporated to dryness in a — 
porcelain dish, the dry residue treated with 50 c.c. of a solution 
1000 c.c. water, 100 g. concentrated hydrochloric acid and 100 g. 
ammonium chloride, filtered and washed with the same solution. 
The precipitate, which, besides the tungstic acid, also contains 
silicic acid and stannic oxide, is dissolved in warm ammonia 
water, the filter well washed with the same reagent, and the 
ammoniacal solution allowed to flow into 15 c.c. of con- 
centrated nitric acid and 45 ec.c. of concentrated hydrochloric. 
acid. This time the liquid is evaporated to complete dryness, 
is taken up with the above mixture of hydrochloric acid and 
ammonium chloride, filtered and washed finally with dilute nitric 





* Z. angew. Chem., 19, 140 (1906). 
{ Z. anal. Chem., 39, 361 (1900). 


TUNGSTEN. 301 


acid (using preferably a Munroe crucible), and ignited in an electric 
oven; or in case the latter is not available, the crucible is placed 
inside a larger one of platinum and ignited over a Teclu burner 
to constant weight. The resulting tungstic acid is said to be 
free from silica and stannic oxide. 

For the determination of the tun, a second 100 c.c. of the original 
alkaline solution is used. It is treated with 45 c.c. of concentrated 
hydrochloric acid, whereby tungstic acid and stannic acid are 
precipitated. At this point 2 or 3 gms. of pure zinc are added 
whereby the tungstic acid is changed to the blue oxide and the 
stannic acid is reduced to metallic tin. The mixture is allowed 
to stand quietly for an hour at a temperature between 50° 
and 60°. The tin then goes into solution as stannous chloride, 


_ and the greater part of the tungsten remains undissolved in the 


form of the blue oxide. The latter is filtered off, and washed. 
In this way the whole of the tin is obtained in an acid solution, 
in the presence of a small amount of tungsten, which does no 
harm. The blue oxide on the filter, however,-is dissolved in hot, 
dilute ammonia solution in order to make sure that it contains 
no trace of metallic tin. If this should be the case, the sma!! 
particle of tin is dissolved in a little hydrochloric acid and the 
_ resulting solution added to the main solution of the tin. 
The solution is now diluted with water and the tin precipitated 
as stannous sulphide by the introduction of hydrogen sulphide 
gas. The precipitate is filtered, ignited in a porcelain crucible 
and weighed as SnOg2 (see p. 233). Or, the moist precipitate of 
stannous sulphide.may be dissolved in potassium hydroxide 
solution and the tin determined by electrolysis (see p. 234). 
According to Donath and Miller * a mixture of stannic oxide 
and tungstic acid may be separated as follows: The mixture is 
ignited with powdered zinc for fifteen minutes in a covered 
porcelain crucible. After cooling, the spongy contents of the 
crucible are heated in a beaker with hydrochloric acid (1:2) until 
there is no more evolution of hydrogen perceptible. The solution 
is then allowed to cool somewhat and some powdered potassium 





* Wiener Monatshefte, 8, 647 (1887). 


302 GRAVIMETRIC ANALYSIS. 


chlorate is added little by little until the blue tungsten oxide is 
completely transformed to yellow tungstic acid, when the liquid 
no longer shows any blue tinge. It is diluted with one and one- 
half times as much water, and after standing twenty-four hours 
the tungstic acid is filtered off, washed first with dilute nitric acid 
and then with one per cent. solution of ammonium nitrate, dried, 
ignited, and weighed as WOs. 
The tin is determined in the filffate as above. 


Separation of Tungstic Acid from Silica. 


When a mixture of tungstic and silicic acid is at hand, such as 
is obtained by evaporation with nitric acid, the silicic acid may be 
removed by treating the ignited residue with hydrofluoric acid 
and a large excess of sulphuric acid. The separation does not 
succeed, however, in the mixture of oxides as obtained after 
precipitation with mercurous nitrate; for in this case the silicic 
acid is so enveloped with tungstic acid that some of the former is 
not volatilized as fluoride. In such cases, as Friedheim * has 
shown, excellent results may be obtained by the 


Method of Perillon.t 


The mixture of the ignited oxides is introduced into a plati- 
num boat and heated to redness in a stream of dry hydrogen 
chloride. Thereby the tungsten is volatilized, probably as an 
acid chloride, which can be recovered in a receiver containing 
dilute hydrochloric acid; the silica remains behind in the com- 
bustion tube. 

Frequently the tungsten is reduced to a blue lower oxide, which 
is not volatile in a current of hydrochloric acid gas. In such cases, 
after the apparatus has been allowed to cool, the hydrogen 
chloride is replaced by air, and the contents of the boat are heated 
‘in a current of air. The tube is again allowed to cool, the air 
replaced by hydrogen chloride, and the tube once more heated 


—_—— 





* Z. anorg. Chem., 45, 398 (1905). 
t Bull. soe. l’industrie miner., 1884. 


VANADIUM. 303 


to redness. The process is repeated if necessary until finally a 
residue of pure white SiO2 is obtained. The molybdic acid hydro- 
chloride, MoO3-2HCl, in the receiver is evaporated to dryness 
with nitric acid, and the precipitated W0Os3 is filtered, ignited 
and weighed. Unless the current of hydrogen chloride gas is 
perfectly free from air, the platinum boat will be strongly 
attacked. 

The bisulphate method of effecting the separation is less 
accurate (See Vol. I). 


VANADIUM, V. At. Wt. 51.2. 


Vanadium is determined as the pentoxide, V,O,. 

The most convenient method for determining vanadium is a 
volumetric process, and will be discussed in the chapter on volu- 
metric analysis. 

If vanadium is present as ammonium or mercurous vanadate, 
it can be easily changed to the pentoxide by ignition; the latter 
is a reddish-brown fusible substance which solidifies as a radiating, 
crystalline mass. If vanadic sulphide is carefully roasted in the 
air, it is also changed quantitatively to the pentoxide. 

In the analysis of most minerals containing vanadium, the 
vanadium is separated from the other metals present by fusing with 
a mixture of six parts sodium carbonate and one part potassium 
nitrate. After cooling, the melt is extracted with water, whereby 
the sodium vanadate goes into solution while most of the metals 
are left behind in the form of oxides or carbonates. If phosphorus, 
arsenic (molybdenum, tungsten), and chromium are present, 
these elements also dissolve on treating the melt with water in 
the form of the sodium salts of the corresponding acids. 

In practice, therefore, the vanadium is usually met with as 
the sodium salt of vanadic acid, and it is a matter of separating it 
from the aqueous solution obtained after fusing with sodium 
carbonate and potassium nitrate, and of separating it from the 
other acids which are likely to accompany it (phosphoric, arsenic, 
and chromic acids). | 


304 GRAVIMETRIC ANALYSIS. 


Precipitation of Vanadic Acid from the Solution of 
Sodium Vanadate. 


There are two good methods for the separation of vanadic 
acid from a solution of an alkali vanadate: the Rose method, 
according to which the vanadium is precipitated as mercurous 
vanadate, and that of Roscoe, by which it is precipitated as lead 
vanadate. The Berzelius-Hauer method,* in which the vanadium 
is precipitated as ammonium metavanadate, was found by Holver- 
scheidt + to give always too low results, but Gooch and Gilbert,f 
as well as E. Campagne,§ obtained correct results by working 
in an ammoniacal solution, which was saturated with ammonium 
chloride. 


1. The Mercurous Nitrate Method of Rose. 


The alkaline solution is nearly neutralized with nitric acid 
and to it is added, drop by drop, a nearly neutral solution of mer- 
curous nitrate || until, after the precipitate has settled, a further 
addition of the reagent causes no precipitation. The liquid is then 
heated to boiling, the gray-colored precipitate is allowed to settle 
and is filtered and washed with water to which a few drops of © 
mercurous nitrate solution have been added. ‘The precipitate is 
dried, ignited under a good hood, and the residue of V,O, is weighed. 

Remark.—In neutralizing the alkaline solution of the vanadate 
with nitric acid, the solution must on no account be made acid, 
for in this case nitrous acid (from the nitrate fusion) will be set 
free and the latter reduces some of the vanadate to a vanadyl 
salt and the latter is not precipitated by mercurous nitrate. In 
order to avoid passing over the neutral point, Hillebrand recom- 
mends fusing with a weighed amount of sodium carbonate and 
adding the amount of nitric acid that has been found necessary 
by a blank test to neutralize this. The method gives good results. 





* Pogg. Ann., 22, 54 and J. prakt. Chem., 69, 388. 

+ Dissertation, Berlin, 1890. 

t Z. anorg. Chem., 32, 175 (1902). 

§ Berichte, 1903, 3164. 

|| The mercurous nitrate used should leave no residue on being ignited. 


VANADIUM. 305 


2. The Lead Acetate Method of Roscoe.* 


Principle.—If a solution weakly acid with acetic acid is treated 
with lead acetate, orange-yellow lead vanadate is quantitatively 
precipitated. The lead vanadate, however, does not possess a 
constant composition, so that the amount of vanadium present ° 
cannot be determined by weighing the precipitate. After being 
washed, it is dissolved in as little nitric acid as possible, the lead 
precipitated as lead sulphate, and the vanadium determined in 
the filtrate by evaporating the latter, driving off the excess of sul- 
phuric acid, and weighing the residual V,O,. | 

Procedure.—The solution from the sodium carbonate and 
potassium nitrate fusion is nearly neutralized as before with nitric 
acid, an excess of lead acetate solution is stirred into it, when the 
voluminous precipitate will collect together, rapidly settle to the 
bottom of the beaker, and the supernatant liquid will appear 
absolutely clear. The precipitate is at first orange-colored, but on 
standing it gradually becomes yellow and finally perfectly white. 
It is filtered off, washed with water containing acetic acid until 
half a cubic centimeter of the filtrate will leave no residue on 
evaporation. The precipitate is now washed into a_ porcelain 
dish, the part remaining on the filter is dissolved in as little as 
_ possible of hot dilute nitric acid, and the solution added to the 
main part of the precipitate, to which enough nitric acid is added 
to completely dissolve it. An excess of sulphuric acid is added 
to the solution, and it is evaporated on the water-bath as far as 
possible, finally heating over the free flame until dense fumes of 
sulphuric acid are evolved. After cooling, from 50 to 100 c.c. of 
water are added, the lead sulphate is filtered off and washed with 
dilute sulphuric acid until 1 c.c. of the filtrate will show no yellow 
color with hydrogen peroxide. The lead sulphate should be 
white and free from vanadium; it will be so provided enough sul- 
phuric acid was used and the mass was not heated until absolutely 
dry before diluting with water. The filtrate containing all the 
vanadie acid is evaporated in a porcelain dish to a small volume, 
transferred to a weighed platinum crucible, evaporated further 
on the water-bath, and finally in an air-bath until all the sulphuric 





* Ann. Chem. Pharm., Suppl., 8, 102 (1872). 


306 GRAVIMETRIC ANALYSIS. 


acid is removed. The open crucible is then ignited for some time # 
at a faint-red heat and its contents finally weighed as V,O,. 

Remark.—Instead of decomposing the lead vanadate by means 
of sulphuric acid, Holverscheidt recommends precipitating the lead 
as sulphide by means of hydrogen sulphide and determining the 
vanadium in the filtrate. For this purpose the blue-colored filtrate 
from the lead sulphide precipitate (which contains some vanadyl 
salt) is boiled to expel the excesgof hydrogen sulphide and the 
deposited sulphur is filtered off. A few drops of nitric acid are. 
added, the solution evaporated to dryness, and the reddish-yellow 
hydrate of vanadic acid is changed by gentle ignition into the 
pentoxide of vanadium. 

Lead may also be separated from the vanadic acid as lead 
chloride. In this case the procedure recommended in the analysis 
of vanadinite (p. 308) is followed. 

The separation of vanadium as the sulphide by acidifying a 
solution of an alkali vanadate that has been treated with an excess 
of ammonium sulphide is not admissible, for only a part of the 
vanadium is precipitated as the brown sulphide, the rest remaining 
in solution in the form of vanadyl salt. H. Rose called the attention 
of chemists to the inaccuracy of this method, but this has not pre- 
vented its being recommended in some of the most recent works 
on analytical chemistry. The author has carefully tested the 
method and found it useless. 


Separation of Vanadium from Arsenic Acid. 


Most minerals containing vanadium also contain arsenic, 
and after extracting the melt, obtained by fusion with sodium 
carbonate and nitre, with water, both elements go into solution, 
For their separation, the solution is acidified with dilute sulphurie 
acid and sulphur dioxide is passed into the hot liquid, whereby the 
vanadic acid is reduced to vanadyl salt and the arsenic to arsenious 
acid. After boiling to remove the excess of sulphur dioxide, the 
solution is saturated with hydrogen sulphide and the precipitate 
of arsenic trisulphide is filtered off. The filtrate is freed from 





* On expelling the sulphuric acid, there is finally formed some green and 
brown crystals of a compound of vanadie acid with sulphuric acid; these are 
decomposed only at a faint-red heat. 


- VANADIUM. 3°97 


hydrogen sulphide by boiling, evaporated with nitric acid in order 
to form vanadic acid again, the solution is then made alkaline with 
sodium carbonate, and the vanadium determined by one of the 
above methods. 


- Separation of Vanadium from Phosphoric Acid. 


If the solution of the soda-nitre fusion contains phosphorie as 
well as vanadiec acid, both are precipitated by mercurous nitrate, 
the precipitate washed with dilute mercurous nitrate solution and 
weighed. In this way the sum of the V,0O,+P,0, is obtained. 
When P,O,; is present the V,O; does not melt, but only sinters 
together. The ignited oxides are fused with an equal weight of 
sodium carbonate, the melt is dissolved in water, the solution made 
acid with sulphuric acid and boiled with sulphurous acid in order 
to reduce the vanadic acid to vanadyl sulphate; the latter will be 
recognized by the pure blue color which the solution assumes. 
Carbon dioxide is passed into the boiling solution until the 
excess of sulphurous acid is removed, when it is allowed to 
cool. To the cold solution, now about 100 c.c. in volume, 200 e.e. 
of a 75 per cent. solution of ammonium nitrate and 50 c.c. of 
ammonium molybdate solution are added (cf. Remark, below), the 
solution is warmed to about 60° C., set aside and allowed to stand 
‘for one hour. The clear liquid is then decanted through a filter, 
washed three times by decantation with 50 c.c. of the proper wash 
liquid (see p. 437), after which the precipitate is dissolved by 
passing 10 c.c. of 8 per cent. ammonia through the filter into the 
the beaker containing the bulk of the precipitate and the filter 
is finally washed with 30 ¢.c. of water. To this solution 20 c.c. of 
a 34 per cent. ammonium nitrate solution and 1 ¢.c. more of ammo- 
nium molybdate are added, the solution heated until it begins to’ 
boil, and the phosphoric acid reprecipitated by the addition of: 
20 c.c. of hot 25 per cent. nitric acid. The phosphoric acid is 
determined by the method of Woy (page 440). The amount of’ 
phosphoric acid found is deducted from the sum of the oxides and 
the difference gives the amount of V,O,. 

Remark.—A. Gressly tested this method in the author’s labora- 
tory and made the interesting observation that if'about 015 gm.’ 
of V,O; was present with 0.1 gm. P,O,, no trace of the latter could 


308 GRAVIMETRIC ANALYSIS. 


be detected according to the procedure of Woy, not even on boiling 
the solution. On the other hand, an immediate precipitation was 
produced if a stronger solution of ammonium molybdate were used 
(75 gms. of ammonium molybdate dissolved in 500 c.c. of water) 
and this solution poured into 500 c.c. of nitric acid, sp. gr. 1:2. 

The above-described separation gives correct results only when 
the vanadium is present as vanadyl sulphate; if vanadiec acid is 
present it is precipitated with the phosphoric acid. If the solu- 
tion is allowed to stand after the addition of the ammonium molyb- 
date, the vanadyl sulphate is gradually oxidized to vanadic acid; 
the precipitate therefore should not be allowed to stand long 
before filtering. 


Separation of Vanadium from Molybdenum. 


The solution containing the alkali salts of the two acids is 
acidified with sulphuric acid and the molybdenum precipitated 
in a pressure-flask by means of hydrogen sulphide, and the pre- 
cipitate filtered off through a Gooch crucible, as described on 
pp. 285 and 286), and weighed as MoO,. After removing the excess 
of hydrogen sulphide from the filtrate, the vanadium is oxidized 
with nitric acid and determined as described under the Separation of 
Vanadium from Arsenic Acid, p. 306. 


Analysis of Vanadinite, (Pb,(VO,),C1). 
Besides lead, vanadic acid, and chlorine, the mineral often con- 
tains arsenic and phosphoric acids. 


Determination of Chlorine. 


About 1 gm. of the finely powdered mineral is dissolved in 
dilute nitric acid (in order to avoid loss of chlorine the solution is 
kept cold), and the solution is diluted with considerable water. 
The chlorine is precipitated with silver nitrate and the weight of 
the silver chloride determined as described on p. 317. 


Determination of Lead. 


The filtrate from the silver chloride is treated with bydveckia gt 
acid. in order to precipitate the excess of silver, filtered, washed. 
with hot water, and the solution thus freed from silver is evaporvisl 


VANADIUM. 309 


to dryness to remove the nitric acid. The dry mass is moistened 
with hydrochloric acid, 95 per cent. alcohol is added in order 
to precipitate completely the lead chloride, and the latter is 
filtered through a Gooch crucible, washed with alcohol, dried 
at 110° C. and weighed as PbCle. 


Determination of Vanadium, Phosphoric Acid, and Arsenic. 


The filtrate from the lead chloride contains the vanadium 
as vanadyl salt. The alcohol is driven off by careful heating 
on the water-bath, nitric acid is added to the solution, and 
the iatter is repeatedly evaporated in order to oxidize the blue 
vanadyl salt to brown vanadium pentoxide. The dry mass is 
washed by means of as little water as possible into a weighed plati- 
num crucible, the residue adhering to the sides of the dish is dis- 
solved in a little ammonia and added to it. The crucible is then 
heated, at first gradually to expel the ammonia, and afterward 
more strongly with ready access of air (uncovered crucible) until 
the reduced, dark-colored oxide is changed over to the brownish- 
red pentoxide. The temperature is then raised until the vanadium 
oxide begins to melt. If phosphoric acid is present, its anhydride 
is weighed with the vanadium and the amount of P.O, is deter- 
mined as described on p. 307; this amount is deducted from the 
weight of the two oxides. 

The determination of arsenic is best carried out in a separate 
portion. For this purpose the mineral is dissolved in hot nitric 
acid, the greater part of the excess of the acid is removed by 
evaporation, the solution is diluted with water, and the lead 
precipitated by the addition of sulphuric acid. From the filtrate, 
the last portions of lead and arsenic are precipitated by hydro- 
gen sulphide, after previous reduction with sulphurous acid. 
The filtered precipitate is digested with sodium sulphide and the 
arsenic precipitated from the solution thus obtained by the addi- 
tion of hydrochloric acid. The arsenic sulphide is then changed to 
arsenic acid, preferably by dissolving in ammoniacal hydrogen 
peroxide, and is precipitated as magnesium ammonium arsenate, 
and determined according to p. 206. 


310 GRAVIMETRIC ANALYSIS. ° 


Determination of Vanadium and Chromium in Iron Ores 
and Rocks. 


Method of W. F.. Hillebrand.* 

As vanadium often occurs in many ores of iron and in rocks, 
although in very small amounts, it is often of interest and of im- 
portance to be able to determine it in such cases. For this purpose 
it is best to proceed as follows: . = 

Five gms. of the finely powdered mineral are mixed with 20 gms. 
sodium carbonate and 3 gms. potassium nitrate and fused over the 
blast-lamp. The green fusion (containing manganese) is extracted 
with water, a few drops of alcohol are added to reduce the man- 
ganese, and the residue is filtered off. 

The aqueous solution contains sodium vanadate and often phos- 
phate, chromate, molybdate, aluminate, and considerable silicate as 
well. First of all, the aluminium and the greater part of the silicic 
acid are removed by nearly neutralizing the alkaline solution with 
nitric acid.{t It is very important that the solution is not made 
acid at this point on account of the reducing action of the nitrous 
acid set free from the nitrite formed during the fusion. The 
almost neutral solution is evaporated nearly to dryness, taken up 
in water, and filtered.§ 

The cold alkaline solution is now treated with an almost neutral 
solution of mercurous nitrate until no further precipitation takes 
place. The somewhat voluminous precipitate contains, besides 
mercurous carbonate, also its chromate, vanadate, molybdate, 
arsenate, and phosphate, if the corresponding elements are present 
in the mineral. If the precipitate is too bulky, a little nitric acid 
is cautiously added, and then a drop of mercurous nitrate in 
order to see if the precipitation is complete. 





* See U. S. Geol. Survey Bull., 411. 
Ff If considerable vanadium is present, the insoluble residue always con- 
tains vanadium and must be fused with soda-nitre again. 
t The amount of nitric acid sana to neutralize 20 gms. of soda is 
determined by a blank test. : 
§ The residue of aluminium hydroxide. oa silicic acid almost never con-. 
tains vanadium, but contains chromium. If it is desired to determine the 
latter, the residue is evaporated to dryness with hydrofluoric and sulphuric 
acids, the dry mass is fused with soda and nitre again, and the aqueous solu- 
tion of the melt added to the main solution. 


VANADIUM. 314 


The liquid is heated to boiling, filtered, the precipitate washed 
with water containing ammonium nitrate, dried, and ignited in a 
platinum crucible at as low a temperature as possible. The ignited 
residue is fused with a little sodium carbonate, the melt extracted 
with water and, if yellow-colored, it is filtered into a 25-c.c. flask 
and the amount of chromium determined colorimetrically by com- 
paring its color with a carefully prepared solution a8 potassium 
chromate. 

The solution is ches slightly acidified with sulphuric acid, and 
the molybdenum, arsenic, and traces of platinum precipitated by 
hydrogen sulphide in a pressure-flask. The precipitated sulphides 
are filtered off, the filter together with the precipitate is carefully 
ignited in a porcelain crucible, 2 few drops of sulphuric acid are 
added and the crucible heated again until the acid is almost com- 
pletely removed. On cooling the mass is colored a beautiful blue if 
molybdenum is present. 

The filtrate from the above precipitate is freed from the ex- 
cess of the hydrogen sulphide by boiling and passing a stream of 
carbon dioxide through it, and the hot solution is then titrated 


to a pink color with a potassium permanganate solution (cf. Vol. 


Anal.). In order to obtain absolutely accurate results, the solu- 
tion is now reduced by means of sulphur dioxide and the titration 
repeated. The mean of the two. experiments gives the vanadium 
value. 


V204+0 = V20s. 


This method gives correct resulis only when the amount of 
chromium present is very small, which is true in the majority of 
cases. 

In case more than 5 mem. of chron are e present a correction 
must be made, for a measurable amount of permanganate is 
used up in oxidizing the chromium. This is determined by tak- 
ing a solution containing the same amount of chromate es the 
analyzed solution, reducing it with sulphurous acid, and titrat- 
ing with permanganate. The amount of permanganate now used 
must be subtracted from the amount used in the analysis, and from 
the difference the amount of vanadium present is calculated. 


312 GRAVIMETRIC ANALYSIS. 


Determination of Vanadium and Chromium in Pig Iron. 


From 5 to 10 gms. of the iron are dissolved in dilute hydro- 
chloric acid in a flask, meanwhile passing a current of carbon diox- 
ide through the liquid. For each gram of iron taken, 5 c.c. of 
hydrochloric acid, sp. gr. 1.12 and 10 ¢.c. of water are used. The 
solution is hastened by warming, finally boiling it until there is no 
more evolution of gas. It is now dilutted with an equal volume of 
water, allowed to cool, and, without filtering off the slight residue, 
an excess of barium carbonate is added and the mixture allowed to 
stand for twenty-four hours with frequent shaking. The residue 
is filtered off, rapidly washed with cold water, dried and ignited in 
a platinum crucible in order to burn off the carbon. Five parts of 
sodium carbonate and one part of nitre are then added to the con- 
tents of the crucible and the mixture is heated to quiet fusion. 

The fusion is leached with water and the solution thus obtained 
contains all the chromium as chromate, and the vanadium as vana- 
date in the presence of alkali silicate and phosphate. The aque- 
ous solution is now nearly neutralized with nitric acid, being care- 
ful not to make the solution acid as the nitrous acid set free will 
reduce some of the vanadium and chromium. The barely alkaline 
solution is then treated with an almost neutral solution of mercu- 
rous nitrate until no further precipitation takes place, the liquid is 
heated to boiling, filtered and washed with water containing a little 
mercurous nitrate. After drying, as much of the precipitate as 
possible is transferred to a platinum crucible, the filter burned by 
itself and itsash added to the main portion of the precipitate, which 
is ignited to remove the mercury. The residue is fused with a little 
sodium carbonate, the melt extracted with water, and the solution 
filtered. In case the filtrate is colored yellow, the amount of 
chromium present is determined colorimetrically * by placing the 
solution in a graduated cylinder and comparing its color with a potas- 





* The colorimetric determination is only suitable when small amounts are 
present. When considerable chromium is present (chrome steel) the results 
obtained by the colorimetric determination may be as much as 2 per cent. too 
high. In such cases the chromic acid is titrated with ferrous sulphate (see 
Vol. Anal.). 


VANADIUM. 313 


sium-chromate solution containing a known amount of chromium. 
The solution is then acidified with sulphuric acid and hydrogen 
sulphide is conducted into the boiling solution to precipitate ar- 
senic and platinum. After filtering, the hydrogen sulphide is re- 


moved and the hot solution titrated with be potassium perman- 


ganate solution (cf. Volumetric Analysis). 


Determination of Vanadium, Molybdenum, Chromium, and 
: Nickel in Steel. Method of A. A. Blair.* 


The method is given in this edition of the book as illustrating 
the removal of the greater part of the iron from a ferric chloride 
solution by shaking with ether. Although it is difficult to 
effect a perfect separation in this way, still when the conditions 
are right, almost all of the iron can be removed so that a large 
sample of steel can be taken for analysis. This particular 
method has not been tested in the author’s laboratory and 
is not given in the German edition. The ether separation, 
however, is discussed in many other text-books and deserves 
mention.—(TRANSLATOR.) 

Molybdenum, in this analysis, follows the iron so that when a 
small amount of the former is present, as is the case in steels, 
the ether solution may be regarded as containing all of the 
molybdenum, as well as the greater part of the iron. 

Procedure.—T wo grams of the steel are dissolved in nitric acid 
with the addition of hydrochloric acid if necessary. The resulting 
solution is evaporated to dryness and the residue dissolved by 
treatment with hot concentrated hydrochloric acid. If silica is 
present, the solution is diluted and filtered. The hydrochloric 
acid solution is evaporated to a sirup, the latter dissolved in a little 
hydrochloric acid, sp. gr. 1.1,f and transferred with the aid of a 
little more of the same acid to a separatory funnel of about 250 
c.c. capacity which is provided with tightly fitting stop-cock 
and glass-stopper. About 80 c.c. of ether are added to the cold 


—_—_—— 





* J. Am. Chem. Soc., 30, 1228. 
{ The separation works best with acid of this concentration. 


(314 _ GRAVIMETRIC ANALYSIS. 


solution * and the mixture is shaken vigorously for half a minute. 
When the two liquids are in equilibrium, the lower layer is 
transferred to a second separatory funnel. The stopper of the 
first funnel is carefully rinsed with hydrochloric acid, sp. gr. 
1.1, the contents of the funnel once more shaken, and the 
lower layer added to the contents of the second funnel. The 
solution in the latter is shaken with 50 c.c. more of ether, and the 
acid solution containing all the vinadium, chromium, nickel, 
manganese and copper, is run into a beaker and freed from 
dissolved ether by evaporating on the water bath nearly to dry- 
ness. An excess of nitric acid is added and the solution again 
evaporated to remove all the hydrochloric acid. When the solu- 
tion is almost sirupy, 20 ¢c.c. of hot water are added and the 
solution is heated with the addition of a little sulphurous acid to 
reduce any chromic acid that may have been formed. The hot 
solution is slowly poured, while stirring vigorously, into a boiling 
10 per cent. solution of sodium hydroxide. After boiling a few 
minutes, the precipitate is allowed to settle, is washed twice by 
decantation, and finally on the filter until the volume of the 
filtrate is about 300 c.c. The precipitate contains the hydrated 
oxides of chromium, nickel and iron with the greater part of the 
manganese and any copper that may have been in the sample. 
The filtrate contains the vanadium, some silicate and aluminate 
(from the reagents) and sometimes a little chromium. It is 
made barely acid with nitric acid, once more made alkaline with 
a few drops of sodium hydroxide, boiled and filtered. 

The vanadium is determined in this last filtrate. It is pre- 
cipitated as lead vanadate by the addition of 10 c.c. of a 10 per 
cent. solution of lead nitrate, eventually adding enough acetic 
acid to make the solution decidedly acid and boiling for several 
minutes. The lead vanadate is filtered and washed with hot 
water. The precipitate is dissolved in hot, dilute hydrochloric 
acid, the solution evaporated nearly to dryness, treated with 50 
e.c. of hydrochloric acid, evaporated again, cooled, treated with 
10 c.c. concentrated sulphuric acid, and evaporated until fumes 





* The warm solution would result in the reduction of some of the iron by 
the ether. 


DETERMINATION OF VANADIUM, MOLYBDENUM, ETC. 315 


of sulphuric anhydride are evolved. When cold 150 c.c. of water 
are added, the solution heated to between 60 and 70° and titrated 
with permanganate. The evaporation of the vanadate solution 
with hydrochloric acid and subsequent treatmen: with sulphuric 
acid results in the reduction of the vanadium from the quinque- 
valent to the quadrivalent condition,* and the titration with 
permanganate makes the vanadium quinquevalent again. Con- 
sequently 2 atoms of V are equivalent to 1 atom of oxygen in 
the titration. The presence of a little iron does not interfere 
when the vanadium-is reduced in the above manner, 

The two precipitates obtained by the sodium hydroxide treat- 
ment contain chromium, nickel and copper besides iron and 
manganese. The two filters are ignited and the precipitates 
fused with about 2 gms. of sodium carbonate and half a gram of 
potassium nitrate. The fused mass is treated with water and 
the solution filtered. The residue contains the nickel, copper, 
iron and part of the manganese; the filtrate contains the chromium 
and the rest of the manganese. To the filtrate, enough ammonium 
nitrate is added to convert all the sodium salts to nitrates, and the 
solution evaporated to small volume with the addition of a few 
drops of ammonia from time to time. The evaporated solution 
is diluted to 50 c.c. and filtered. The precipitate consists of 
hydrated manganese dioxide (alumina and silica from the 
reagents). Tae filtrate is boiled to drive off the ammonia, sul- 
phurous acid added to reduce any chromic acid, the excess of 
reducing agent boiled off, and the chromium precipitated by the 
careful addition of ammonia to the boiling solution. The precip- 
itate is filtered off, washed and weighed as Cr2O3. 

The filter containing the insoluble residue from the above 
fusion is returned to the same crucible in which the fusion was 
made and ignited. The ignited oxides are dissolved in hydro- 
chloric acid, the solution diluted and the copper precipitated by 
hydrogen sulphide. The filtrate is evaporated with sulphuric 
acid until the hydrochloric acid is all expelled, whereupon it is 





* Campagne, Compt. rend., 137, 570 (1903). 


316 GRAVIMETRIC ANALYSIS. 


diluted and treated with a large excess of ammonia and the 
nickel determined by electrolysis (see p. 136). 

The ether solution from the two separatory funnels is 
shaken with water, which causes the separation of an ether 
layer from the solution containing the iron and molybdenum. 
The lower layer is, therefore, drawn off; the solution of ferric 
chloride containing all the molybdenum is evaporated nearly to 
dryness, the cold solution treated *with 10 c.c. of concentrated 
sulphuric acid and evaporated until the sulphuric acid fumes 
freely. The cold sulphuric acid solution is diluted with 100 c.c. 
of water, reduced by the careful addition of ammonium acid 
sulphite, the excess of sulphurous acid boiled off, and the cold 
solution saturated with hydrogen sulphide in a 200 c¢.c. pressure 
bottle. The bottle is stoppered and heated on the water bath for 
several hours. After slowly cooling, the precipitate is filtered into 
a Gooch crucible and washed with water containing a little sul- 
phuric acid and finally with alcohol. The Gooch crucible is 
placed within a larger porcelain crucible so that the bottom of the 
former does not touch that of the latter, covered with a watch-. 
glass and heated gradually until there is no more odor of sulphur 
dioxide. The watch-glass is then replaced by a porcelain crucible 
cover and the heating is continued until the ignited precipitate 
becomes bluish white in color. 

The Gooch crucible is then heated to faint redness, cooled and 
weighed. The heating and weighing is repeated until the pre- 
cipitate ceases to lose in weight. The crucible is then placed in 
the suction bottle and washed with ammonia until the washings 
are free from molybdenum. The crucible is again heated and 
weighed. The difference in weight corresponds to the amount 
of molybdenum trioxide. A small amount of ferric oxide always 
remains on the felt. r 


SILVER. 317 


METALS OF GROUP I. 
SILVER, LEAD, MERCUROUS MERCURY (AND THALLIUM). 


The determination of lead and mercury has already been 
considered; it remains for us to discuss the determination of silver. 


SILVER, Ag. At. Wt. 107.88. 
Forms: AgCl and Ag. 
Determination as Silver Chloride, AgCl. 


The solution, slightly acid with nitric acid, is heated to boiling 
and the silver precipitated by the addition of hydrochloric acid, 
drop by drop, until no more precipitate is formed. The precipitate 
is allowed to settle in a dark place, filtered through a Gooch cruci- 
ble and washed, first with water containing a little nitric acid until 
the chloride test can no longer be obtained, then twice with alcohol 
or water in order to remove the nitric acid. The precipitate is 
dried first at 100° C. and finally at 130° C. till a constant weight is 
obtained. If it is not desired to use a Gooch crucible for this 
determination, the silver chloride can be filtered upon an ordinary 
washed filter, washed as before and dried at 100°C. As much 
of the precipitate as possible is transferred to a weighed porcelain 
crucible, the filter burned (as described on page 21) in a platinum 
spiral whereby some of the silver chloride adhering to it will be 
reduced to metal. The ash of the filter is added to the main por- 
tion of the precipitate. It is moistened with a little nitric acid and a 
drop or two of concentrated hydrochloric acid, dried on the water- 
bath and then heated over a free flame until the silver chloride 
begins to melt. After cooling in a desiccator it is weighed. 

Solubility of Silver Chloride.* One liter of water dissolves 
0.00154 g. AgCl at 20° and 0.0217 g. at 100°. In water con- 
taining a little hydrochloric acid, the AgCl is less soluble than in 
pure water but as the quantity of hydrochloric acid is increased, 
the solubility of AgCl rises rapidly. Thus one liter of 1 per cent. 
HCl dissolves only 0.0002 g. AgCl at 21°, but 11. of 5 per cent. HCl 
dissolves 0.0003 g. and 11. of 10 per cent. HCl dissolves 0.0555 g. 


AgCl. 





* G.S. Whitby, Z. anorg. Chem., 67, 108 (1910). 


318 GRAVIMETRIC ANALYSIS 


Determination as Metal, Ag. 


Metallic silver is obtained by the ignition of silver oxide, car- 
bonate, cyanide or the salt of an organic acid. In the latter case, 
the substance must be heated very cautiously at first in a covered 
crucible. When the organic substance is completely charred, 
the cover is removed from the crucible and the contents are ignited 
until the carbon is completely burned, and the crucible then weighed. 

From the chloride, bromide (but not the iodide) and sulphide, 
the metal may be obtained by igniting in a current of hydrogen. 
The reduction of the chloride, bromide, and iodide may be effect- 
ed very conveniently by passing the electric current through 
the substance after it has been melted together. The porcelain 
crucible containing the silver halide is placed in a crystallizing 
dish and near it is placed a second crucible containing a little 
mercury and a small piece of zine. Upon the silver salt is placed 
a small disk of platinum foil, which is fastened to a platinum wire; 
the other end of the wire dips in the mercury in the other crucible. 
The crystallizing dish is filled with dilute sulphuric acid (1:20) 
so that the crucible is entirely covered with the acid and it is then 
allowed to stand over night. Next morning all of the silver salt 
will be found to be reduced. The crucible is removed from the 
acid, washed with water, dried, ignited, and weighed. By this 
simple method, E. Lagutt obtained excellent results. If the 
silver halide has not been fused to a compact mass small particles 
of the silver precipitate are likely to float around during the opera- 
tion, and escape reduction. . 

Silver can also be deposited electrolytically, but this method 
will not be described in this book, for it offers no particular advan- 
tages over the determination as silver chloride with a Gooch cru- 
cible. 


Separation of Silver from Other Metals. 


As almost all metal chlorides * are soluble in dilute hydro- 





* Thallium chloride is difficultly soluble in water. If thallium is present 
the silver is precipitated from a nitrate solution by means of H,S, ignited 
in a stream of hydrogen, and weighed as metal. To determine the thallium, 
the filtrate is evaporated to dryness, the residue dissolved in a little water 


SILVER, : 319 


 chloric acid, siver is separated from the other metals by the addi- 
tion of hydrochloric acid to the solution. If the solution contains 
mercurous salts these are oxidized before the addition of the 
hydrochloric acid by boiling with nitric acid. 

For the separation of silver from gold and platinum in alloys 
consult pages 259 and 270. 





and the thallium precipitated by the addition of potassium iodide. The 
thallium iodide precipitate is washed with dilute potassium iodide solution, 
then with alcohol, dried at 150° and weighed as TII. 


GRAVIMETRIC DETERMINATION OF THE 
METALLOIDS (ANIONS). 


GROUP I. 


HYDROCHLORIC, HYDROBROMIC, HYDRIODIC, HYDROCYANIC, 
HYDROFERROCYANIC, HYDROFERRICYANIC, SULPHOCY- 
ANIC, AND HYPOCHLOROUS ACIDS. 


HYDROCHLORIC ACID, HCl. Mol. Wt. 36.47. 
Form: Silver Chloride, AgCl. 


We can distinguish between two cases: 

A. The chloride solution is present either as free hydrochloric 
acid or as a chloride soluble in water. 

B. It is present in the form of an insoluble chloride. 


A. The Chloride is Present in Aqueous Solution. 


If only metals of the alkali or alkaline earth groups are present, 
the cold solution is made slightly acid with nitric acid, and silver 
nitrate is slowly added with constant stirring until the precipitate 
collects together and further addition of the reagent produces no 
more precipitation. The liquid is now heated to boiling, the pre- 
cipitate allowed to settle in the dark, filtered through a Gooch 
crucible, and then treated exactly as described in the determina- 
tion of silver, page 317. 

If the aqueous solution contains a chloride of a heavy metal, 
it is not always possible to follow the above procedure. If, for 
example, substances are present which on boiling are changed to 
insoluble basic salts, it is evident that the precipitate of silver chlo- 
ride would be contaminated with these substances and too high 
results would be obtained. This is particularly true of stannic 


and ferric salts. Ferrous salts, on the other hand, in case only little 
320 


HYDROCHLORIC ACID. 321 


nitric acid is present, reduce silver nitrate to metallic silver on 
heating the solution; if enough nitric acid is present to prevent 
the reduction to silver, the danger of forming basic salts still 
remains to be feared. In such cases the precipitation is effected 
as before from a cold solution and the poliaeee ae: heating is 
omitted. 

In all cases, however, it is better to first remove the heavy 
metal by precipitation with ammonia, caustic soda or sodium 
carbonate. 


Example: Analysis of Commercial Tin Chloride. 


Tin chloride is obtained either as a solid salt corresponding 
to the formula SnCl,+5H,O, or as a concentrated aqueous solu- 
tion. 

As both the solid salt and its concentrated bblntion are very 
hygroscopic, it is necessary to weigh out the portion for analysis 
from a stoppered vessel. It is best to proceed as follows: 

A large sample of the substance (about 10 gm.) is placed in a 
tared weighing beaker, closed and weighed. About 10 c.c. of water 
are added, the salt is completely dissolved to a homogeneous 
syrup by shaking, and the beaker is again weighed. Four more 
weighing beakers are now tared and into each is placed about 
2 c.c. of the syrup. Each beaker is quickly stoppered and then 
weighed. 

Determination of Tin.—The contents of one of the weighing 
beakers is washed into a 400-500-c.c. beaker, diluted to about 
300 ¢.c. and a few drops of methyl orange are added, whereby the 
liquid is colored red. Ammonia solution (free from chloride) is 
now added until the color of the solution is changed to yellow (an 
excess of ammonia must be carefully avoided for tin hydrox- 
ide is somewhat soluble in ammonia). The solution is then treated 
with ammonium nitrate (5 c.c. of concentrated ammonia exactly 
neutralized with nitric acid, sp. gr. 1.2), boiled for one or two min- 
utes, filtered, and washed with water Sos teaeuias ammonium nitrate, 
and weighed as SnO,. 

Determination of Chlorine.—The filtrate from the tin hydroxide 
precipitate is acidified with nitric acid, and precipitated in the cold 
with silver nitrate. The solution is then heated to boiling and, 


322 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


after the precipitate has settled, it is filtered through a Gooch 
crucible, washed with cold water containing a little nitric acid, 
then with cold water or alcohol, dried at 130° C., and weighed. 

The amount of tin and chlorine present is computed as follows: 


Weight of Solid Salt= A. 

Weight of the Solid Salt+ Water= B. 

Weight of the Solution taken for Analysis=a. 
Weight of the SnO, found= p. 

Weight of the AgCl found= p’. 


Since B gm. of the solution contain A gm. of the solid salt, 
then the amount a of the solution taken for analysis will contain: 





B: A=a:2 
= ae wt. of substance taken. 
This amount of substance yielded p gm. SnO,, corresponding 
to: 
SnO,:Sn=p:2’ 
, Sn-p 
SnO, 
and in percentage? 
Aa Sapo 4; 
BF oe tet 


100Sn p-B 
47 PREDATES LRM PEN 
a0 BaOe ata ee 


In the same way the amount of chlorine present is found to be: 


100C1 p’-B_ 
Sl Ga 


This analysis may be accomplished much more rapidly by a 
volumetric process. (Consult Vol. Anal.) 

If antimony or stannous compounds are present, the above 
procedure cannot be used. It has been proposed to add tartaric 
acid to the solution, then dilute with water and precipitate the 
chlorine with silver nitrate. It is better, however, to proceed as 
follows: The antimony is precipitated by hydrogen sulphide as its 


HYDROCHLORIC. ACID. 323 


sulphide, the excess of the latter is removed by passing carbon 
dioxide through the solution, after which the precipitate is filtered 
and washed. The filtrate containing all the chlorine is made 
slightly ammoniacal, a little hydrogen peroxide or potassium per- 
carbonate is added (both reagents must be free from chloride) and 
the solution boiled until the excess of the peroxide is destroyed. 
By this treatment traces of hydrogen sulphide remaining in the 
solution are oxidized to sulphuric acid. After cooling, the solu- 
tion is acidified with nitric acid, and the chlorine determined as 
silver chloride as described above. 

According to this method, chlorine may ee determined in the 
presence of large amounts of hydrogen sulphide without difficulty. 

It is less practical to proceed as follows: The solution is satu- 
rated with ammonia and the hydrogen sulphide is precipitated by 
the addition of ammoniacal silver nitrate solution, the deposited 
silver sulphide is filtered off, washed with ammonia, and the silver 
chloride precipitated from the filtrate by acidifying with nitric 
acid, 


B. Analysis of an Insoluble Chloride. 


The substance is boiled with sodium carbonate solution * (free 
from chloride), and the chlorine determined in the filtrate as before. 

Many chlorides, e.g. silver chloride, many minerals such as 
apatite,t sodalite, and rocks containing the latter, are not decom- 
posed by boiling them with sodium carbonate. In such cases, the 
substance must be fused with sodium carbonate. | 

Silver chloride should be mixed with three times as much 
sodium carbonate and heated in a porcelain crucible until the 
mass has sintered together. The mass is treated with water, the 
insoluble silver filtered off, and the chlorine determined in the fil- 
trate as under (a). 

For the determination of chlorine in rocks, 1 gm. of the finely- 





* Mercurous chloride is decomposed only slowly by sodium carbonate 
solution, but readily acted upon by potassium or sodium hydroxide. 

+ According to Jannasch, chlorine in apatite may be determined by treat- 
ing the finely powdered mineral with nitric acid and silver nitrate on the 
water-bath. Everything goes into solution with the exception of silver 
chloride, which is filtered off and weighed. (This does not apply to a sample 
of apatite contaminated with silica or silicates.) 


324 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


powdered material is fused with four or five times as much sodium 
carbonate (or with a mixture of equal parts sodium and potassium 
carbonates) at first over a Bunsen burner, afterward over a Teclu 
burner or the blast-lamp. The melt is extracted with hot water. 
After cooling, methyl orange is added, the solution is acidified 
with nitric acid and allowed to stand overnight. If silicic acid 
has precipitated out by the next morning, a little ammonia is 
added, the solution is boiled, filteredy and washed with hot water. 
The cold filtrate is acidified with nitric acid and the chlorine 
determined as above. 

If there is no separation of silicic acid on acidifying the water 
extraction of the fusion with nitric acid,* the chlorine is precipi- 
tated at once from the cold solution. 


Free Chlorine. 


If it is desired to determine gravimetrically the amount of 
chlorine in a sample of chlorine water, it is not feasible to simplify 
add silver nitrate, for all of the chlorine is not precipitated as silver 
chloride; a part of it remains in solution as soluble silver chlorate: 


6Cl-++ 3H,0-+ 6AgNO, =5AgCl+ AgClO,+ 6HNO,. 


The chlorine, therefore, must be changed to hydrochloric acid 
or to one of its salts before attempting to precipitate with silver 
nitrate. This may be accomplished in several ways: 

1. A definite amount of the chlorine water is transferred by means 
of a pipette to a flask containing ammonia and after mixing the solu- 
tion is heated to boiling. After cooling the liquid is acidificd with 
nitric acid and precipitated by silver nitrate. The ammonia con- 
verts the chlorine partly to ammonium chloride and partly to am- 
monium hypochlorite. The latter is decomposed partly in the eold 
and quantitatively on warming into ammonium chloride and 
nitrogen: 

(a) 2NH,OH+2Cl=NH,Cl+ NH,4O0CI1+4+ H20; 
(b) 3NH,OCI]+2NH3=3H20+ N2+3NH,CL. 





* According to W. F. Hillebrand, there is no separation of silicic acid to 
be feared from 1 gm. of the substance. 


HYDROCHLORIC ACID. 325 


2. The chlorine water is treated with an excess of sulphurous 
acid, the solution is made ammoniacal, hydrogen peroxide is added, 
and the liquid boiled until the excess of hydrogen peroxide is re- 
moved. After cooling the solution is acidified with nitric acid, 
diluted with water, and the chlorine precipitated by means of silver 
nitrate. 

3. The chlorine water is treated with dilute caustic soda solu- 
tion, an aqueous solution of sodium arsenite is added (arsenic 
trioxide dissolved in sodium carbonate) until a drop of the liquid 
will not turn a piece of iodo-starch paper blue. It is then acidi- 
fied with nitric acid and the chlorine precipitated by a soluble 
silver salt. 

If the solution contains both free chlorine and hydrochloric 
acid, the total chlorine is determined by one of the above methods, 
while the free chlorine is determined in a separate sample by a 
volumetric process (see Iodimetry). 


Determination of Chlorine in Non-electrolytes (Organic 
Compounds). 


A. Method of Carius.* 


Principle—The method is based upon the fact that all organic 
compounds are decomposed by heating with concentrated nitric 
acid at a high temperature under pressure. If the substance con- 
tains halogen, sulphur, phosphorus, or arsenic, it is first set 
free as such, but on account of the reducing action of the nitrous 
acid formed it is then changed over into its hydrogen compound. 
The latter, however, is partly oxidized by the nitric acid. The 
reaction is therefore a reversible one. If, on the other hand, the 
substance is heated under the same conditions with nitric acid in 
the presence of silver nitrate, the halogen hydride is converted into 
silver halide as fast as it is formed and the halogen is in this case 
quantitatively changed into its silver salt. Sulphur, phosphorus, 
and arsenic are oxidized in the same way to sulphuric, phosphoric, 
and arsenic acids and any metals present form nitrates. 





* Ann. d. chiee u. Pharm. (1865), 136, p. 129, and Zeit. f. anal. Chem. 
4, p. 451. 


326 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Procedure for the Halogen Determination. 


A glass tube made of difficultly fusible glass (about 50 em. 
long, 2 cm. in diameter, with walls about 2 mm. thick) is sealed at 
one end, thoroughly cleaned and dried by sucking air through it. 

About 0.5 gm. of powdered silver nitrate (or in the case of 
substances rich in halogen as much as 1 gm. may be used) is trans- 
ferred to the tube by pouring the powder 
through a cylinder made by rolling up a 
piece of glazed paper and shoving the paper 
into the tube until it reaches about the 
middle of it. About 40 ¢.c. of pure nitric 
acid (sp. gr. 1.5) free from chlorine are 
poured into the tube through a funnel 
whose stem is about 40 cm. long. In this 
way only the lower half of the tube is wet 
with the acid. The tube is then inclined 
to one side and from 0.15-0.2 gm. of the 
substance contained in a small glass tube 
closed at one end is introduced into it (this 
smaller tube should be about 5 em. long 
and 5 mm. wide). As soon as the tube 
containing the substance has- reached the . 
acid, it remains suspended (Fig. 53, a). It 
is very important that the substance should not come in contact 
with the acid before the tube is closed at the upper end, as other- 
wise there is likelihood of some halogen escaping. 

The upper end of the tube is now heated very cautiously in 
the flame of the blast-lamp until the tube begins to soften and 
thicken (Fig. 53, b). It is then drawn out into a 3-5 em. long, 
thick-walled capillary and the end fused together (Fig. 53, c). 

After the tube has become cold, it is enveloped in asbestos 
paper, carefully shoved into the iron mantle of a ‘‘bomb furnace,” 
and gradually heated. Aliphatic substances are usually decom- 
posed by heating four hours at 150-200° C; substances of the 
aromatic series usually require from eight to ten hours’ heating at 
250-300° C., while in some cases an even longer heating at a higher 
temperature is necessary. The time and temperature must be 





Fig. 53. 


HYDROCHLORIC ACID. 327 


found out for each substance by experiment. The decomposition 
is complete when on cooling the contents of the tube neither 
erystals nor drops of oil are to be recognized.* The heating is so 
regulated that after three hours the temperature of about 200° C. 
is reached, after three hours more 250-270° C., and finally after 
another three hours a temperature of about 300°C. is attained.} 
After the heating is finished, the tube is allowed to cool completely 
in the furnace, the iron mantle together with the tube is then 
removed, and by slightly inclining the mantle the capillary of the 
tube is brought out into the open air. In most cases a drop of 
liquid will be found in the point of the latter. In order not to lose 
this, the outer point of the capillary is carefully heated with a 
free flame, and by this means the liquid is driven back into the 
other part of the tube. The point of'the capillary is now more 
strongly heated t until the glass softens, when it will be blown out 
in consequence of the pressure within the tube. The gas escapes 
with a hissing sound. When the contents of the tube are at the 
atmospheric pressure, a scratch is made upon it with a file just 
below the capillary, and this is touched with a hot glass rod, whcreby 
the tube usually breaks and the upper part can be removed. The 
contents of the tube are then carefully poured into a fairly large 
beaker without breaking the little tube in which the substance was 
‘weighed out, and the inner part of the tube as well as its capillary 
is washed out with water. The liquid in the beaker is diluted to 
about 300 ¢.c. and heated to boiling. After cooling, the insoluble 
silver halide is filtered off through a Gooch crucible, and after 
washing and drying at 130° C. its weight is determined. 

If it is thought that the precipitate is contaminated by frag- 
ments of broken glass, as is often the case even with careful work, 
the clear liquid is decanted through a filter, the residue washed by 





* Sometimes, with substances rich in sulphur, crystals of nitrosyl sulphuric 
acid are formed and adhere to the sides of the tube. They are easily distin- 
guished from crystals of the undecomposed substance. 

+ Such a high pressure is often attained that the tube bursts as soon as it 
is heated very hot. In such cases it should be heated to only 200° C., allowed 
to cool, the capillary opened and the gas set free. It is then fused together 
again and heated to the desired temperature. 

t Before heating, the tube and the hand should be wrapped in a towel to 
avoid accidents. 


328 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


decantation with very dilute nitric acid to the disappearance of 
the silver reaction, and the residue (except when it is silver iodide) 
is dissolved in warm ammonia water. The solution is filtered 
through the same filter, but the filtrate is this time collected in a 
fresh beaker. After washing the filter with dilute ammonia, the 
filtrate is acidified with nitric acid, heated to boiling, and after 
allowing the silver chloride or bromide to settle in the dark, it is 
filtered through a Gooch crucible, dfed at 130° C., and weighed. 

In the case of silver iodide, it cannot be dissolved in ammonia 
and in this way separated from splinters of glass. In this case the 
substance, together with the glass, is filtered through an ordinary 
washed filter (not a Gooch crucible), completely washed with dilute 
nitric acid, then once with alcohol in order to remove the nitric 
acid, and dried at 100° C. As much of the precipitate as possible 
is transferred to a watch-glass, the filter burned, and its ash placed 
in a weighed porcelain crucible. A little dilute nitric acid is added 
(in order to change any reduced silver into the nitrate), the liquid is 
evaporated on the water-bath, a few drops of water and a drop of 
pure hydriodic acid are added, and the contents of the crucible are 
again evaporated to dryness, when the main part of the precipitate is 
added, heated until it begins to fuse, and then weighed. The mass 
in the crucible is then covered with pure dilute sulphuric acid, a 
piece of chemically pure zine is added, and the crucible allowed to 
stand overnight. After this time the silver iodide will be com- 
pletely reduced to metallic silver. The zinc is removed, and the 
residue washed by decanting several times with water until the 
iodine reaction can no longer be detected. The residue is then 
warmed with dilute nitric acid upon the water-bath, in order to 
dissolve the silver, the solution is filtered through a small filter; 
and the latter is washed with water and dried. This filter is ig- 
nited in a crucible and the residue (the glass) is weighed. This 
second weight deducted from the former gives the amount of silver 
iodide present. , 

This method is also suitable for obtaining lead and mercury 
from organic compounds in a form which can be precipitated by 
hydrogen sulphide. — 

The method of Carius is by far the best for the determination of 
halogens in organic substances when only one of the halogens is 


HYDROBROMIC ACID. 329 


present. If two or three of them are present at the same time. 
the ‘‘ lime method ” is to be preferred. 
: The Lime Method. 

Into a glass tube made of difficultly fusible glass (about 40 em. 
long, 1 em. wide and closed at one end), a layer of lime (free 
from chloride) from 5 to 6 cm. long is introduced, then about 0.5 
gm. of substance, and finally 5 cm. more of lime. The substance is 
then mixed thoroughly with the lime by means of a copper wire 
whose end is wound into a spiral. The tube is nearly filled 
with lime, placed on its side, and gently tapped so that a small 
canal is formed above the lime. The tube is then placed in a small 
combustion furnace (cf.Carbon) and heated. First of all the front 
end of the tube, free from substance, is heated to a dull redness, 
then the back end, and afterward the other burners are lighted one 
after another until finally the whole tube is at a dull-red heat. After 
cooling, the contents of the tube are transferred to a large beaker 
and the lime dissolved in dilute nitric acid free from chlorine. The 
carbon is filtered off, and the halogen precipitated with silver nitrate. 

If the lime contains calcium sulphate, this is reduced to sul- 
phide, so that some silver sulphide is likely to be precipitated with 
the silver halide. In this case the solution is treated with hydrogen 
peroxide (free from halogen) before enough nitric acid has been 
added to make the solution acid, the liquid is boiled to remove 
the excess of the reagent, then acidified, filtered, and precipitated 
with silver nitrate.* In the analysis of substances rich in nitrogen, 
it is possible that some soluble calcium cyanide will be formed. 
In this case care must be taken that the silver precipitate con- 
tains no silver cyanide (cf. Separation of Cyanogen from Chlorine- 
Bromine, and Iodine, p. 339). 


HYDROBROMIC ACID, HBr. Mol. Wt. 80.93. 
Form: Silver Bromide, AgBr. 
Hydrobromic acid is determined exactly the same as hydro- 
chloric acid. This is also true of the determination of free bro- 
mine, and bromine in non-electrolytes. 


_ * W. Biltz (Chem. Ztg. , 1903, Rep. 142), separates the halides from sulphide 
by treating the precipitated silver salts with an ammoniacal sodium thiosul- 
phate solution, whereby the silver halide goes into solution, from which the 
silver is precipitated as silver sulphide, by adding ammonium sulphide, and 
determined as silver. 





330 GRAVIMETRIC DETERMINATION OF THE METALLOIDS., 


HYDRIODIC ACID, HI. Mol. Wt. 127.93. 


Forms: Silver Iodide, AgI, and Palladous 
Iodide, PdI.. 


(a) Determination as Silver Iodide. 


The determination of hydriodic acid is carried out in exactly 
the same way as the analysis of hydrochloric acid. If it is de- 
sired to filter the silver iodide through an ordinary washed filter 
instead of through a Gooch crucible, the procedure described on 
p. 328 is used, converting the reduced metal to iodide by dissolv- 
ing in nitric acid and adding hydriodicacid. In case there is no 
hydriodic acid at one’s disposal, the main portion of the precipitate 
is placed in a weighed porcelain crucible and heated until it begins 
to melt and then weighed. The filter ash is placed in another 
crucible, and treated with nitric and hydrochloric acids, whereby 
the silver and any unreduced iodide are changed to silver chloride. 
The silver chloride is weighed and the equivalent amount of silver 
iodide is added to the weight of the main part of the precipitate. 

Example.—Suppose a grams substance gave p grams silver 
iodide and p’ grams silver chloride, then 


AgCl: Agl=p’:z, 


el 
v= Kecl 
: AgI , ‘ 
We have, therefore, in a grams substance p+- Aecl? grams silver 


iodide, and the amount of iodine present may be calculated in the 
usual manner. 


(b) Determination as Palladous Iodide. 


This important method for the separation of iodine from 
bromine and chlorine is carried out as follows: 
The solution is acidified with hydrochloric acid, and palladous 
chloride solution is added until no more precipitate is formed. 
After standing one or two days in a warm place, the brownish- 
black precipitate of palladous iodide is filtered through a Gooch 


SEPARATION OF IODINE FROM CHLORINE, 331 


crucible, or through a tared filter that has been dried at 100° C., 
washed with warm water, dried at 100° C., and weighed as Pdl,. | 

According to Rose, the PdI, may be changed to palladium by 
igniting in a current of hydrogen, and from the weight of the palla- 
dium the amount of iodine calculated. 


SEPARATION OF THE HALOGENS FROM ONE ANOTHER, 


1. Separation of Iodine from Chlorine, 


(a) The Palladous Iodide Method. 


The iodine is determined as above as palladous iodide, and in a 
second sample the sum of the chlorine and iodine is determined 
from the weight of their insolwble silver salts. 


(b) Method of Gooch. 


This method depends upon the fact that in a dilute solution of 
the three halogens, nitrous acid sets free iodine alone: 


2KI+ 2KNO,-+ 4H,SO,=4KHSO,+2NO+ 2H,0+I,, 


which escapes from the solution on boiling. In one sample, there- 
fore, the halogens are precipitated together in the form of their 
silver salts, in a second sample the amount of the chlorine is deter- 
mined after setting free the iodine by means of nitrous acid, and 
the amount of iodine determined by difference. In order to obtain 
correct results by this method, the solution must be very dilute 
when it is boiled to expel the iodine; otherwise some chlorine 
escapes. : 

Procedure.—The mixture of the halogen salts (about 0.5.gm. 
of the substance should be dissolved in 600-700 ¢.c. water in a 
liter flask) is treated with 2-3 c.c. of dilute sulphuric acid, 
- 0.5-1 gm. of solid potassium nitrite (free from halogen) is added, 
and the solution is boiled until entirely colorless; in most cases this 
is accomplished in about three-quarters of an hour. The contents 
of the flask are now treated with silver nitrate solution, and the 
resulting precipitate is allowed to settle. It is filtered through a 
Gooch crucible, and weighed, 


332 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


(c) Method of Jannasch.* 


Jannasch proceeds in exactly the same way as Gooch, but in- 
stead of letting the iodine escape, he collects in it a mixture of 
caustic soda and hydrogen peroxide, whereby it is transformed to 
sodium iodide and is subsequently determined as silver iodide. 
In the other solution the chlorine is determined in the usual way. 

Procedure.—The solution contaiping the two halogens is placed 
in a 14-liter round-bottomed flask and diluted to a volume of 600-700 
c.c. Like a wash-bottle, this flask is provided with one glass tube 
reaching to the bottom, through which vapor can. be conducted 
into the flask, and with another shorter tube for the escape of gas. 
This second tube is connected with an Erlenmeyer flask for a re- 
ceiver, and this is in turn connected with a Péligot tube. About 
50 c.c. of pure 5 per cent. caustic soda solution are placed in the 
Erlenmeyer flask, an equal volume of hydrogen peroxide free from 
chlorine is added, and the mixture cooled by surrounding the flask 
with ice orsnow. The Péligot tube is likewise filled with a suitable 
amount of caustic soda and hydrogen peroxide. From 5-10 c.c. 
of dilute sulphuric acid (1:5) and 10 ¢.c. of 10 per cent. sodium 
nitrite solution are now added to the solution containing the halo- 
gens, the flask is immediately closed, and the contents of the flask 
are heated over a free flame while water vapor is at the same time 
conducted into it. As soon as the liquid begins to boil, the space 
above is filled with violet vapors of iodine, which are gradually 
driven over into the Erlenmeyer flask, where, with evolution of 
oxygen, they are completely absorbed by the hydrogen peroxide 
solution. The iodine is changed into sodium iodide and sodium 
hypoiodite by means of the dilute alkali: 


| 21+ 2NaOH = Nal-+ NaOI+ H,O. 
The sodium hypoiodite, however, is reduced by the hydrogen 


peroxide to sodium iodide: 
NaOI+ H,O, = H,0 + O,+ Nal. 


When all the iodine is driven over into the receiver (which is 
always the case after the solution in the flask has become colorless 





* Zeit. fiir anorg. Chem. I, p. 144, and Prakt. Leit. der Gewichts-analyse, 
p. 182 et seq. 


SEPARATION OF IODINE FROM CHLORINE. 333 


and has been boiled for twenty minutes longer), the delivery-tube 
between the distilling-flask and the Erlenmeyer flask is removed, 
the liquid within it is washed with hot water into the Erlenmeyer 
and the current of steam is stopped. The contents of the Péligot 
tube are added to the Erlenmeyer flask and the solution heated to 
boiling in order to remove the excess of hydrogen peroxide. After 
cooling, the liquid is acidified with a little sulphuric acid; this always 
causes a yellow coloration due to free iodine.* The solution, there- 
fore, is treated with a few drops of sulphurous acid, whereby it is 
completely decolorized. An excess of silver nitrate and a little 
nitric acid are then added, the liquid is boiled, and the silver 
iodide filtered through a Gooch crucible and weighed. 

For the chlorine determination, the contents of the distilling- 
flask are transferred to a beaker and the chlorine determined as 
silver chloride. 

Remark.—This method has been carefully tested in the author’s 
laboratory by O. Brunner, and in the above form has been found 
to give very exact results. 

Jannasch recommends a slightly different procedure. He 
adds silver nitrate directly to the alkaline solution in the Erlen- 
meyer flask. In this way accurate results are obtained provided 
there is no iodate formed by the absorption of the iodine. In 
the latter case, due to insufficient cooling of the contents of the 
receiver, the addition of silver nitrate results in the formation of 
some silver iodate, and this amount of iodine escapes determination, 
for silver iodate is soluble (though difficultly so). In such cases _ 
the results obtained are too low. If the solution is acidified, how- 
ever, the presence of the iodate is shown by the separation of a 
little iodine, and this can be changed: by sulphurous acid to iodide, 
and accurate results will be obtained. — 





* If the above directions are closely followed, there should not be much 
separation of iodine. It may be caused by the presence of a small amount 
of nitrous acid which is not oxidized to nitric acid by hydrogen peroxide or 
if the contents of the Erlenmeyer flask are not kept cool, appreciable amounts 
of sodium iodate (NaIO,) are formed, and the latter is not reduced by hydro- 
gen peroxide. In this case there isa separation of .a:considerable amount of 
iodine on acidifying the solution, but the addition of sulphurous acid changes 
it to iodide without loss. 


334 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Determination of the Halogens by Indirect Analysis. 


(a) Determination of Bromine together with Chlorine. 


Principle.—In this method the sum of the weights of the silver 
salts of the two halogens is first determined and afterwards the 
silver bromide converted to silver chloride by heating in a current 
of chlorine. | | 

Procedure.—The solution contfining about 0.5 gm. of the 
halogen salt is acidified with a little nitric acid (free from chlorine) 
and precipitated in the cold by the addition of a slight excess of 
silver nitrate. The liquid is heated to boiling, with frequent stir- 
ring, and after cooling again, the precipitate is filtered through 
a 15 cm. long, asbestos filter-tube made of difficultly fusible glass, 
The precipitate is dried at 150° C. and weighed after cooling. 

For the transformation of the bromide into chloride, the asbes- — 
tos is shoved forward a little in the tube by means of a glass rod 
_ (in order that the gas may pass through it more readily), the tube 
is fastened in a slightly inclined position, and a current of dry 
chlorine gas is passed through it. At the same time the tube is 
heated cautiously by moving a small flame back and forth. During 
the first half hour the precipitate should not be heated hot 
enough to melt it; finally, however, the temperature is raised © 
until it begins to melt, after which the chlorine is replaced by air, 
and after cooling the residue is again weighed. 

If p represents the combined weight of the two silver salts, 
and gq the weight after the silver has been completely changed 
to chloride, then 

AgCl AgBr 
lex+y =p 
2.2 + my = q (AgCl) 


and from this it follows: 


1 
3. Y= 7 (P-O- 


AgCl 
FS Tie 0.7633. 


If this value is substituted in equation (3), we obtain 
(AgBr) y=4.224 (p—q) 


In this equation m= 


DETERMINATION OF IODINE TOGETHER WITH CHLORINE: 335 


and 
(AgC) r=p—y 
from which the amount of bromine and chlorine may be calculated. 


(b) Determination of Iodine together with Chlorine. 


The same procedure is used as above described , 
If p represents the weight of silver iodide+silver chloride and 
q the weight after the silver has been converted to chloride, then 


AgCl . AgI 
Lith y= Pp 
2.2 + my = q (AgCl) 


and from this it follows: 


_If this value is substituted in equation (3), we obtain 


(Agl) y=2.567(p—q) 
and : 
(AgCl) x=p—y. 


(c) Determination of Bromine in the Presence of Iodine. 


In this case p represents the weight of the silver bromide and 
silver iodide, and q as before the corresponding weight of silver 
chloride: | , 

AgI AgBr 
lLzt+y=p 
2. mz + ny = Qg(AgCl) 





and 
n 1 
‘ nm name 
in which » 
mae _ 0.6108 and n= se! _0.7633. 


AglI AgBr 


336 GRAVIMETRIC DETERMINATION OF THE METALLOIDS, 


If these values for m and nm are substituted in equation (3), 
we obtain 
(Agl) x=4.996- p—6.545-q 
and 

(AgBr) y=p—z. 


(d) Determination of Iodine, Bromine, and Chlorine in the Presence 
of One Anether. 


In one portion of the substance the total weight (P) of the 
halogen salts is determined, and this is.changed over into chloride 
whose weight (Q) is obtained. In asecond portion of the substance, 
the iodine is determined as palladous iodide, whose weight is (¢). 

If (¢) is multiplied by 1.303, the corresponding weight of silver 
iodide is obtained (p). | 

If (p) is subtracted from (P), the sum of the weights of the 
silver bromide and silver chloride is obtained (P—>p). 

Again, if (4) is multiplied by 0.7951, the corresponding weight 
of silver chloride is obtained (q), and if this is subtracted from (Q), 
the amount of silver chloride (Q—gq) will be obtained which corre- 
sponds to the amount that would be obtained from the weight 
(P—p). 

If, then, the amount of silver chloride is designated by x and 
the amount of silver bromide by y, we have: 


AgCl AgBr 
lz+ y=(P-p) 
2. x + my = (Q—9q) 


from which follows from p. 334, (a): 


3. y= [(P—) -(Q-9)} 


(AgBr) y=4.224[(P—p) —(Q—q)] 
and 
(AgCl) x=(P—p)-y. 


Instead of determining the iodine as palladous iodide it may | 
be removed as on page 331, 6, by treatment with nitrous acid 
and the weight of the: silver bromide + silver chloride obtained. 


HYDROCYANIC ACID. 337 


The amount of chlorine, bromine, and iodine follows from the 
above calculation. 

For the determination of bromine and iodine volumetrically 
consult Part II, Iodimetry. 


HYDROCYANIC ACID, HCN. Mol. Wt. 27.02. 


Forms: Silver Cyanide, AgCN, and Metallic 
Silver, Ag. 


Free hydrocyanic acid as well as the cyanides of the alkalies 
and alkaline earths are decomposed quantitatively by silver nitrate 
with the formation of insoluble silver cyanide. 

If, therefore, it is desired to determine gravimetrically the 
amount of cyanide present in an aqueous solution of hydrocyanic 
acid or of an alkaline cyanide, the cold solution is treated with an 
excess of silver nitrate, stirred, a little dilute nitric acid is added, 
the precipitate allowed to settle and it is then filtered through a 
weighed filter, dried at 100°C. and weighed. To confirm the 
result, the silver cyanide is placed in a porcelain crucible, the filter 
burned in a.platinum spiral, its ash added to the main portion 
of the precipitate, and the contents of the crucible ignited, at 
first gently and finally until the silver begins to melt; it is then 
weighed. 

By the decomposition of the silver cyanide, difficultly volatile 
paracyanide is formed, but this is gradually burned away by 
igniting the contents of the open crucible. 

Example: Determination of Hydrocyanic acid in Bitter-almond 
Water.—Bitter-almond water contains cyanogen as free hydrocyanic 
acid and as ammonium cyanide, but the greater part is present as 
-mandelic acid nitrile, C,H;CH(OH)CN. The latter compound is not 
decomposed in aqueous solution by means of silver nitrate, but is 
readily acted upon by the latter if the solution is made ammoniacal 
after the addition of the silver nitrate and then made acid. 

The gravimetric determination of the cyanogen present is per- 
formed according to the method of Feldhaus * as follows: 

100 gms. of bitter-almond water are treated with 10 c.c. of a 10 
per cent. silver nitrate solution, 2-3 ¢.c. of concentrated ammonia 


* Z. anal. Chem. IIT (1864), p. 34. 





338 GRAVIMETRIC DETERMIANTION OF THE METALLOIDS. 


are added, the solution is immediately acidifisd with nitric acid, 
the precipitate allowed to settle, and the HCN determined ag 
described above. 

 Liebig’s volumetric method is much more busistotOry for this 
determination (see Part II, Precipitation Analyses). 

If it is desired to determine the amount of cyanogen in a solid 
alkali cyanide, a weighed amount of the salt is dissolved in water 
containing silver nitrate, and theysolution then acidified with 
nitric acid and the precipitate treated as above. 

If the cyanide is dissolved in water before the addition of the 
silver nitrate, there is always a slight loss of hydrocyanic acid. 

Some complex cyanides are quantitatively decomposed by 
silver nitrate, e.g. those of nickel, zinc, and vopper (the latter 
only slowly); while others such as the ferro- and ferricyanides of 
the alkalies (and mercuric cyanide) are not. 


Determination of Cyanogen in Mercuric Cyanide, Method of Rose. 


Mercurie cyanide is a non-electrolyte and is consequently not 
precipitated by silver nitrate, but it is acted upon by hydrogen 
sulphide with the formation of insoluble mercuric sulphide and 
hydrocyanic acid: 


He(CN), +H,S=HgS+2HCN. 


This reaction, however, cannot take place in neutral or acid 
solutions on account of the volatility of the hydrocyanic acid; it 
must be performed in an alkaline solution. In order to avoid 
the introduction of an excess of hydrogen sulphide into the solu- 
tion, the following procedure is necessary: 

The solution of the mercuric cyanide is treated with ee 
twice as much zine sulphate dissolved in ammonia. If this should 
cause a turbidity, enough ammonia is added to clear it up and 
hydrogen sulphide water is then slowly poured in. This causes at 
first a brown precipitate which becomes black on stirring. The 
hydrogen sulphide water is added until the upper liquid shows 
a pure white precipitate of zinc sulphide. The zine sulphate, 
therefore, serves, as it were, as an indicator, inasmuch as the 
pure white precipitate will not be formed until the mercury is 
completely precipitated. The precipitated sulphides are now 


$ 


SULPHOCYANIC ACID. 339 


filtered off and washed with dilute ammonia. The filtrate con- 
tains all of the hydrocyanic acid and is treated with an excess of 
silver nitrate, acidified with nitric acid filtered and the weight of 
the silver cyanide determined as described on page 337. 


Determination of Hydrocyanic Acid and Halogen Hydride in the 
Presence of One Another, according to Neubauer and — 
Kerner.* 


The solution is treated with silver nitrate in the cold, the 
precipitate filtered, dried at 100°C. and in this way the total 
weight of the silver salts is determined. A definite amount of 
the precipitate is placed in a porcelain crucible, heated until it 
is completely melted, and then reduced with zine and sulphuric 
acid as described on page 328. The metallic silver and para- 
cyanogen are filtered off and the halogen determined in the fil- 
trate according to page 320 et seq. 

The above separation can be more satisfactorily effected by 
means of a volumetric process (See Precipitation Analyses). 


SULPHOCYANIC ACID, HCNS. Mol. Wt. 59.09. 
Forms: Cug(CNS)s, Ag(CNS), BaSO,. 
1. Determination as Cuprous Sulphocyanate, Cu. (CNS)>. 


The solution of the alkali sulphocyanate, which is neutral or 
slightly acid with hydrochloric or sulphuric acid, is treated with 
20 to 50 c.c. of a saturated solution of sulphurous acid, and copper 
sulphate solution is introduced with constant stirring until a 
slightly greenish tint is imparted to the liquid. After standing 
a few hours, the precipitate is filtered into a Munroe crucible, 
washed with cold water containing sulphurous acid, then once 
with alcohol, and dried at 130° to 140° to constant weight. The 
results are good. 





* Ann. d. Chem. u. Pharm. (1857), 101, p. 344. 


340 GRAVIMETRIC DETERMINATION OF THE METALLOIDS, 


2. Determination as Silver Sulphocyanate, Ag(CNS). 


This excellent method for estimating sulphocyanic acid is only 
applicable in the absence of the halogen acids, or hydrocyanic 
acid. | 
The dilute solution of the alkali sulphocyanate is treated in 
the cold with a slight excess of silver nitrate solution, which has 
been slightly acidified with nitric acid. After stirring well, the 
precipitate is filtered into a Munroe crucible, washed with water, 
then with a little alcohol, dried at. 130° to 150° and weighed. 

R. Philipp obtained good results by this method. 


3. Determination as Barium Sulphate. 


In the absence of all other compounds containing sulphur, 
thiocyanic acid may be determined with accuracy by oxidizing 


it and precipitating the sulphuric acid formed as barium sul- 
phate. Bromine water is the most suitable oxidizing agent for 
this purpose. The alkali sulphocyanate solution is treated with 
an excess of bromine water, heated for from thirty minutes to an 
hour on the water-bath, the solution acidified with hydrochloric 
acid, and the sulphuric acid precipitated (according to the direc- 
tions on p. 464 et seq.) by means of barium chloride, and 
weighed as barium sulphate. 

Instead of bromine, nitric acid may be employed as the 
oxidizing agent. | 

It will not do at all, however, to treat a solid alkali sulpho- 
cyanate with strong nitric acid in an open vessel, for on account of 
the violent action some of the hydrocyanic acid is volatilized and 
escapes oxidation. It is better, as E. Heberlein found in the 
author’s laboratory, to dissolve the alkali sulphocyanate in water 
(Heberlein used 20 c.c. of a one-tenth normal potassium sulphc- — 
cyanide solution). and add 10 c.c. of fuming nitric acid, keeping the 
beaker surrounded with ice. The solution is at first colored yellow, 
then deep red, reddish brown, and finally becomes colorless. The 
sulphur is then by no means entirely oxidized to sulphuric acid; 
to accomplish this the solution must be kept boiling gently 


SULPHOCYANIC ACID. 342 


for two hours. It is then evaporated almost to dryness, taken 
up in 200 c.c. of water, precipitated hot with barium chloride 
solution and the barium sulphate filtered off and weighed. Heber- 
lein found 99.79—99.94 per cent. of the potassium sulphocyanate 
taken. The oxidation is more certain, if the solution of the alkali 
sulphocyanate is placed in a flask connected with a return-flow 
condenser, treated with an excess of fuming nitric acid, boiled 
two hours and then treated as above. In this way Heberlein 
found 100.1 and 100.2 per cent. of the theoretical amount of sul- 
phocyanic acid. The oxidation of the sulphocyanic acid is still 
better effected by first precipitating the acid in the form of its 
silver salt * and filtering it off (it is only necessary to wash the 
precipitate when a sulphate is also present). The funnel contain- 
ing the precipitate is then placed over a small flask, the apex of 
the filter is broken with a glass rod and the precipitate washed 
into the flask by means of a stream of nitric acid (sp. gr. 1.37-1.40). 
In this way there is no violent reaction and no loss of sulphocyanic 
acid to be feared. The contents of the flask are heated to boil- 
ing for three-quarters of an hour. If at the end of this time, red 
vapors are still evolved from the flask (usually due to small par- 
ticles of filter paper) it makes no difference; the oxidation of the 
sulphocyanic acid is sure to have been complete. The contents of 
the flask are evaporated to a small volume in order to remove the 
excess of nitric acid, taken up with water and the silver precipitated 
as chloride and filtered off. The sulphuric acid is precipitated in 
the filtrate as barium sulphate and the latter is weighed.j 
Hydrogen peroxide in ammoniacal solution also oxidizes sulpho- 
cyanic acid completely to sulphuric acid but the oxidation requires 
more time than in the case of nitric acid. By this method, accord- 
ing to Heberlein, the alkali sulphocyanate is treated with a large 
excess of 3 to 4 per cent. hydrogen peroxide (for 20 c.c. of one-tenth 
normal sulphocyanate solution, 120 c.c. of 3 to 4 per cent. hydrogen 
peroxide are used), the solution made ammoniacal, allowed to stand 
twenty-four hours at the ordinary temperature, then heated two 
hours on the water-bath, and finally boiled. After acidifying with 
* W. Borchers, Repertorium der anal. Chemie, 1881, p. 130. 


+ Borchers precipitates the sulphuric acid without removing the silver 
by means of barium nitrate. The procedure given here is better. 





x 
342 GRAVIMETRIC DETERMINATION OF THE ME TALLOIDS. 


hydrochloric acid the sulphuric acid is precipitated with barium 
chloride and the barium sulphate formed is weighed. 

The oxidation is effected even more slowly by potassium per- 
carbonate. 


Determination of Sulphocyanic and Hydrocyanic Acids in the 
Presence of One Another (Borchers).* 


The amount of silver nitrate necessary to precipitate both of 
the acids is determined volumetrically in one sample of the substance 
(see Precipitation Analysis) and in a second portion the weight of 
the barium sulphate formed after the oxidation of the sulpho- 
cyanic acid is determined. From the latter weight the amount 
of sulphocyanic acid present can be computed and also the amount 
of silver nitrate that would be required to precipitate it. If this 
amount is subtracted from the amount of silver nitrate required 
to precipitate both of the acids, the amount of silver nitrate 
equivalent to the hydrocyanic acid present is obtained. 


Determination of Sulphocyanic Acid together with Halogen 
Hydrides (Volhard). 


In one portion the amount of sulphocyanic acid present is deter- 
mined as barium sulphate after oxidation with nitric acid. A 
second portion is heated in a closed tube with concentrated nitric 
acid and silver nitrate (Carius Method,}+ page 325) after which the 
halogen silver salts are filtered off, weighed, and subsequently 
changed to silver chloride as described on page 334. A third por- 
tion is fused with sodium carbonate and potassium nitrate and 
the iodine determined from the melt as palladous iodide (see 
page 330). From the data thus obtained, the three halogens are 
computed (see page 336). 


HYDROFERROCYANIC AcID, H4Fe(CN),. Mol. Wt. 215.9. 
Form: Silver Cyanide, AgCN. 
The most accurate procedure for the analysis of cyanides is to de- 
termine the carbon and nitrogen by elementary analysis (which see). 
*: 708i Clbecc ; 
+ Instead of using the Carius method, the halogens and sulphocyanide may ~ 


be precipitated by silver nitrate, filtered through a Gooch crucible, dried at . 
160° and weighed. 





- HYDROFERROCYANIC ACID. : 343 


Determination as Silver Cyanide (Rose-Finkener). 


This method depends upon the fact that all salts of hydroferro- 
cyanic acid on being heated with yellow mercuric oxide give up 
their cyanogen to the mercury, forming soluble mercuric cyanide, 
“while the iron is changed to insoluble ferric hydroxide. Thus 
Prussian blue is decomposed as follows: 


Fe,’ Fe” (CN) ¢]; + 9HgO0 + 9H,0= 
—9Hg(CN),-+4Fe(OH),+3Fe(OH),. 


A weighed amount of the substance is treated with an excess 
of mercuric oxide and the liquid is boiled until the blue color has 
completely disappeared, when the precipitate is filtered off. 

On filtering off the insoluble oxides, at first a clear filtrate is 
obtained, but on washing some of the precipitate usually passes 
through the filter. By washing with a solution containing a dis- 
solved salt, preferably mercuric nitrate, it is possible to obtain, how- 
ever, a clear filtrate. ven then the operation is tedious, so that 
the attempt has been made to avoid the washing of the precipitate 
by diluting the liquid containing the precipitate suspended in it 
to a definite volume, filtering through a dry filter, measuring off 
a definite volume of the filtrate, and subsequently determining 
the cyanogen as silver cyanide after first precipitating out the 
mercury as sulphide (see p. 338). The amount of cyanide found 
is then calculated over into the amount that would have been 
obtained in case the whole of the solution had been used for the 
analysis. In this way an error is introduced which in some 
cases is considerable. Let us assume that the Prussian blue was 
decomposed in a 100-c.c. flask and after the decomposition was 
complete, the liquid was diluted up to the mark; and that in 
50 c.c. of the filtrate p gms. of cyanide were found. 

The amount of cyanide in the portion weighed out is not 2 pgms., 
for the volume of the liquid before filtering was not 100 c.c., but 
100—» c.c., where v is the volume of the suspended oxides. 
This volume v can be determined only approximately, so that the 
cyanogen determination by this method will never be abso- 
lutely certain. In order to obtain exact results, the first-men- 
tioned proczdure should be followed: or, better still, the amount 


344 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


of carbon and nitrogen should be determined by elementary 
analysis. 

Soluble ferrocyanides may be satisfactorily determined by 
titration with potassium permanganate (cf. Part II, Oxidation 
and Reduction Methods). For the determination of the iron 
and other metals, the substance is heated with concentrated sul- 
phuric acid, the residue after evaporation is dissolved in water, 
and the solution analyzed as usual.« 


HYDROFERRICYANIC ACID, H,Fe(CN),. Mol. Wt. 214.9. 


The ferricyanides are analyzed in the same way as the ferro- 
cyanides. 


HYPOCHLOROUS ACID, HCIO. Mol. Wt. 52.47. 


Hypochlorous acid is always determined volumetrically and 
will be discussed in Part II of this book, under Oxidation Methods, 


GROUP II. 
NITROUS, HYDROSULPHURIC, ACETIC, CYANIC, AND HYPO- 
PHOSPHOROUS ACIDS. 


NITROUS ACID, HNO». Mol. Wt. 47.02. 


Nitrous acid is either determined volumetrically, gasometric- 
ally, or colorimetrically. The two former methods will be dis- 
cussed in Parts II and III of the book. 


Colorimetric Determination, of Peter Griess. 


This method serves only for the determination of extremely 
small amounts of nitrous acid (e.g., in drinking-waters), and 
depends upon the formation of intensively colored azo-dyes. 

Inasmuch as azo-compounds are formed only when nitrous 
acid is present, they can all be used in testing for this acid, but 
the different substances do not prove equally sensitive as reagents. 
Thus in the production of tri-amido-azo-benzene (Bismarck brown) 
not less than ;3?,; mgm. of nitrous acid in a liter can be detected, 
while according to the following procedure ;,45 mgm. in a liter 
can be detected with certainty. To carry out the determination 
two solutions are necessary, one of sulphanilic acid and one of 


NITROUS ACID. : 345 


a-naphthylamine. Both substances are dissolved in acetic acid * 
and prepared according to the directions of Ilosvay + as follows: 

1. 0.5 gm. of sulphanilic acid is dissolved in 150 c.c. of dilute 
acetic acid. i 

2. 0.1 gm. of solid a-naphthylamine is boiled with 20 c.c. of 

water, the colorless solution is poured off from the bluish-violet 
residue, and 150 c.c. of dilute acetic acid are added. 
_ These two solutions are now mixed.{ It is not necessary to 
protect the reagent from the action of light, but it is desirable to 
keep impure air away from it. As long as the solution remains 
colorless it can be-used. If it comes in contact with nitrous acid, 
which is often present in the air, the reagent becomes red, and in 
this case it must be decolorized by shaking with zinc-dust before 
using. 

Besides the above reagent, it is necessary to prepare a solution 
of sodium nitrite of known strength. For this purpose a concen- 
trated solution. of commercial potassium nitrite is treated with 
silver nitrate solution, the precipitated silver nitrite is filtered off 
and washed a few times with cold water. In order to obtain abso- 
lutely pure silver nitrite the precipitate is dissolved in as little hot 
water as possible and quickly cooled. The mass of crystals is placed in 
a funnel provided with a platinum cone, and after being sucked free 
from mother-liquor, it is washed with asmall amount of distilled 
water. The silver nitrite is placed in a calcium chloride desiccator 
and allowed to dry in the dark. As soon as it has become dry 
(shown by its having assumed a constant weight) exactly 0.4047 gm. 
of it is weighed out into a liter flask and dissolved in hot distilled 
water. About 0.2 to 0.3 gm. of pure sodium chloride is added (i.e., 
a little more than the theoretical amount) in order to convert the 
silver nitrite into silver chloride and sodium nitrite. After becom- 
ing cold, the solution is diluted to exactly one liter with pure 
water, then thoroughly shaken, and the precipitate allowed to settle. 
After this, 100 c.c. of the clear liquid are pipetted into a second 





* P. Griess used dilute sulphuric acid to set free the nitrous acid. Tlosvay 
showed that if acetic acid were used the reaction was much more sensitive. 

+ Bull. chim. [2] 2, p. 317. 

t Lunge, Zeitschr. f. angew. Chem. 1899, Heft 23. 


346 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


liter flask and diluted up to the mark with water free from nitrous 
acid. 1c.c. of this solution contains 0.01 mgm. N,Og. 


Procedure jor the Determination. 


50 c.c. of the water to be examined are placed in a cylinder, 
such as is shown on p. 61, treated with 5 c.c. of the reagent, 
and the contents of the cylinder fhixed with the aid of the 
stirrer shown in Fig. 25; the cylinder is placed in water at 
about 70-80° C. If as much as 7/55 mgm. of nitrous acid is pres- 
ent in a liter of the water tested, the red coloration will appear 
within one minute; with relatively larger amounts (e.g., as much 
as 1 mgm. per liter) the solution is simply colored yellow, unless 
a concentrated solution of naphthylamine is used. Meanwhile 
in three other cylinders are placed respectively 0.1 ¢.c., 0.5 ¢.c., and 
1 c.c. of the solution containing a known amount of sodium nitrite; 
each is diluted with water up to the mark and treated with the 
reagent in the same way. As soon as a distinct red coloration is 
apparent, the colors are compared with that produced by the water 
to be analyzed. If the color of the unknown water lies between 
two of the standards—e.g., between that produced with 0.1 and 0.5 
c.c. of the standard—then three more standards are prepared con- 
taining, say, 0.2, 0.3, and 0.4 ¢.c. of the known solution. When 
the color of the unknown solution is.matched, then the water con- 
tains the same amount of nitrous acid as the standard. 

If the water contains considerable nitrous acid (e.g., over 0.3 
mgm. per liter), then the red coloration will be so dark that the 
colorimetric determination cannot be performed with certainty. 
In this case a definite volume of the water is diluted with distilled 
water and the nitrous acid present in this diluted water is deter- 
mined as before. 

Tromsdorff recommends for the determination of nitrous acid 
in drinking-water the use of zine iodide of starch, and comparing 
the blue color produced by the nitrous acid (cf. Vol. I, p. 287). If 
345 mgm. of nitrous acid is present in a liter, the blue color produced 
can be distinctly seen; with ;4; mgm. per liter, however, the color 
is so intense that it is unsuited for a colorimetric determination. 
This method is not to be recommended because in the first place 


HYDROSULPHURIC ACID. 347 


it is far less sensitive than the Griess method, and second because 
it can easily lead to error inasmuch as a blue color will be often 
produced when there is no nitrous acid present. Traces of hydrogen 
peroxide or ferric salts, which are likely to be present’in a drinking- 
water, will also cause the solution of zinc iodide of starch to turn 
blue. 


HYDROSULPHURIC ACID, H.S. Mol. Wt. 34.09. 


Formst Barium Sulphate, BaSO,, Hydrogen Sulphide, H,S, 
and colorimetrically. 


There are four cases to be considered: 


I. The determination of free hydrogen sulphide. 
II. The determination of sulphur in sulphides soluble in water. 
Ill. The determination of suiphur in sulphides insoluble in 
water but decomposable by dilute acids with evolution of hydro- 
gen sulphide. 
IV. The determination of sulphur in insoluble sulphides. 


I. Determination of Free Hydrogen Sulphide. 
(a) Determination of Hydrogen Sulphide in Gas Mixtures. 


In case it is desired to know the per cent. of hydrogen sulphide 
present in a mixture of gases, the analysis is best made volumetri- 
cally (see Part II, lodimetry), but it is possible to accomplish the 
same end by a gravimetric process. 

The source of the gas is connected by means of rubber tubing 
with the ten-bulb absorption-tube shown in Fig. 55, page 359, * which 
contains a solution of ammoniacal hydrogen peroxide free from sul- 
phuric acid. The other end of the absorption-tube is connected with 
an aspirator, i.e. a large bottle of about 4-5 liters capacity filled 
with water and closed by means of a double-bored stopper. Through 
one hole of the stopper is passed a right-angled glass tube which 
reaches just below the bottom of the stopper in the bottle, and ‘its 
other end is connected with the absorption-tube. Through the other 
hole in the stopper is placed a glass tube reaching to the bottom 





* Usually two of these tubes are used in order to make sure that none of 
the gas escapes absorption. 


348 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


of the bottle. The upper end of this tube is likewise bent, and is 
connected with a rubber tube to serve as a siphon; on the lower 
end of the rubber tube is a screw-cock. 

Before beginning the experiment, the air in the rubber tubing 
between the source of gas and the absorption-tube is removed 
by conducting the gas to be analyzed through it. When this is 
‘accomplished the tubing is connected with the absorption-tube. 
Water is now allowed to run slowl¥ from the aspirator into a 
vessel graduated in liters; after from 2-5 liters of the water have 
run out, the aspirator is closed by screwing up the cock on the 
siphon arm. The contents of the absorption-tube are poured into 
a beaker, slowly heated to boiling, and kept at this temperature 
for from five to ten minutes. The solution is then evaporated on 
the water-bath to a small volume, a little hydrochloric acid is 
added, the solution filtered if necessary, and the sulphuric acid pre- 
cipitated at a boiling temperature with a boiling solution of 
barium chloride. After the precipitate has settled, it is filtered 
off, ignited wet in a platinum crucible, and weighed as barium 
sulphate. 3 

Both at the beginning and end of the experiment it is necessary 
ta note the temperature of the room and the barometer reading. 
The mean of these readings is used for the calculation. The 
amount of hydrogen sulphide present in the gas is computed as 
follows: | 

The volume of water which has flowed out of the aspirator 
represents the volume of the gas that has been sucked through 
the apparatus less the amount absorbed by the ammoniacal hy- 
drogen peroxide solution. Let V represent the volume of water in 
liters which has flown from the aspirator and p the weight of 
barium sulphate found. 

Since one gram molecule of barium sulphate corresponds to 
one gram molecule of hydrogen sulphide and the latter assumes at 
0° GC. and 760 mm. pressure a volume of 22.159 liters,* we have: 


BaSO4:22.159: p: V4; 


Wis ee ciee volume of the hydrogen. sulphide absorbed. 
BaSO.4 

* According to Leduc, Comptes rendus, 125, 571 (1897) the density of H,S 

(referred to air=1) is 1.1895, from which the molecular volume is computed 


as 22.159 liters. 





DETERMINATION OF SULPHUR IN SULPHIDES. 349 


_ Now the volume (V) of the gas that passed through the apparatus 
_wwas at ¢° and B mm. pressure, while V; is measured at 0° C. and 
760 mm. pressure. It is necessary, therefore, to reduee V to 0°C 
and 760 mm. pressure. | 





V _V-(B—w)273 
°  760(273 +t) ° 
The volume of the gas drawn through the apparatus is then: 
BPS oF 


and we have: 
(V,+V,):V,=100:z, 


_ V,-100 = the per cent. by volume of hydrogen sulphide present. 


en LY, 





(b) Determination of the Amount of Hydrogen Sulphide Present 
in Solution. 


By means of a pipette a definite volume of the solution is 
measured out and allowed to run into ammoniacal hydrogen per- 
oxide with constant stirring of the latter by means of the pipette 
itself. After heating to boiling and acidifying with hydrochloric 
acid, the amount of sulphuric acid formed is determined as barium 
sulphate. 


II. Determination of Sulphur in Sulphides Soluble in Water. 


(a) The solution is treated with an excess of ammoniacal 
hydrogen peroxide water, slowly heated to boiling and kept at 
that temperature until the excess of the reagent is destroyed, when 
the sulphuric acid is precipitated with barium chloride and weighed 
as barium sulphate. 

(2) The solution is treated with bromine water until a perma- 
nent brown color is obtained,. when it is warmed, acidified with 
hydrochloric acid, and the sulphuric acid determined as barium 
sulphate. 

If the solution contains thiosulphate, sulphide, and sulphate, 
as is likely to be the case after standing in the air for some time, 
the sulphide sulphur is precipitated by means of cadmium acetate 
and the sulphur in the precipitate is determined as under III, or 
the cadmium sulphide is oxidized with either bromine water or 


350 GRA VIME TRIC DETERMINATION OF THE METALLOIDS. 


fuming nitric acid, and the sulphuric acid formed determined as 
barium sulphate. | 

The determination of thiosulphate, sulphide, and sulphite 
sulphur will be discussed in Part II of this book.under Iodimetry. 


III. The Determination of Sulphur in Sulphides Soluble in 
Dilute Acids. . | 


Principle-—The hydrogen sulphide is evolved by treatment 
of the sulphide with dilute acids, and absorbed in ammoniacal hydro- 
gen peroxide solution as under I; or the hydrogen sulphide is 
absorbed in caustic soda solution. and the sodium sulphide formed 
analyzed according to II; or finally the gas may be absorbed in a 
weighed tube containing pumice soaked with copper sulphate 
solution, in which case the gain in weight represents the amount of 
gas absorbed. : 


Evolution and Absorption of the Hydrogen Sulphide. 


In the case of sulphides rich in sulphur 0.25-0.50 gm. of the 
substance should betaken for the analysis, whereas of those contain- 
ing less sulphur a correspondingly larger amount should be taken. 
The substance is placed in an Erlenmeyer flask (Fig. 54, a) the con- 
nection between the flask and the receiver is broken and the air is 
expelled from K by conducting hydrogen gas through the delivery 
tube and out through the open stop-cock of 7. After a rapid 
current of hydrogen has passed through the apparatus for about 
five minutes, the receivers V and P are partly filled with an 
ammoniacal solution of hydrogen peroxide * (about 3-4 per cent. 
H,0,); placing about 100 c.c. of the solution in V and about 
10-20 c.c. in P. 

The receiver, V, is now connected with the delivery-tube from 
the evolution-flask K, and hydrogen is conducted from 7 throughout 
the whole apparatus for five minutes more in order to remove as 





* In case hydrogen peroxide is not at hand, the receivers should contain 
100 ¢c.c. of dilute sodium hydroxide solution (250 gm. to 1 liter). After the 
decomposition is complete the contents of the receiver are transferred to a 
beaker, 30-50 c.c. of bromine water are added, the solution acidified with 
hydrochloric acid (sp. gr. 1.19) and boiled while carbon dioxide is passed 
through it until the excess of bromine is completely expelled. The sulphuric 
acid formed is then precipitated with a hot solution of barium chloride. 
Instead of oxidizing the sodium sulphide to sodium sulphate it can be 
titrated with iodine (cf. lodimetry). 


ANALYSIS OF SULPHIDES. 351 


much as possible of the air from the receivers. After this, about 
20 c.c. of boiled water are introduced into K through 7’ so that the 
substance is entirely covered, then dilute hydrochloric acid (1 vol. 
concentrated acid + 1 vol. of boiled water) is slowly added to the 
contents of the flask and the decomposition is promoted by warm- 


3 
C0, 





le 
\ Fae 








Fig. 54. 
ing somewhat. When the evolution of the gas has ceased, the 
contents of K are heated to a gentle boiling and a slow current 
of hydrogen * is conducted through the apparatus from 7’ for 
twenty minutes, when the flame is removed and the current of 
hydrogen: is continued for fifteen minutes longer. At the end of 


this time, the hydrogen sulphide will surely completely be driven 
over into V.+ 





* The hydrogen is evolved from zinc and sulphuric acid in a Kipp gener- 
ator. The gas is washed first with an alkaline lead solution in order to remove 
traces of hydrogen sulphide and then with water. 

t By the absorption of the hydrogen sulphide in the ammoniacal solution 


352 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


The contents of the two receivers are washed into a beaker 
and slowly heated to boiling in order to effect the complete oxi- 
dation of the thiosulphuric and sulphurous acids and to expel the 
excess of the hydrogen peroxide. The solution is finally acidi. 
fied with hydrochloric acid and the sulphuric acid determined as 
barium sulphate. 

This method yields excellent results and can be appial 
to the . 


Determination of Sulphur in Iron and Steel. 


Inasmuch as the amount of sulphur present is so small, a large 
amount of the substance must be taken for the analysis. For 
pig iron 2-5 gms. are sufficient, while with steel 5 gms., and with 
wrought iron, as much as 10 gms. should be used. 

The determination is carried out in the same way as before, 
except in this case a stronger acid should be used (HCI sp. gr. 1.19); 
this is allowed to act upon the iron at once without first cover- 
ing it with water, and the boiling is continued for at least twenty 
minutes after the gas evolution has ceased. 

Instead of collecting the evolved hydrogen sulphide in ammo- 
niacal hydrogen peroxide, it is often more convenient to absorb 
it in ammoniacal cadmium solution, or in caustic soda solution, 
and determine the sulphur volumetrically by an iodimetric process. 
See Appendix I. 

Remark.—The sulphur present in steel or cast iron made by 
the Thomas-Gilchrist, or basic Bessemer, process can as a rule be 
determined accurately by this method. In the case of certain 
other steels and cast irons, however, the results are likely to be 
low. In order to carry out an accurate determination in such 
cases, a tube made of difficultly fusible glass (about 30 em. long 
and 1 cm. wide) is inserted between the evolution flask K and the 
absorption flask V (Fig. 54). After the air has been replaced by 
of hydrogen peroxide the latter is always colored somewhat yellow owing to 
the formation of a little ammonium disulphide. This yellow color can be 
distinctly seen in the delivery-tube, where it dips into the solution in the 
receiver and later disappears owing to further oxidation: 

(N Hy) 22 (NH4)25203—>(NH,4)2803—> (Ng) O04. 
When the color can no longer be detected, it is a sign that the greater part of 
the hydrosen sulphide has been driven over. 





r 


DETERMINATION OF SULPHUR IN IRON AND STEEL. 353 


hydrogen, this tube is heated to dark redness by means of a small 
furnace of from four to six burners, whereby the sulphur in the 
methyl sulphide passing through the tube is converted completely 
into hydrogen sulphide. 

When the use of this tube is adopted, care must be taken that 
no drops of water enter the red-hot tube. To this end, the liquid 
in the flask K should only be boiled very gently, or better, the flask 
should be connected with a return flow condenser. (Cf. p. 381.) 

The insoluble residue which is obtained especially in the case of 
irons containing considerable silicon, often contains an appreciable 
amount of sulphur. The residue is, therefore, filtered off, washed, 
dried, fused with sodium carbonate and potassium nitrate (cf. 
p. 357), the melt extracted with water, the resulting solution evap- 
orated with -hydrochloric acid, any deposited silicic acid filtered 
off, and the sulphuric acid in the final filtrate determined as 
barium sulphate in the usual way. 

Phillips and Blair have shown* that sulphur present in dif- 
ferent kinds of iron and steel, especially cast iron, may be present 
in four different conditions: 

1. By far the greater part is evolved as hydrogen sulphide 
when the metal is treated with hydrochloric acid. 

2. Another part is evolved probably as methyl sulphide (CHs3) 28, 
an extremely stable sulphur compound which is not very much 
affected byammoniacal hydrogen peroxide, bromine in hydrochloric 
acid, or aqua regia. The sulphur in this compound is changed 
completely into hydrogen sulphide: on being passed through a 
tube heated to redness, in which hydrogen is also present. 

3. Another part of the sulphur present is not volatilized by 
the action of hot dilute hydrochloric acid, but can be oxidized to 
sulphuric acid by treating the contents of the evolution flask with 
nitric acid or aqua regia. 

4. Another very small part of the sulphur may be present in 
the form of an insoluble sulphide which is not oxidized by nitric 
acid or aqua regia and can only be obtained in solution after 
fusion with sodium carbonate and potassium nitrate. 

More recent workt has shown, however, that annealing the 


* Cf. J. Am. Chem. Soc., 19, 114 (1897). 
+ T. G. Elliott, Chem. News, 104, 298 (1911), mixes 5 gms. of the iron or 





354 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


sample converts all the sulphur into a condition such that it is 
evolved as hydrogen sulphide when the metal is treated with 
hydrochloric acid, sp. gr. 1.19. 


Bamber Method for Determining Sulphur in Iron and Steel. 


On account of the uncertainty in obtaining all the sulphur 
present in iron or steel by the above evolution method, the Com- 
mittee on Standard Methods for the "Analysis of Iron—American 
Foundrymen’s Association, have recommended the following 
method, which is that proposed by Bamber. 

A 3-gm. sample of drillings is dissolved in concentrated nitric 
acid. After the iron is completely dissolved, 2 gms. of potassium 
nitrate are added, the solution is evaporated to dryness in a 
platinum dish and the dry residue is ignited over an alcohol 
lamp at a red heat. After the ignition, 50 c.c. of a 1 per cent. 
solution of sodium carbonate are added, the liquid boiled for a 
few minutes, and then filtered, washing the precipitate with hot 
1 per cent. sodium carbonate solution. The filtrate containing 
all the sulphur is evaporated to dryness with hydrochloric acid, 
the residue thus obtained is taken up in 50 e.c. of water and 2 c.e. 
of concentrated hydrochloric acid, and the resulting solution is 
filtered. The filtrate is diluted to a volume of about 100 c.c. and 
precipitated hot with barium chloride solution. 

During the determination great care should be taken to virtvel 
the absorption of fumes containing sulphur. For this reason a 
gas flame should not be used at any stage in the process. 


Colorimetric Determination of Sulphur in Iron and Steel.* 
Principle.-—The hydrogen sulphide evolved from a weighed 
amount of iron is passed through a cloth which has been wet with a 
solution of cadmium acetate. The hydrogen sulphide reacts with the 
cadmium salt to form yellow cadmium sulphide and the intensity 
of the color is proportional to the amount of hydrogen sulphide. 
If a grams of substance produce a certain shade then it would 
take 2a grams of a substance containing half as much sulphur to 





steel with 0.25 gm. of anhydrous potassium ferrocyanide, wraps the mixture 
in filter paper, places it in a porcelain crucible, and anneals at 750°-850° for 
20 minutes in a muffle furnace. 

* J. Wiborgh; Stahl and Eisen, 6 (1866), p. 240. 


DETERMINATION OF SULPHUR IN IRON AND STEEL. 355 


duplicate it, or in other words, the relations hold, as=a’s’, where 
a and a’ represent the amount of substance taken for the analysis 
and s and s’ the percentage of sulphur present. In the first place, 
then, a scale must be prepared of different shades. representing 
different percentages of sulphur. For this purpose, Wiborgh 
uses the apparatus shown in Fig. 55. It consists of a 250-300-c.c. 
H Erlenmeyer flask A with a side- 
nna arm funnel 7’ and with a ground- 
glass connection between the cylin- 
der B. The latter is about 20 cm. 
long, and is from 5.5-6.0 em. wide 
at the top and about 8 mm. at the 
bottom. The upper edge of the 
cylinder is rounded over and ground 
perfectly smooth. Upon this upper 
edge are placed two rubber rings 
of the same inner diameter as the 
glass cylinder. Between these two 
rings is laid a circular piece of cloth 
C that has been dipped in a solution 
of cadmium acetate, and upon the 
upper rubber ring is placed a wooden 
ring H which is held firmly against 
the edge of the cylinder by means 
of three clamps K (only two are 
shown in the illustration). 

The flask A is filled not quite half full with distilled water, the 
contents boiled a few minutes to remove the air, the flame is re- 
moved, and a weighing-tube containing a definite amount of a 
sample whose sulphur content is known is thrown into the 
flask. The cylinder, with the cadmium acetate cloth in position, 
is placed upon the flask, and the gentle boiling is continued until 
the cloth is uniformly moistened with the aqueous vapor which 
is seen to pass through it. The water must not be boiled too 
strongly and the cloth must not be allowed to puff up, for in that 
case it will become distorted and afterward an unevenly colored 
surface will be obtained. After boiling for three or four minutes 
sulphuric acid (1:5) is cautiously added, drop by drop, to the 
contents of the flask (3 ¢.c. for each 0.1 gm. of iron) through the 





Fia. 55. 


356 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


funnel 7. The evolution of hydrogen sulphide begins at once 
and is recognized by the cadmium acetate cloth becoming yellow. 
After the acid has all been added, the boiling is continued until 
there is no more gas evolved from the substance, and then for 
ten minutes more in order to completely expel it from the solu- 
tion. _ : 

The piece of cloth is now removed and placed upon a piece of 
white filter-paper, so that the side whach was toward the flask is on 
top. In the same way a scale of six different shades is prepared 
corresponding to the following table: 


Tint No. 1. Tint No. 4. 
Amount Per Cent. Amount Per Cent. 
Weighed Sulphur Weighed Sulphur 

Out. Present. Out. Present. 
0-8 0-0025 0-8 0-015 
0-4 0-005 0-4 0-030 
0-2 0-010 0-2 0-060 
0-1 0-020 0-1 0-120 
0-08 0-025 0-08 0-150 
0-04 0-050 0-04 0-300 
0-02 0-100 0-02 0-600 

Tint No, 2. Tint No. 5. 
0-8 0-005 0-8 0-025 
0-4 0-010 0-4 0-050 
0-2 0-020 | 0-2 0-100 
0-1 0-040 0-1 0-200 
0-08 0-050 0-08 0-250 
0-04 0-100 0-04 0-500 
0-02 0-200 0-02 1-000 

Tint No. 3. Tint No. 6. 
0-8 0-01 0-8 0-035 
0-4 0-02 0-4 0-070 
0-2 0-04 0-2 0-140 
0-1 0-08 0-1 -0-280 . 
0-08 0-10 0-08 0-350 
0-04 0-20 0-04 0-700 
0-02 0-40 0-02 1-400 


To illustrate the use of this table, suppose we wish to prepare 
the scale from a sample of steel containing exactly 0.17 per cent. 
of sulphur. How much of it should be weighed out in order to 
prepare Tint No. 1? 

From the table we know that this shade can be prepared by 


DETERMINATION OF SULPHUR IN INSOLUBLE SULPHIDES. 357 


weighing out 0.8 gm. of an iron containing 0.0025 per cent. sul- 
phur, and it follows from what has been said: 


0.8X 0.0025 =X 0.017 
0.8X 0.0025 _ 
=O 017 =0.0118 gm. 





We must, therefore, weigh out 0.0118 gm. of the steel in order 
to prepare Tint No. 1. . 

In the same way the amount necessary to produce Tint No. 2 
will be found to be 0.0235 gm., ete. For the determination proper, 
from 0.1—0.8 gm. of the substance (according to its supposed sul- 
phur content) is weighed out and treated in the same way. If 
with a sample of 0.2 gm. a shade corresponding to Tint No. 5 is 
obtained, the table shows us that 0.1 per cent. of sulphur is present. 

Remark.—The above process is very simple and to be recom- 
mended in case a large number of sulphur determinations are to be 
made, as is the case in iron and steel laboratories. It is to be 
noted, however, that an accurate value is obtained only when all 
the sulphur is present in a form such that it is evolved as hydrogen 
sulphide on treatment. with acid. 


IV. Determination of Sulphur in Insoluble Sulphides. 


For this analysis the sulphur is either oxidized to sulphuric 
acid and determined as barium sulphate, or the sulphide is acted 
upon in a suitable apparatus with nascent hydrogen, whereby the 
sulphur is evolved as hydrogen sulphide and is absorbed by one of 
the above-described methods. 

The oxidation of the sulphide can take place: 


(a) In the Dry Way. 
(b) In the Wet Way. 


(A) OXIDATION IN THE DRY WAY. 
1. Fresenius’ Method: Fusion with Sodium Carbonate and 
Potassium Nitrate. 
About 0.5 gm. of the finely powdered sulphide is intimately 
mixed in a spacious nickel crucible with twelve times as much 
of a mixture of four parts sodium carbonate and one part 


358 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


potassium nitrate,* covered with a thin layer of the mixture and 
heated at first gently, then gradually increasing the temperature 
until the contents of the crucible are melted; it is then kept at this 
temperature for fifteen minutes. After cooling, the melt is 
extracted with water, filtered, the residue boiled with pure dilute 
sodium carbonate solution and finally washed with water to the 
disappearance of the alkaline reaction. The filtrate is treated in a 
covered beaker with an excess of d@ydrochloric acid boiled to 
expel the carbon dioxide, and evaporated to dryness. In order 
to remove all of the nitric acid, the dry mass is treated with 10 c.c. 
concentrated hydrochloric acid and again evaporated to dryness. 
This last residue is moistened with 1 c¢.c. concentrated hydro- 
chloric acid, treated with 100 c.c. water, and filtered if necessary. 
The filtrate is diluted to 450 c.c., heated to boiling and precipitated 
with 24 c.c. of normal barium chloride solution} which is diluted 
to 100 ¢.c. and added as quickly as possible while stirring vigor- 
ously (cf. sulphuric acid). 

Remark.—This is the most reliable method for determining 
the total amount of sulphur in insoluble sulphides and serves for 
testing values obtained by other methods. 

It is important, however, to conduct the fusion in such a man- 
ner that none of the combustion products of the sulphur in the 
illuminating-gas comes in contact with the contents of the crucible. 
This is accomplished, as suggested by Léwe,t by placing the cruci- 
ble in an inclined position within a hole in a piece of asbestos board. 


2. Method of Béckmann. 


In order to avoid the tedious operation of destroying the nitrate 
which is necessary in the method of Fresenius, Bckmann fuses 0.5 
gm. of the substance with 25 gms. of a mixture of six parts sodium 
carbonate and one part potassium chlorate. The contents of the 
crucible are heated gently at first and finally until there is no more 





* Glaser recommends sodium peroxide as an oxidizing flux, in which 
case a nickel or iron crucible must be used. See Chem. Ztg., 18, 1448, and 
Z. anal. Chem., 34, 594 (1895) and List, Z. angew. Chem., 1908, 414. Glaser’s 
method is given in Appendix I. 

+ 122 gm. of the solid BaCl,-2H20 dissolved in a liter of water. 

t Z. anal. Chem., 20, 224 (1881). 


DETERMINATION OF SULPHUR IN INSOLUBL& SULPHIDES: 359 


evolution of oxygen. After cooling the melt is extracted with 
water, the filtrate acidified with hydrochloric acid aiid precipitated 
at a boiling temperature with barium chloride. 

This method is held to be less accurate than that of Fresenius, 
but according to the author’s experience it is equally good. 


3. Oxidation by Chlorine (Rose). 


This very important method is used less to determine the amount 
of sulphur present in insoluble sulphides than it is to effect the 
solution of the sulphide for the separation and determination 
of the metals. As an example of this sort of an analysis we will 
consider the 


Analysis of Tetrahedrite (Fahlerz). 


Tetrahedrite is a sulpho-salt corresponding to the general for- 
mula 4MS-R,S, in which M is Cu,, Ag,, Fe, Zn, or Hg,, and R is 
As, Sb, or Bi. , 

From 0.5-1 gm. of the finely-powdered mineral is introduced 
by means of a long weighing-tube into the bulb of the tube R, Fig. 56, 
which is 30 cm. long and 14 cm. wide and made of difficultly fusible 
glass. 











Fic. 56. 


In the receivers V and Z are placed about 100 ¢.c. of hydrochloric 
acid (1:4) to which 3.5 gms. of tartaric acid have been added, and a 
slow but steady stream of chlorine * is conducted through the appa- 
ratus. 





* The chlorine is generated in a Kipp apparatus from chloride of lime and 
hydrochloric acid. In order to purify the gas it is passed through the wash- 
bottles A, B, and C. The first contains water and the other two contain con- 


360 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


As soon as the chlorine reaches the substance in R, the decom- 
position begins. The contents of & become heated and the volatile 
chlorides collect in the front part of the tube. When the action 
begins to diminish, the decomposition is assisted by heating R 
with a small flame kept in constant motion. The heating is con- 
tinued until only brown vapors of ferric chloride are given off; as 
little as possible of these should pass into the receiver. The easily 
volatile chlorides, however, are driven over into V as much as 
possible by carefully heating with the flame. After allowing to 
cool in an atmosphere of chlorine, the tube FR is broken by first 
scratching with a file near the drawn-out part and then touch- 
ing it with a hot glass rod. Over each of the open ends of the 
tube a clean, moist test-tube is placed and allowed to stand this 
way overnight; in this way the sublimate absorbs water and can 
be easily washed off in the morning. The contents of V and Z 
are poured into a beaker and the drawn-out part of & is washed out 
with hydrochloric acid containing tartaric acid. 


The Residue A 


consists of silver, lead, and copper chlorides, almost all of the zinc, 
lead, considerable amounts of iron, and the gangue. 


The Solution B 


contains all of the sulphur as sulphuric acid, the bismuth as chloride, 
the arsenic and antimony as their pentoxide compounds, a part 
of the iron and zine and often small amounts of lead. 


Treatment of the Residue A. 


This is warmed for a long time with dilute hydrochloric acid, — 
diluted with water, allowed to settle, and the residue consisting of 
silver chloride and the gangue is filtered off, washed thoroughly 
with hot water in order to make sure that all lead chloride is re- 
moved, treated with ammonia on the filter and the silver precipi- 
tated from the ammoniacal filtrate by acidifying with hydrochloric 
acid, and determined as the chloride. The residue, insoluble in 
ammonia, is ignited wet in a platinum crucible and weighed. 





centrated sulphuric acid. It is also well to introduce a calcium chloride tube 
filled with pieces of calcite between C and RF in order to remove traces of acid. 


DETERMINATION OF SULPHUR IN INSOLUBLE SULPHIDES. 361 


Into the filtrate from the silver chloride, hydrogen sulphide is 
passed until the solution is saturated with the gas, the precipitate 
consisting of copper and lead sulphides is filtered off, and the lead 
separated from the copper as sulphate according to p. 200. The 
filtrate from the hydrogen sulphide precipitate is combined with 
- that obtained from Solution B after hydrogen sulphide has been 
passed into it. 


Treatment of Solution B. 


A stream of carbon dioxide is passed through the solution for 
some time in order to remove the greater part of the excess of chlo- 
rine, and hydrogen sulphide is then passed into it at the tempera- 
ture of the water-bath. The precipitate, consisting of sulphides 
of arsenic, antimony, mercury, and possibly bismuth, is filtered 
oft after standing twelve hours, and the arsenic and antimony 
separated from the mercury and bismuth by means of ammonium 
sulphide as described on p. 235. From the ammonium sulphide 
solution the arsenic and antimony are precipitated by acidifying 
with dilute hydrochloric or sulphuric acid, the precipitated sul- 
phides filtered off and the arsenic separated from the antimony as 
described on p. 241 et seq. 

The precipitate insoluble in ammonium sulphide usually consists 
almost entirely of mercuric sulphide and sulphur, in which case 
it is washed first with alcohol, then a few. times with carbon 
bisulphide, then with alcohol again, dried at 100° C. (preferably 
in a Paul’s drying-oven) and weighed. If bismuth is present, how- 
ever, the mixture of the two sulphides is treated with nitric 
acid of sp. gr. 1.2—-1.3, boiled, an equal volume of water added, 
the residue filtered and the bismuth determined in the filtrate 
according to p. 157, while the mercury is determined as above 
described. ; 7 

The filtrate from the hydrogen sulphide precipitate contains 
iron and zine and is combined with the corresponding filtrate 
from the Residue A, which likewise contains these metals. These 
are precipitated by the addition of ammonia and ammonium 
sulphide, filtered off, dissolved in hydrochloric acid, the solution 
oxidized with nitric acid, and the iron separated from the zine, 
preferably by the barium carbonate method (see p. 150). 


362 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


It is best to determine the sulphur in a separate portion by . 
fusion with sodium carbonate and potassium nitrate as described 
on p. 308. 

The determination of the sulphur in an aliquot part of the 
Solution B is not to be recommended on account of the fact that 
the metals present are likely to contaminate the precipitate of 
barium sulphate. 

> 


(B) OXIDATION OF SULPHUR IN THE WET WAY. 


For this purpose aqua regia, fuming nitric acid, bromine, 
hydrochloric acid and potassium chlorate, and, in some cases, 
ammoniacal hydrogen peroxide have been proposed. 

Aqua regia is most frequently used in practice and in the pro- 
portion first reeommended by J. Lefort,* viz., 3 volumes of nitric 
acid of sp. gr. 1.4 and 1 volume of hydrochloric acid of sp. gr. 1.2. 
As an example we will cite the 


Determination of Sulphur in Pyrite, G. Lunge’s Method.t 


First of all, the sample must be ground to an impalpable 
powder, or sulphur will separate out during the solution of the 
sample. Of the fine powder, 0.5 gm. is treated with 10 ¢.c. of a 
mixture consisting of 3 parts nitric acid, sp. gr. 1.42, and 1 part 
hydrochloric acid, sp. gr. 1.2, in a 300 ¢.c. beaker which is covered 
with a watch-glass. At first the acid is allowed to act upon the 
pyrite in the cold, but at the last the reaction is completed by 
heating upon the water-bath. Then the solution is transferred 
to a porcelain evaporating dish and evaporated to dryness on the 
water-bath. The residue is treated with 5 c.c. of concentrated 
hydrochloric acid and again evaporated to dryness. The dry 
mass is now treated with 1 c.c. of concentrated hydrochloric 
acid and 100 c.c. of hot water, the solution filtered through a 
small filter and the residue washed first with cold water and then 
with hot water. The hot filtrate, of not more than 150 c.c. at 

* J. de Pharm. et. de Chimie [IV], Vol. 9, p. 99, and Zeit. fir anal. Chem., 
IX, p. 81. 


+ The procedure is given here as recommended by the Report of the Sixth 
International Congress of Applied Chemists, Rome, 1906, Vol. VI, p. 15. 





DETERMINATION OF SULPHUR IN INSOLUBLE SULPHIDES. 363 


the most, is treated with 20 c.c. of 10 per cent. ammonia and 
kept at about 70° for fifteen minutes. The ferric hydroxide 
precipitate is filtered and. washed with hot water, whereby the 
precipitate is well “churned,” until a volume of about 450 c.c. 
is reached. The filtrate is neutralized with hydrochloric acid, 
using methyl orange as indicator, and 1 ¢.c. of concentrated hydro- 
chloric acid added in excess. Thereupon the solution is heated 
until it begins to boil, when it is treated-with a boiling-hot solu- 
tion made by taking 24 c.c. of 10 per cent. barium chloride 
solution and diluting to 100 ¢.c. The reagent is added as quickly 
as possible at one time while stirring the solution vigorously. 

The barium sulphate precipitate is washed three times by 
decantation with boiling water, then transferred to a filter and 
washed free from chlorides, dried, ignited and weighed. 

To test the ammonia precipitate for sulphur, transfer it from 
the filter into a beaker by means of a stream of water from the 
wash bottle and dissolve it by the addition of as little hydrochloric 
acid as poasible. The resulting solution is precipitated with 
ammonia, filtered, and the filtrate and washings treated as in the 
case of the main analysis. Should any barium sulphate be 
obtained in this way, it should be filtered off and weighed with the 
main part of the barium sulphate precipitate. 

Remark.—It is still better to filter the precipitate through a 
Munroe crucible. After washing, the precipitate is dried as much 
as possible by suction, the crucible placed within a larger porcelain 
or platinum crucible, heated gently and weighed. 

The above method gives excellent results, which as a rule agree - 
closely with those obtained by the Fresenius method. If the 
pyrite, however, contained barium or any considerable amount of 
lead, some sulphate will always remain undissolved with the 
gangue. In such cases the Lunge method will give lower results 
but on the other hand it represents more nearly the quantity of 
sulphur in the pyrite which is available for the manufacture of 
sulphuric acid. In spite of the strong oxidizing power of the above 
mixture of nitric and hydrochloric acids, it is not sufficient to 
permit the determination of sulphur in roasted pyrite, on account 
of the danger of losing some sulphur as hydrogen sulphide. Such 


364 = GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


products are fused with a mixture of two parts sodium carbonate | 
and one part potassium nitrate and the analysis carried out as 
described under the Fresenius method. 


Determination of Sulphur in Cast Iron and Steel. (Noyes and 
| Helmer *). | 


About 5 gms. of iron or steel, in the form of fine borings, are 
introduced gradually into an Erlenmeyer flask containing a cooled 
mixture of 200 c.c. water and 8 c.c. bromine, free from sulphur. 
As soon as all the metal has dissolved, the contents of the flask are 
heated to boiling, in order to expel the slight excess of bromine, 
and the solution is filtered from any residue. Inasmuch as the 
latter frequently contains an appreciable amount of sulphur, it is 
dried, transferred to a platinum crucible, the ash of the filter paper 
added, and a fusion is made with 2 gms. sodium carbonate. 
The crucible should be inclined within an asbestos shield to pro- 
tect its contents from being contaminated with any sulphur 
from the gas flame. After the sodium carbonate has melted, the 
crucible is allowed to cool somewhat, a crystal of potassium nitrate 
is added, and the heating is continued. After cooling, the melt 
is dissolved in water, the resulting solution filtered, the filtrate 
acidified with hydrochloric acid, heated to boiling, and the 
sulphuric acid precipitated by the addition of 5 e.c. barium 
chloride solution. In the following calculation, the weight of this 
precipitate is called p. 

The original solution of the iron, containing the greater part of 
the sulphur, is poured, while constantly stirring, into 130 c.c. of 
10 per cent. ammonia water which is contained in a 500-c.c. 
calibrated flask. The contents of the flask are well shaken, 
diluted with water up to the mark, mixed by pouring back and forth 
several times into a dry beaker, and then filtered through a dry 
filter, rejecting the first few c.c. of the filtrate. From the strongly 
ammoniacal filtrate, 300 c.c. are transferred by a pipette into a 
new beaker, evaporated to 100 c.c., while avoiding any contamina- 
tion from a gas flame, treated with five or six drops of dilute 





* J. Am. Chem. Soc., 28, 675 (1901). 


DETERMINATION OF SULPHUR IN INSOLUBLE SULPHIDES. 365 


hydrochloric acid and the sulphuric acid precipitated by the 
addition of 5 c.c. of hot, normal barium chloride solution. The 
weight of this precipitate is taken as p; in the following computa- 
tion. 

The sulphur content of the sample of iron or steel, weighing a 
gms., 8 then found to be 





Cp a ees oS 13.73 x LP itp =per cent. sulphur. 

Remark.—This method is accurate and easily carried out. All 
the sulphur is obtained with the exception of that small amount 
which is combined with an organic radical. A great advantage 
is gained by not having to wash the ferric hydroxide precipitate. 
A very slight error is introduced by neglecting the volume of the 
ferric hydroxide precipitate, but this is negligible in the deter- 
mination of such small amounts of sulphur. The iron must be 
introduced into the bromine in very small portions in order to 
prevent overheating which would result in the formation of a 
basic salt that is hard to get back into solution. | 


Determination of Sulphur in Iron and Steel. Method of Krug.* 


Procedure.—5 gms. of borings are treated in a 500-c.c. round- 
bottomed flask with 50 c.c. concentrated nitric acid, sp. gr. 1.4 
and the contents of the flask are gently heated. After the reddish- 
brown vapors cease to form, the acid is gradually heated up to the 
boiling point. When at the end of an hour or two the solution of 
the iron is complete, 0.25 gm. of potassium nitrate, dissolved in a 
little water, is ad2ed, the liquid evaporated to dryness, and the 
residue ignited until no more brown fumes are evolved. After 
cooling, the ferrie oxide is dissolved by heating with 50 c.c. 
concentrated hydrochloric acid, the solution evaporated nearly 
to dryness, and the treatment with hydrochloric acid and evapora- 
tion repeated until no more chlorine is evolved. The hydro- 





* Stahl und Eisen, 25, 887 (1905). 


366 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


chloric acid solution is then rinsed into a beaker, and any residue 
of silica, carbon, etc., is filtered off into a porcelain evaporating 
dish. The filtrate is evaporated until a film of ferric chloride 
forms on the solution, which is redissolved by the addition of a 
few drops of hydrochloric acid. After cooling the ferric chloride 
solution is introduced into a double separatory funnel, washing 
out the dish with hydrochloric acid, sp. gr. 1.1, but keeping the 
volume below 60 c.c. 30 c.c. of faming hydrochloric acid and 
ether mixture (prepared by gradually pouring ether into co'd con- 
centrated hydrochloric acid, sp. gr. 1.2, solution until a little layer 
of ether is formed on top) and 100 c.c. of ether. The mixture is well 
cooled under the water tap and thoroughly shaken. The upper 
olive-green ether layer contains nearly all of the iron, the lower 
light yellow solution containsall the sulphuric acid. The lower layer 
is carefully withdrawn into the other separatory funnel and the 
ether solution is washed once with a few c.c. of dilute hydrochloric 
acid, sp. gr. 1.1 which has been saturated with ether. The ether 
solution is shaken with this last mixture and after standing until 
two layers again separate, the lower one is added to the contents 
of the other separatory funnel. 75 c.c. of pure ether are now 
introduced into the second separatory funnel and the contents 
well shaken, this time the cooling is unnecessary. The upper 
layer will contain an ether solution of practically all the remaining 
iron, whereas the lower hydrochloric acid layer will contain all the 
sulphuric acid and some dissolved ether. The lower layer is 
withdrawn to a porcelain evaporating dish, and the ether contained 
in it is removed by evaporating on the water bath to dryness. To 
the residue, a few drops of hydrochloric acid and a little water 
are added. The solution is filtered and the hot filtrate treated 
with hot barium chloride solution. 

Remark.—In testing this method, Dr. Krug established the 
fact that a mixture of pure ferric chloride, corresponding to 5 gms. 
iron, could be treated with 10 c.c. of tenth-normal sulphuric acid, 
and all of the latter recovered after the ether separation. The 
results were found to be more reliable than those obtained by 
an evolution method. 


DETERM.NATION OF SULPHUR IN INSOLUBLE SULPHIDES. 367 


(c) EXPULSION OF HYDROGEN SULPHIDE FROM INSOLUBLE 
SULPHIDES. 


(a) The Iron Method.* 

In 1881, M. Gréger showed that by heating pyrite with iron 
out of contact with the air the former is quantitatively changed 
into ferrous sulphide, 

FeS, + Fe=2FeS, 


and from the latter all of the sulphur will be given off as hydrogen 
sulphide on treatment with hydrochloric acid. In 1891 the author 
independently came to the same conclusion and worked out a 
methed which permits of the determination of sulphur not only 
in pyrite but in all other insoluble sulphides. 

Procedure.—First of all the finely powdered sulphide is heated 
out of contact with the air with iron powder. In this way 
part of the sulphur is usually given up to the iron, and the com- 
pound itself is reduced to compounds which are acted upon by 
hydrochloric acid with evolution of hydrogen sulphide; the latter 
is absorbed in ammoniacal hydrogen peroxide solution, as de- 
scribed on p. 347. The heating with iron is accomplished in a 
small glass crucible about 30 mm. long and 10 mm. in diameter 
(Fig. 54, b), which can be easily made from an ordinary piece of 
combustion tubing. About 3 gms. of iron powder that has been 
previously ignited in hydrogen is placed in the crucible, from 0.3- 
0.5 gm. of the sulphide is thoroughly mixed with it, and the mix- 
ture is finally covered with a thin layer of iron powder. The cru- 
cible is now placed in the opening of the piece of asbestos board A 
(Fig.54, b) and upon it is placed the gas-delivery tube B which has 
been prepared from difficultly fusible glass. A stream of dry 
carbon dioxide f is passed through the apparatus for a few min- 





* Berichte, XXIV, p. 1937. 

+ The carbon dioxide is prepared from marble and hydrochloric acid in a 
Kipp generator. To purify the gas it is passed through a wash-bottle con- 
taining water, then through one containing potassium permanganate, then 
through a tube filled with pumice soaked in copper sulphate solution, and 
finally through a calcium chloride tube. Potassium permanganate and cop- 
per sulphate serve to remove traces of hydrogen sulphide that the carbon 
dioxide might contain. 


368 | GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


utes and the crucible is gently heated with a small flame. Usu- 
ally there is a distinct glowing visible, but no trace of the sulphur 
is lost. by volatilization. As soon as the contents of the crucible 
have ceased to glow, the temperature is raised until a dull-red heat 
- is obtained, and the crucible is kept at this temperature for ten 
minutes. 

After cooling in the carbon dioxide, the crucible together with 
its contents is placed in the 400-c.c. flask K and is connected with 
the absorption vessels V and P as shown in the figure. The rest 
of the procedure is carried out as described on p. 350. 

Remark.—Commercial iron powder always contains a small 
amount of sulphur, so that a blank experiment is first made with 
a weighed amount of the same, and for the experiment proper the 
same amount of iron is used. The amount of sulphur found to 
be present in the iron is subtracted from the amount found in 
the analysis. } 

The author was disappointed in not being able by this method 
to distinguish between the sulphur present in insoluble sulphides 
' as sulphide and that present as sulphate (barium sulphate). If 
the amount of sulphate present is small, it is completely reduced 
to sulphide by this method, while if a large amount of sulphate 
is present, it is often only partially reduced. As, however, the 
amount of barium sulphate* present in insoluble sulphides is 
usually small, this method serves for the determination of the 
total amount of sulphur. . 


(b) The Tin Method.+ 


Principle—Almost all insoluble sulphides on being treated 
with metallic tin and concentrated hydrochloric acid give off all 
their sulphur as hydrogen sulphide. Harding,{ who first studied 
this method, used tin and hydrobromie acid. 

Procedure.—Into the evolution tube (Fig. 57), which is 20 em. 
long and 2.5 cm. wide, is placed a layer of finely-powdered tin (g) 
about 0.5 em. thick. Upon this the substance is placed enclosed 
in tinfoil (s) and then a layer of granulated tin about 6 cm. deep 


“— 





* Only barium sulphate is reduced with difficulty, the sulphates of the 
heavy metals are easily reduced. 

t Berichte, XXV, p. 2377. 

t Berichte, XIV, p. 2085. 


DETERMINATION OF SULPHUR IN INSOLUBLE SULPHIDES. 369 


(Z) is added. A current of pure hydrogen is conducted through 
the apparatus for about five minutes, after which the stop-cock 








Fig. 57. 


is closed and the tube connected with the receivers P and V, as 
shown in the figure. The flask V contains an ammoniacal solution 
of hydrogen peroxide, while P contains 2 to 3 em. of water in order 
to remove any stannous chloride that may be carried over with 
the gas. Concentrated hydrochloric acid is now added through 
the drop-funnel until the tin is at the most half covered with the 
acid. The contents of the tube are then warmed slightly, prefer- 
ably by placing it in a small paraffin bath. The capsule of tin 
soon dissolves, and the substance is seen to be floating in the acid. 
It dissolves after about fifteen minutes, and the acid becomes 


370 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


perfectly clear. The heating is now continued until there is no 
more yellow coloration to be detected in the delivery-tube which 
dips into the receiver V. More acid is then added to the contents © 
of the tube, until the tin is completely covered with it and the 
heating is continued for half an hour, meanwhile first heating 
the contents of P to boiling and passing a current of hydrogen 
through a. By this means all of the sulphur will be driven over 
into V * and is there held in solutton as ammonium sulphate and 
analyzed as described on p. 350. 

Remark.—This method affords an accurate means for deter- 
mining the sulphur present in insoluble sulphides as sulphide in 
the presence of sulphate. Thus the amount of pyrite in clay-slate 
_ that contains gypsum can be determined by this method, although 
usually the treatment with aqua regia or fusion with soda and 
nitreisused. By these last two methods, however, the total sulphur 
is determined. More accurate values for the pyrite present in such 
cases may be obtained by decomposition in a current of chlorine 
(see p. 359), in which case only the sulphide sulphur is determined, 

Finally, it may be mentioned that arsenic sulphide may be de- 
composed by the above method, although a longer time is required 
than is the case with pyrite, copper, chalcopyrite, galena, cinnabar, 
etc. Arsenopyrite, on the other hand, is either unacted upon of 
only decomposed with difficulty, while the iron method effects the 
decomposition with ease. 


Determination of Sulphur in Non-electrolytes. 


In order to determine the amount of sulphur present in organic 
compounds, it is oxidized to sulphuric acid and determined as 
barium sulphate. 

The oxidation is effected 

(a) In the Wet Way. 
(b) In the Dry Way. 


(a) Oxidation in the Wet Way (Carius). 


This operation is conducted in precisely the same manner ag 
was described on p. 325 for the determination of halogens, except 





* With large amounts of sulphur, one receiver is often insufficient. In 
such cases the tube b is connected with a Péligot tube containing ammoniacal 
' hydrogen peroxide as shown in Fig. 54, p. 351. 


ACETIC AND CYANIC ACIDS. 371 


in this case there is no silver nitrate added to the contents of the 
tube. After the closed tube has been heated and opened, its 
contents are transferred to a beaker, hydrochloric acid is added, 
and the liquid is evaporated to a small volume in order to remove 
the nitric acid; it is then diluted with water to a volume of about 200 
c.c. and precipitated hot with a boiling solution of barium chloride 
and weighed as barium sulphate. 


(b) Oxidation in the Dry Way (Liebig). 

A mixture of eight parts potassium hydroxide (free from sul- 
phate) and one part of potassium nitrate is melted in a large silver 
erucible with the addition of a little water. After cooling, a weighed 
amount of the substance is added and the contents of the crucible 
again heated very gradually, frequently stirring the mixture with 
a silver wire until the organic substance is completely decomposed. 
After cooling, the melt is dissolved in water, acidified with hydro- 
chloric acid and the sulphuric acid formed is precipitated and 
weighed as barium sulphate. 

This method is particularly suited for the determination of sul- 
phur present in difficultly volatile substances, e.g., in wood-cements. 

CH, 
ACETIC ACID, | - Mol. Wt. 60.03. 
. COOH 

Free acetic acid is always determined volumetrically. For 
the analysis of acetates, the substance is heated with phosphoric 
acid when the free acetic acid distils over and is then titrated 
(cf. Part II, Acidimetry). The carbon and hydrogen of the acetate 
ean be determined by Elementary Analysis (which see). 


CYANIC ACID, HOCN. Mol. Wt, 43.02. 


The only method for examining cyanates consists of deter- 
mining. the amount of carbon and nitrogen present by a combus- 
tion (see Elementary Analysis). 


Determination of Cyanic Acid, Hydrocyanic Acid, and Carbonic 
Acid in a Mixture of their Potassium Salts. 
In one portion of the substance the carbonic acid is deter- 
mined by the addition of calcium chloride to the ammoniacal solu- 
tion and weighing the ignited precipitate as calcium oxide. 


372 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


In a second portion the cyanogen of the cyanide is deter- 
mined as silver cyanide by treating the aqueous solution with an 
excess of silver nitrate, then acidifying with nitric acid and deter- 
mining the weight of the silver cyanide as described on p. 328. 

In a third portion the potassium is determined by evaporating 
with sulphuric acid and weighing the residue of potassium sulphate 
as described on p. 41. If from the total amount of potassium 
present the amount present as potassium carbonate and potassium 
cyanide is deducted, the difference gives the amount of potassium 
combined with the cyanic acid. 


HYPOPHOSPHOROUS ACID, H,PO,. Mol. Wt. 66.06. 


Forms: Mercurous Chioride, Hg,Cl,; Magnesium Pyrophosphate, 
Mg,P.0.. 


(a) Determination as Mercurous Chloride. 


The solution of the salt, which is slightly acid with hydro- 
chloric acid, is treated with an excess of mercuric chloride; by 
this means insoluble mercurous chloride is precipitated. After 
standing for twenty-four hours in a warm, dark place the precip- 
itate is filtered through a Gooch crucible, washed with water 
dried at 110° C., and from the weight of the mercurous chloride the 
amount of hypophosphorous acid present is caiculated as follows: 

H,PO, + 2H,0 + 4HgCl, = 2H¢,Cl, + 4HCl+ H,PO, 
2Hg,Cl,: H,PO,= p:x 
__ HsPO.-p 
2Hg,Cl, 
in which p is the weight of the Hg,Cl, obtained in the analysis. 


(b) Determination as Magnesium Pyrophosphate. 


First of all, the phosphorous acid is converted into phosphoric 
acid by adding 5 c.c. of concentrated nitric acid to the aqueous 
solution of from 0.5-1 gm. of the substance in about 100 ec.c. of 
water,* evaporating on the water-bath to a small volume, adding 
a few drops of fuming nitric acid and again heating. After this the 
phosphoric acid is precipitated by magnesia mixture and the pre- 





* If the hypophosphite were at once treated with nitric acid, metaphos- 
phoric acid would be obtained; by the addition of water the ortho-salt is 
formed. 


SULPHURGUS ACID. 373 


cipitate is weighed as magnesium pyrophosphate as described 
under Phosphoric Acid. 


GROUP III. 


SULPHUROUS, SELENOUS, TELLUROUS, PHOSPHOROUS, CAR- 
BONIC, OXALIC, IODIC, BORIC, MOLYBDIC, TARTARIC, META- 
AND PYROPHOSPHORIC ACIDS. 


SULPHUROUS ACID, H,SO,. Mol. Wt. 82.09. 
Form: Barium Sulphate, BaSO,,. 


The sulphite or free sulphurous acid is first oxidized to sul- 
phuric acid and then precipitated with barium chloride..- 

The oxidation can be accomplished by means of chlorine, 
bromine, hydrogen peroxide, or potassium percarbonate. 


Oxidation with Chlorine or Bromine. 


Chlorine or bromine water is allowed to flow gradually into 
the aqueous solution of sulphurous acid, or of a sulphite, the excess 
of the reagent is subsequently removed by boiling and the sulphuric 
acid is precipitated with barium chloride. 


Oxidation with H ydrogen Peroxide.* 


The solution of sulphurous acid or of a sulphite is treated 
with an excess of ammoniacal hydrogen peroxide, heated to boil- 
ing in order to remove the excess of the peroxide, acidified with 
hydrochloric acid and precipitated with barium chloride after 
making acid with hydrochloric acid. 

With potassium percarbonate a similar procedure is used. 
The alkaline solution of the sulphite is treated in the cold with 
solid potassium percarbonate, gently heated, after which the tem- 
perature is gradually raised till the boiling-point is reached. 
The solution is then acidified with hydrochloric acid and pre- 
cipitated with barium chloride. : 





* The hydrogen peroxide must always be tested to see if it contains sul- 
phuric acid; if it is found to be present, the amount is determined and 
afterward an accurately measured quantity is used for the oxidation. The 
amount of sulphuric acid from the peroxide is deducted from the total value 
found in the analysis. 


374. GRAVIMETRIC DETERMINATION OF THE. METALLOIDS. 


Sulphurous acid may be determined very accurately by a volu- 
metric analysis (cf. Part Il, lodimetry). 


Selenous and Tellurous Acids. 


The analysis of these acids was discussed under Selenium and 
Tellurium. 


PHOSPHOROUS ACID, H;PO;. Mol. Wt. 82.06. 


Forms: Mercurous Chloride, Hg,Cl,, and Magnesium Pyro- 
phosphate, Mg,P,0.. 
_ This determination is effected exactly as that of hypophos- 
phorous acid (cf. page 372). 


In this case, however, it is to be noted that 1 mol. Hg,Cl, cor- 
responds to 1 mol. H,PO,: 


H,PO,+2HeCl,+H,0=H PO, +2HC1+He,0h, 


Determination of Phosphorous and Hypophosphorous Acids. 


In this case an indirect analysis must be made. After oxidizing 
one portion of the substance to phosphoric acid, the total amount 
of magnesium pyrophosphate is found; mercuric chloride is allowed 
to act upon a second portion and the weight of the mercurous 
chloride formed is determined. From these data the amount of 
each acid present can be calculated as follows: 

Suppose we have a solution containing the two acids. Let us | 
denote by x the amount of hypophosphorous acid present in V e¢.e. 
of the solution, and let ox represent the amount of mercurous 
chloride produced from it and mz the amount of magnesium pyro- 
phosphate. Further, let y represent the amount of phosphorous 
acid present in the same volume of the solution and vy the cor- 
responding amount of mercurous chloride and ny that of mag- 
nesium pyrophosphate. The total amount of the mercurous 
chloride is q, while the total amount of magnesium pyrophosphate 
is p. We ich then 

mx + ny =p 
0x + VY = 
from which it follows » 


nN Vv . 
an one 





CARBONIC ACID. 375 


and — 
ra) m 
Yt Dee oa eae ss li q-0.1742. 





In these equations, m, n, 0, and v have the following values: 
ai Mg2P207 ie MgeP20,7 


pe 2HgeClo ey pe Hg Cle vs? 
So P Oe: 5 14.26 CO eaRO. eer 


CARBONIC ACID, H,CO,. Mol. Wt. 62.02. 


Carbonic acid is determined gravimetrically as CO,; but a 
more accurate determination is effected by expelling this gas and 
measuring its volume. 


1. Gravimetric Determination of Carbon Dioxide. 


This analysis may be accomplished in two ways. First, we 
may weigh the substance, expel the carbon dioxide and then weigh 
it again, when the difference will represent the amount of gas. 
Second, the carbon dioxide may be expelled from a weighed 
amount of the substance and then absorbed in a suitable appa- 
ratus; in this case the carbon dioxide is weighed directly. 


A, DETERMINATION OF CARBONIC ACID BY DIFFERENCE. 
(a) Determination in the Dry Way. 


For the analysis of a carbonate, or a mixture of carbonates 
which contains no volatile constituent other than the carbon 
dioxide, 1 gm. of the substance is weighed into a platinum 
crucible and gradually heated to a high temperature.* In case 
calcium, strontium, or magnesium is present a final heating over 
the blast-lamp is necessary, while with other carbonates the heat 
of a good Teclu burner is sufficient; even the difficultly decom- 
posable cadmium carbonate can be analyzed by this method. The 
carbonates of barium and the alkalies, on the other hand, do not 
lose their carbon dioxide on ignition. 





* Carbonic acid cannot be determined by this method when the residual 
oxide suffers change, as, for example, in the case of FeCO, and MnCO, where 
an oxidation would take place. 


376 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


If the substance contains water besides carbon dioxide then 
the sum of the water+ carbon dioxide is determined by the loss on 
ignition, and the amount of carbon dioxide is determined in a sec- 
ond portion by (0). 


(b) Determination in the Wet Way. 


Principle-—The weighed carbonate is placed in an apparatus 
containing acid, but in such a way that the former does not at first 
come in contact with the latter. The 
whole apparatus is then weighed, after 
which the acid is allowed to act upon the 
_ substance, when carbon dioxide is evolved 
and escapes from the apparatus. (Care 
must be taken that no moisture escapes 
with the gas.) By afterward weighing 
the apparatus and subtracting this weight 
from that first obtained, the weight of 
the carbon dioxide is ascertained. 

Procedure.—This analysis is easily 
accomplished, and a large number of 
different forms of apparatus have been 
devised for this purpose. In this book, 
however, only one of these so-called alka- 
limeters will be described, namely, that 
of Mohr, which in an improved form is 
shown in Fiy. 58, although it must be 
stated that many other forms (e.g., those 
of Bunsen,* Shrodtter, Geissler, I rese- 
nius-Will, etc.) answer the purpose equal- 
ly well. 

The alkalimeter consists of the small, wide-mouthed, flat-bot- 
tomed flask F', which has a ground-glass connection with the tubes | 














*In the German edition of this book, Bunsen’s alkalimeter is described ‘ 
instead of Mohr’s. The above apparatus has the advantage of having a stop- 
cock to separate the acid compartment from the flask, besides having a flat 
bottom, upon which it will rest unsupported. It is all made of ‘very thin - 
glass and weighs comparatively little. 


CARBONIC ACID BY DIFFERENCE. 377 


Aand B. At the bottom of Bis placed a loose wad of cotton; a 
cylinder of glazed paper about 3 em. wide is introduced into the 
neck of the tube, and through this cylinder some pieces of sifted 
calcium chloride* are poured. The paper cylinder is removed 
after the tube is about three-quarters full of calcium chloride, and 
care is taken to see that none of the latter adheres to the glass 
above the filled portion. Another wad of cotton is then placed 
in the tube, the top is placed upon it, and the tube is closed tempo- 
rarily at d by means of a piece of stirring-rod within rubber tubing. 
The tube should be kept closed when not in use to prevent the 
gradual absorption of moisture from the air. Two ordinary cal- 
cium chloride tubes are filled in the same way about two-thirds 
full, but in this case softened cork stoppers are placed at the end 
of the tubes after the second wad of cotton. Through a hole in 
each stopper a short piece of glass tubing with rounded ends is 
introduced, and the cork is shoved far into the tube with the help 
of a stirring-rod, leaving the outer 2 or 3mm.empty. This space 
in the tube is filled with molten sealing-wax, so that a perfectly 
air-tight connection is made. These tubes are also closed, when 
not in use, by stirring-rod within rubber tubing. 

Before beginning the determination the apparatus must be 
clean and dry. It is not advisable to dry the flask by washing 
with alcohol and ether, but it should be gently heated while a cur- 
rent of dry air is sucked through it. As aspirator an inverted 
wash-bottle may be used, from which the water is allowed to run 
out slowly through the shorter tube. During the aspiration the 
small calcium chloride tubes are connected with c and d respect- 
ively, so that no moisture can enter the flask. 

When all is ready the finely-powdered substance, which has 
been dried at. 100° C. and cooied in a desiccator, is placed in a 
weighing-tube, from 1 to 1.5 gms. are transferred to the flask and 
a little water is added.j The tube A is now filled two-thirds full 

* As commercial calcium chloride always contains a little free lime, some 
carbon dioxide will be absorbed by it and consequently low results obtained 
in the analysis, unless the calcium chloride is saturated with carbon dioxide 
before the analysis is made (see foot-note, page 380). 

+ This method is often used for the determination of the carbonic acid 


in. baking-powders. Such substances are decomposed by water so that they 
should be kept dry until after the apparatus has been weighed. 





378 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


with hydrochloric acid (1 part concentrated acid to 4 of water) by 
means of a small funnel or thistle tube, and the stop-cock T 
must be turned so that none of the acid will run into the flask. 
The whole apparatus, as shown in Fig. 58, is now placed upon 
the balance-pan and accurately weighed. The stop-cock T is then 
opened so that the acid in A slowly drops into the flask. As soon 
as the evolution of carbon dioxide begins to take place quietly, the 
apparatus is allowed to stand withdut watching for about half an 
hour. At the end of this time all of the acid will have passed into 
the flask, and the decomposition will be nearly complete in most 
cases. It now remains to remove all carbon dioxide absorbed by 
the liquid and contained in the apparatus. This is effected by 
gently heating the solution by means of a small flame until the acid 
just begins to boil, meanwhile aspirating a current of dry air through 
ce and out at d. Not more than three or four bubbles of air per 
second should be allowed to pass through the flask. As soon as 
the boiling begins, the flame is removed and the slow current of 
air is still passed through the apparatus until it iscold. Itis then 
stoppered and allowed to stand near the balance for half an hour 
or more, after which it is again weighed without the stoppers. The 
loss in weight represents the amount of carbon dioxide originally 
present in the substance as carbonate. 

Remark.—This method affords excellent results in the esti- 
mation of large amounts of carbonic acid, but it is unreliable for 
the analysis of small amounts such as are present in cements, 
etc. In such cases the Fresenius-Classen or Lunge-Marchlewski 
method is better. (See pp. 380 and 388.) 

The objection to this method lies in the fact that owing to 
the size and weight of the apparatus, there is likely to be an 
error in making the two weighings.* On the other hand, it is 
unquestionably true that it is easier to expel carbon dioxide 
from a solution than it is to absorb it quantitatively. 


B, DIRECT DETERMINATION OF CARBON DIOXIDE. 
Here again the determination can be carried out both in the 


dry and wet ways. 


* There is some danger of losing a little hydrochloric-acid gas during the 
operation. To prevent this the calcium chloride may be replaced by pumice 
impregnated with anhydrous copper sulphate, or the carbonate may be 
decomposed by means of sulphuric acid. 





DETERMINATION OF CARBONIC ACID BY DIFFERENCE, 379 
(a) Determination in the Dry Way. 


From one to two grams of the substance are weighed out into a 
porcelain boat, and the latter is shoved into the middle of a horizon- 
tally held glass tube which is about 20 cm. long and 1-1.5 em. wide, 
and made of difficultly fusible glass. Both ends of the tube are 
provided with calcium chloride tubes connected with it by means of 
tightly-fitting rubber stoppers. ‘Through one of the calcium chloride 
tubes a slow stream of air (free from carbon dioxide)* is conducted 
and the other is connected with two weighed soda-lime tubes (cf. 
p. 380). The substance is heated gradually until it glows strongly, 
meanwhile r¢assing a slow but steady current of air through the 
apparatus. When there is no further heat effect to be detected 
in the soda-lime tubes, the substance is allowed to cool in 
the current of air and the soda-lime tubes are subsequently 
weighed. The increase of weight represents the amount of carbon 
dioxide. | | 

Remark.—This method can be employed for the analysis of 
all carbonates with the exception of those of barium and the 
alkalies,t though, of course, no other volatile acid can be present. 
at the same time. Water is kept back by the calctum chloride 
tube. : 

Example: Analysis of White Lead.—White lead, provided it is 
free from acetate (which must be shown by a qualitative test), 
can be accurately and expeditiously analyzed by the above 
method. It is a basic carbonate of lead and contains, therefore, 
lead oxide, carbon dioxide, and water, while it is often contami- 
nated with sand. 

The analysis is conducted as above described except that in- 
this case the calcium chloride tube which is connected with the 
soda-lime tubes is weighed. ‘The gain in.weight of the former repre- 
sents the amount of water in the substance, the gain in weight of 
the latter shows the amount of carbonic acid present, while if the 





* The air is passed through two wash-bottles containing caustic potash | 


solution. 
.. | Even the carbonates of the alkalies and of barium can be analyzed in 
this way if they are mixed with potassium bichromate. _ 


380 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


residue in the porcelain boat is weighed the amount of lead oxide 
is determined. After weighing the latter the lead oxide is treated 
with hot, dilute nitric acid, when it will dissolve to a clear solution 
if pure, while any sand will remain behind as an insoluble residue. 
If there is a residue it is filtered off, ignited, and weighed. By 
deducting the latter from the original weight of the residue in the 
porcelain boat, the weight of the pure lead oxide is obtained. 
> 
(b) Determination in the Wet Way, after Frésenius-Classen. 


The apparatus necessary for this determination is shown in 
Fig. 59. It consists of a decomposition-flask of about 400 c.c. 
capacity provided with a condenser and connected with the drying- 
tubes a, b, and c and with the weighed soda-lime tubes d and e; * 7 is 
a protection tube whose left arm is filled with calcium chloride and 
whose right arm contains soda-lime. ‘The first drying-tube, a, 
contains glass beads wet with concentrated sulphuric acid, while 
b and ¢ contain granular calcium chloride.f 

Procedure.—The substance is weighed out into the dry flask, 
covered with a little water in order to prevent loss of the substance 
and a slow current of air (free from carbon dioxide) is conducted 
through the apparatus in order to remove any carbon dioxide 
that may be present in the flask or in the three drying-tubes. 
While the air is being led through the apparatus, the soda-lime 
tubes are carefully wiped with a linen cloth and weighed. The 
current of air is now stopped, the weighed tubes are connected with c 





* The right arm of the last soda-lime tube is one-third filled with calcium 
chloride in order to absorb the water set free by the absorption of the carbon 
dioxide by the soda-lime, 2NaOH + CO,= Na,CO,+ H,0. 

+ As commercial calcium chloride always contains lime which will absorb 
carbon dioxide, it must be saturated with this gas before the determination 
is made For this purpose a dry current of the gas is conducted through 
the tubes for one or two minutes, the outer end of the tube is then closed 
by means of a glass rod within a piece of rubber tubing and the other end 
is kept connected with the Kipp generator for twelve hours. At the end 
of that time the excess of carbon dioxide is removed by passing air through 
the tubes for twenty minutes, The air is freed from carbon dioxide and 
dried by passing through two bottles containing concentrated caustic potash 
solution and then through a Jong tube filled with calcium chloride. 


DIRECT DETERMINATION OF CARBON DIOXIDE. 381 


on the one hand and with f on the other, after which a slow stream 
of hydrochloric acid (1:3) is allowed to flow upon the substance 
from the funnel 7, causing an immediate evolution of carbon 
dioxide gas. The stream of acid is regulated so that not more 
than 3-4 bubbles per second of gas pass through a. When all of 
the acid has been added, the contents of the flask are slowly heated 
to boiling and while the solution is boiling gently, a slow current of 
air is passed through the apparatus so that not more than 2-3 











Fie. 59. 


bubbles per second pass through a. During the whole operation, 
cold water is allowed to flow through the condenser; in this way 
the water vapor is condensed and flows back into the flask instead 
of reaching the sulphuric acid in a; consequently the contents of 
the latter tube seldom have to be renewed. Almost all of the car- 
bonie acid is absorbed in the first soda-lime tube, as may be ascer- 
tained by the heat effect there. The second tube, e, should remain 
perfectly cold provided not more than 0.5-1 gm. of the carbonate 
was taken for the analysis. When all of the carbon dioxide has 
been absorbed the tube d quickly cools. As soon as this has 
taken place, the flame is removed and a somewhat more rapid 
current of air is conducted through the apparatus for twenty 
minutes more. The soda-lime tubes are then removed, stoppered, 
and allowed to stand in the balance case for twenty minutes, in 


382 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


order to acquire the temperature of the balance; they are then 
weighed. 

Remark.—The results obtained by this method are wartoaal 
satisfactory. For the analysis of substances containing small 
amounts of carbonate, from 3-10 gms. are taken for the 
analysis. 

If the substance contains besides the carbonate a sulphide 
which is decomposable with acid, a tube containing pumice 
impregnated with copper sulphate * is introduced between a‘ 
and b, and this serves to absorb all of the hydrogen sulphide 
avoid. 

The Fresenius-Classen method is suitable not only for the 
determination of carbon dioxide in solid substances, but also for 
the analysis of carbonates in solution provided little or no free 
carbonic acid is present. In case considerable amounts of the 
latter are to be estimated, as in the case of many mineral waters, 
the analysis is conducted as follows: 


Determination of the Total Amount of Carbonic Acid mn 
Mineral Waters. 


From 3 to 4 gms. of freshly-burnt lime ¢ and the same amount 





* Sixty gms. of pumice in pieces about the size of a pea are placed in 4 

porcelain dish and covered with a concentrated solution of 30-35 gms. of 
copper sulphate. The solution is evaporated to dryness with constant stirring 
and the residue allowed to remain in the hot closet at 150-160° C. for four 
or five hours. At this temperature the copper sulphate is partly dehydrated 
and in this condition it absorbs hydrogen sulphide more readily than when 
in the hydrous condition. It cannot be heated higher than the above tem- 
perature as otherwise some sulphur dioxide is formed which would be absorbed 
by the soda-lime. 
_ {To prepare this lime absolutely free from carbonate, the lime is placed 
in a tube of difficultly fusible glass and heated in a small combustion furnace, 
meanwhile passing a. current of dry air free from carbon dioxide over it. 
In this way 4 gms. of commercial lime can be freed from carbonate in 
one-half to three-quarters of an hour. That the carbon dioxide is actually 
removed can be shown at the end of that time by passing the escaping air 
through baryta water; there should be no turbidity. A blank. experiment 
should always be made with this lime. If it is desired to use commercial lime 
for the determination, the amount of carbonate present is detérmined and 
an accurately weighed amount is used for the analysis. 4 


DIRECT DETERMINATION OF CARBON DIOXIDE. 383 


of crystallized calcium chloride* are placed in each of from four 
to six Erlenmeyer flasks whose necks are of such a size that 
they will each fit the apparatus shown in Fig. 59. The flasks are 
closed by means of tightly-fitting rubber stoppers and accurately 
weighed. A double-bored rubber stopper is taken of such a size 
that it will fit into. the neck of each of the above flasks and through 
one of the holes is fitted a short glass tube which reaches about 3 cm. 
above the stopper and the same distance below, while through the 
other hole a glass. tube about 50 cm. long is passed which likewise 
reaches about 3 cm. below the stopper. ‘To fill the weighed flasks 
with the water to be analyzed, they are taken to the spring and 
are treated one after another as follows: The solid rubber stopper 
is quickly replaced by the one fitted with the two tubes, the thumb 
is placed over the shorter of the tubes, and the flask is dipped well 
below the surface of the water, but so that the longer tube still 
reaches into the air above. The thumb is now removed from the 
shorter tube, when the spring-water will pass into the flask and the 
replaced air will escape through the long tube. As soon as the 
flask is almost full, the shorter tube is again closed with the thumb, 
the flask is removed from the water, and the stoppers are once 
more quickly interchanged. To make sure that the solid stopper is 
not loosened while carrying the flask back to the laboratory, it is 
covered by a piece of parchment paper, and fastened by means of 
string to the neck of the flask. The flasks are then allowed to 
stand several days with frequent shaking, when the precipitate is 
allowed to settle and the flask and contents are weighed. The gain 
in weight represents the amount of water taken for the analysis. 
The supernatant liquid is quickly poured off through a folded filter, 
the filter is immediately thrown into the flask, and the latter is 
now connected with the apparatus shown in Fig. 59. The carbon 
dioxide is determined as in the previous process. 

This method is capable of yielding excellent results provided 
the flasks can be filled as above described. Often, however, the 
spring is not easily accessible, so that the flasks must be filled by a 





* The addition of calcium chloride serves to decompose any alkali car- 
bonate. This is not quantitatively decomposed by lime alone, particularly 
when magnesium carbonate is present. 


384. GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


different method and usually a small amount of carbonic acid is 
lost during the operation. A much more expeditious and accurate 
procedure which can be performed within one hour at the spring, 
consists in the determination of the total amount of carbonic 
acid present in mineral waters by measuring the volume of the 
gas.* : 


2. Gas-volumetric Determination of Carbonic Acid. 


(a) Method of O. Pettersson. 


This excellent method, upon which the two following pro- 
cedures are based, consists of evolving carbon dioxide from 
carbonates by the action of acid, collecting the gas over mercury 








Fig. 60. 


and computing its weight from its volume. Petterson’s apparatus 
is shown in Fig. 60, and was used by him for the determination 
of the carbonic acid in sea-water (Skagerrak), in carbonates, and 
also for the determination of carbon in iron and steel. The method 





* Cf. the modified method of Pettersson on p. 393. 
+ Berichte, 28 (1890), p. 1402. 


GAS-VOLUMETRIC DETERMINATION OF CARBONIC ACID. 385 


- of determining ‘the carbonic acid in a water containing small 
amounts of free earbonic acid but considerable carbonate will suffice 
to show how the apparatus is used. The decomposition-flask K 
is filled with distilled water up to the mark just below the side- 
arm. (the mark is not shown in the illustration). By weighing the 
flask both empty and with this amount of water, the volume of the 
flask when filled to the mark is obtained. The flask is now filled 
up to this mark with the water to be examined, a small piece of 
aluminium wire * is thrown in, and the flask is connected with the 
rest of the apparatus as shown in the figure. All of the rubber 
tubing should be firmly fastened to the glass by means of wire. 
The cozks a, b, and d are closed, cis opened, and the air in the 
measuring-tube is removed by raising M until the mercury rises 
in the capillary up to the crossing point. After this c is closed, 
a is opened, M is brought very low, and the screw-cock d is 
slowly opened. By this means the hydrochloric acid in N is intro- 
duced into the flask K. The acid is allowed to run into the flask 
until the upper part of the apparatus is reached, when d is closed 
and then a. The air in the measuring-tube (which does not con- 
tain an appreciable amount of carbon dioxide) is removed by 
opening c and raising M, after which cis again closed. Nowais 
once more opened, M is lowered, and the liquid in K is heated by 
‘means of a flame. 

A lively evolution of gas at once ensues. As soon as the meas- — 
uring-tube is almost filled with the gas, a is closed, the flame is 
removed from K, M is raised until the mercury within it stands 
level with that in the measuring-tube, and its position in the latter 
is then read. At the same time the barometer reading must be 
noted as well as the temperature of the cold water which surrounds 
the measuring-tube. After this b is opened and WM is raised, 
whereby the gas passes into the Orsat tube O which contains 
caustic potash solution (1:2). As soon as the mercury has reached 
the juncture of the horizontal and vertical tubes, b is closed and 
the gas is allowed to remain in the Orsat tube for three minutes. 
The unabsorbed gas is once more brought into the measuring- 
tube, taking care that none of the caustic potash solution comes 
with it (the latter should not quite reach the stop-cock b). After | 
bringing the gas once more to the atmospheric pressure, its volume 





* (0142 gm. aluminium evolves 20 c.c. of moist hydrogen at 720mm. and 15°C. 


386 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


as well as the temperature and barometer reading is noted. Asa 
rule, these readings of the barometer and thermometer remain 
constant, otherwise it is necessary to reduce the gas volumes in 
each case to 0° C. and 760 mm. pressure. The differenée between 
the two volumes represents the amount of the carbonic acid 
gas. The unabsorbed gas is removed through ¢ and this whole 
operation of collecting the gas and absorbing the carbon dioxide 
is repeated until finally no more gas is given off from the liquid 
in K. 

In case it is desired to determine the amount of carbonate in 
a solid substance, a smaller decomposition-flask should be used. 
The aluminium wire is added to the weighed substance and the 
apparatus is exhausted by repeatedly lowering M, closing a, 
opening c, and then raising M. Finally the acid is allowed to 
act upon the substance and the determination is carried out exactly 
as described above. 

Computation of the Analysis.—Let us assume that from a 
gms. of substance V c.c. of carbon dioxide were obtained, which 
was measured moist at #? C. and B. mm. pressure. First of all 
the volume is reduced to 0° C. and 760 mm. pressure by the 
following formula: 

V (B—w) -273 
760(273+ t) ° 

In this formula, w represents the tension of aqueous vapor ex- 
pressed in millimeters of mercury. 

In order to compute the weight of the carbon dioxide from this 
volume, we start with the fact that the density of carbon dioxide 
is 1.529 * referred to air as unity. 1 c.c. of air at 0° and 760 mm. 
pressure weighs 0.001293 gm.t. consequently at 0° and 760 mm. 


1 c.c.. COg weighs 0.001293 x 1.529 =0.1977 gm. 


and Vo c.c. weigh V X0.001977 gm. The percentage of COg in 
the original substance is then 


Vo X0.1977 
a 





V.= 





=per cent. COs. 





* Cf. Lord Rayleigh, Proc. Roy. Soc., 62, 204 (1897). 
{ Landolt-Bérnstein, Phys. chem. Tabellen. 


GAS-VOLUMETRIC DETERMINATION OF CARBONIC ACID. — 387 


Remark.—The addition of aluminium is absolutely necessary. 
By boiling an acid solution, carbonic acid is not completely 





a 
tr 
Y, 
Vie, 
ow 
IN 
Ne 
GY 


ari oe io oo laa 











95 








ll 
sil 


lal 
teal be 


fel 


* 
bat 
bes 








MCT ATT it if castint wal ennatin min 
nad a Fett foe se “teat 
caste ts oat lt wb —'— 


Pree rece eee 





d\ 








6 Tolar totatuatuaunrtshaia aun kn 
nd os dP bt Ft a a nt eb es a 


b 
_ Fig. 61. 
expelled; this is only effected when a different gas simultane 
ously passes through the solution. Formerly it was customary 


388 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


to pass air through the apparatus, but Petterson accomplished 
the same purpose by generating hydrogen within the liquid 
itself, 


(b) Method of Lunge and M archlewskti.* 


Lunge and Marchlewski carry out the determination according . 
to the same principle as that of the above process; i.e., by simul- 
taneously evolving hydrogen (alunfnium and hydrochloric acid), 
measuring the gas, and absorbing the carbon dioxide by means of 
caustic potash in an Orsat tube. 

The apparatus which they recommend is shown in Fig. 61, b. 
It consists of the 40-c.c. decomposition-flask N, the 140-c.c. 
measuring-tube A, the compensation-tube C, and the levelling- 
tube B; the three last are connected together as shown in the 
figure. 

In the case of all gas-volumetric methods, the volume of the 
measured gas must be reduced to 0° C. and 760 mm. pressure, 
which ordinarily requires a knowledge of the temperature and the 
barometric pressure. In this method the reduction is accom- 
plished without paying any attention to the actual readings of 
the thermometer and barometer by means of the compensation- 
tube C, which contains a known amount of air, viz., that amount 
of air which in a dry condition assumes a volume of 100 c.c. at 
0° C. and 760 mm. pressure. If, therefore, this amount of air has 
a volume of V’ at ¢° and atmospheric pressure P’ (with the mer- 
cury at the same level in B and C), we know that this volume of 
any gas would be equal to 100c.c. at 0° C. and 760 mm. pressure. 
By raising the levelling-tube B so high that V’ ¢.c. is compressed to 
100 c.c., we have accomplished the reduction in a mechanical way. 
If, however, there is a gas volume V” in the measuring-tube A 
under the same pressure as that in the compensation-tube (this is 
the case when the mercury level is the same in A and C), we can 
reduce this volume to the standard conditions by simply raising 
B until the volume of the gas in C is just 100 ¢.c., taking care that © 
the mercury remains at the same height in the tubes A and C. 
The volume of the gas V,’” in A corresponds, therefore, to the vol- 





— 


* Zeitschr. f. angew. Chem., 1891, p. 229. 


GAS-VOLUMETRIC DETERMINATION OF CARBONIC ACID. 389 


ume of this gas at 0° C. and 760 mm. pressure, for it has been com- 
pressed to the same degree as the gas in C. This is apparent when 
we remember that at a constant temperature the product of the 
pressure into the volume remains a constant for any gas. 

In the compensation-tube we have the volume V’ at atmos- 
pheric pressure P’, and after compression the volume becomes 
V,’=100 c.c. and the pressure is P,, from which it follows: 


1, V'P’=V¥'P,. 


In the measuring-tube A, we have the volume V” at the atmos- 
pheric pressure P’, and after compression this volume becomes 
V,’, and the pressure P,, so that 


ei Mth eV OP. 
By dividing equation 1 by equation 2 we have; 


VP Mel PR; 
V".pP’ eo V,/’: fies 





. We eve te 
and V,” is, therefore, the reduced gas volume that is desired. 

Before using the apparatus for the determination, it is necessary 
to fill the compensation-tube with the correct amount of air; this 
is accomplished as follows: 

First of all a calculation is made to determine what would be 
the volume of 100 c.c. of dry air measured at 0° C. and 760 mm. 
pressure when measured moist at the temperature of the laboratory 
and the prevailing barometric pressure. To illustrate, let us 
assume 


t=17.5° C.; B=731 mm. ; w=14.9 (tension of aqueous vapor), 


then 
100760 290.5 
», 273(731,.—14.:9) 


Accordingly 112.9 c¢.c. of air are introduced into the tube 
C by removing the stopper and lowering the: levelling-tube until 
the mercury in the compensation-tube stands at exactly 112.9 c.e. 
A drop of water is added by means of a pipette, the tube is im- 


¥ ae LEZ.9 Cie: 





390 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


mediately stoppered, and an air-tight seal is made by covering the 
latter with mercury. A rubber stopper containing a glass tube is 
then pressed down into D. After this the temperature and pres- 
sure May vary as much as it will; the reduced volume of the air 
in C' will always be equal to 100 c.e. 

Procedure for the Analysis.—About 0.08 gm. of aluminium wire, 
i.e., enough to furnish approximately 100 c¢.c. of hydrogen, is 
weighed out into the decomposition- 
flask. Such an amount of the substance 
to be analyzed is added, that at the 
most 30 ¢.c. of carbon dioxide will be 
generated, and the flask is connected 
with the funnel tube M, and capillary n. 
Connection is also made with the tube A 
after it has been completely filled with jam 
mercury by raising B. The air from Nis § 1} 
now exhausted by lowering B, opening h } 
so that eis connected with A, then clos- 
ing h by turning it 90°, and carefully | 
raising B until the mercury stands at | 
an equal height in A and B; after this |. 
h is turned so that A is connected | 
with the capillary d, and the air in 
A is expelled. After repeating this | 3) ) 
process three or four times until finally \| 
only two or three centimeters of air re- — 
main in A, B is lowered, the hydro- Fic. 62. 
chloric mond (1:3) is added to M, h is 
carefully opened, then m until 10 ¢.c. of the acid have run 
into the flask N, when m is once more closed. The carbon 
dioxide evolution begins at once and the mercury level quickly 
falls in A. The contents of the flask are heated to boiling over a 
flame and kept at this temperature until all of the aluminium has 
dissolved. During the whole operation the mercury level in B 
_must be kept lower than that in A. In order to transfer the gas 
remaining in the flask N into the tube A, M is filled with distilled 
water, m is slowly opened and the water is allowed to run into N 
until the stop-ccck h is reached, when the latter is immediately 





GAS-VOLUMETRIC DETERMINATION OF CARBONIC ACID. 391 


closed. The gas is then compressed by raising the tube B until 
the mercury stands at the same height in A and C and so that the 
level in the latter tube is exactly at the 100-c.c. mark. The reduced 
volume is then read. After this the capillary d is connected 
with an Orsat tube filled with caustic potash (1:2) (Fig. 62), the 
gas in A is driven over into the latter, allowed to stand there for 
three minutes, and again transferred to A, where its volume at 
0° C. and 760 mm. pressure is determined as before. The difference 
in the two readings represents the volume of the carbon dioxide, 
and the per cent. ean be computed according to the formula 


Per cent. CO,=0.1977. 


inwhich V is the amount of carbon dioxide absorbed in the Orsat tube 
and a represents the amount of substance taken for the analysis. 

_ Remark.—This is the most exact of all methods for the deter- 
mination of carbon dioxide in solid substances and is accom- 
plished most quickly. It is particularly to be recommended 
where carbon dioxide determinations must be made daily, as, for 
example, in cement factories. It is necessary, however, to test 
the volume of the gas in the compensation-tube from time to time 
in order to make sure that it really corresponds to 100 ¢.c. of air 
under the standard conditions of temperature and pressure. 

For a single determination the author prefers to dispense 
with the compensation-tube. In this case, however, the col ected 
gas must be kept surrounded by water at a definite temperature, 
as in. the Petterson method, and the temperature and pressure 
must be observed. It is also well to make these readings in the 
above-described procedure, in order to be sure that the volume in 
the compensation-tube has remained constant. 


Pe (c) Method of Lunge and Rittener.* 

. In the decomposition flask K, Fig. 63, is placed 0.14—0.15 g. of 

calcite, or a corresponding amount of any other carbonate, and 

a small piece of aluminium wire, weighing about 0.015 g., is fas- 

tened to the neck of the flask. About 1 ¢.c. of water is allowed 

to flow through the funnel, 7’, and then the capillary is connected 
* Z. angew. Chem., 1906, 1849. 





392 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


with the dry Bunte burette. The stop-cock of the funnel T is 
closed and the two cocks of the Bunte burette are opened. The 
lower stop-cock, h,, is now connected with the suction pump, 
and a partial vacuum is produced in the burette by letting the 
pump run two or three minutes, after which h, is closed. Now 
from the funnel 7’, hydrochloric acid 
(1:4) is allowed to flow upon the sub- 
stance wntil the latter is decomposed 
completely; then the liquid is heated 
to boiling,* taking care that no water 
gets into the burette. Acid is then 
added from 7 until the aluminium 
wire is reached and the flask is heated 
again. The hydrogen now evolved 
serves to expel the last traces of carbon 
dioxide from the flask. As soon as all 
the aluminium is dissolved, hydro- 
chloric acid is added through the funnel 
until the liquid reaches the stop-cock h, 
which is then closed at once. The 
lower end, a, of the burette is now 
Fic. 63. connected by rubber tubing with the 

levelling tube N, which contains a 

saturated solution of common salt. By carefully opening the lower 
stop-cock h, the salt solution is allowed to rise in the burette until 
the liquid there stands at the same height as in the levelling tube, 
whereupon the stop-cock h, is closed. The apparatus is allowed to 
stand for 20-25 minutes so that the temperature of the gas will 
be that of the surroundings and then, by suitably raising or low- 
ering the levelling tube with h, open, the burette reading is taken 
and the temperature as well as barometer reading noted. The 
funnel 7'+ of the burette is filled with strong caustic potash 















ig 


ll 











" 


cers 








*In the case of carbonates, such as magnesite, dolomite, or siderite, 
they are decomposed so slowly by cold, dilute acid that it may be added — 
much more quickly than prescribed above. 3 

{t Provided the temperature and pressure are the same as before the 
absorption of the CO,. If this is not the case, both volumes must be reduced 
to 0° and 760 mm. pressure before the difference is found. 


GAS-VOLUMETRIC DETERMINATION OF CARBONIC ACID. 393 


solution (1:2) and a partial vacuum is produced in the burette, 
by lowering the levelling tube and opening the stop-cock h,. 

The caustic potash solution is allowed to run into the burette 
by opening the upper stop-cock h, which is closed before the last 
few drops of liquid leave the funnel, and the contents of the 
burette mixed by shaking. By repeating the operation it is 
easy to tell whether the absorption of carbon dioxide has been 
complete. The residual volume is read with the usual precau- 
tions and the difference between the two readings * gives the 
volume of the carbon dioxide. 

The computation of the weight of carbon dioxide is carried 
out exactly as described on p. 386, except that the vapor tension 
of the saturated salt solution only amounts to 80 per cent. of the 
tension of pure water at the same temperature. 


Example: Weight of substance=a Temperature = ?°, 
Volume of hydrogen+air+CO,=V; Barometer=B mm. 
Hydrogen+air=V;,’ Tension of aqueous vapor 
a =w mm. 
CO,=V;—V;,’ Tension of salt solu- 
tion=0.8w mm. 


The volume reduced to 0° and 760 mm. is, therefore: 


Yo 760 (273 +2) 2 





and the percentage of CO, in the substance (see p. 386) is, 


V,X0.1977 
a 





= per cent. CO.,. 


For the determination of carbon dioxide in mineral waters 
this apparatus is not suited; for this purpose the author has modi- 
fied the Pettersson apparatus as shown in Fig. 64. 


(d) The Modified Method of Pettersson. 


For decomposition-flasks, Erlenmeyers of from 70-200 c.c. 
capacity are used (according to the supposed amount of carbonic 


394 GRAVIMETRIC DETERMINATION OF THE METALLOWS. 


acid) and the exact. capacity of each flask is etched upon it. To 
determine this, each flask is provided with a tight!y-fitting stopper 
of gray (not red) rubber containing one hole, through which the 
small tube R is introduced. The bottom of # is fused together, 
but near the bottom a small opening is blown. 








The tube R is shoved into the stopper until the smail opening 
can be seen just below the bottom of the rubber stopper, and the 
latter is pressed as far as possible into the Erlenmeyer flask full of 
water. By this means some of the water passes from the flask 
into the tube R. The latter is then raised as is shown in Fig. 
64; b, and in this way an air-tight seal is made. . 

The water in Ff is now removed by filter-paper and the flask 
and contents weighed to the nearest centigram. By deducting 


GAS-VOLUMETRIC DETERMINATION OF CARBONIC ACID. 395 


from this, the weight of the empty flask together with the rubber 
stopper and R, the weight of the water, i.e. the volume of the 
flask, is obtained. By means of a piece of gummed paper fast- 
- ened to the flask, the position of the lower edge of the rubber 
stopper is noted. The flask is emptied, dried, and the neck of the 
flask as well as the paper strip is covered with a thin coating of 
wax. Along the edge of the paper where the bottom of the rubber 
stopper came on the flask, a sharp line is cut in the wax by means 
of a knife and the capacity is written upon the wax with a 
pointed file. ‘These lines are etched upon the flask by exposing 
them to the action of hydrofluoric acid for two minutes. The 
excess of the latter is then washed off, the flask dried, and the wax 
melted and wiped off with filter-paper. The flask is now ready to 
be used for the analysis. 

About 0.04 gm. of aluminium is placed in the flask, which is 
then filled by dipping into the spring. When this is not possible, 
a piece of rubber tubing is placed in the. bottle containing the 
water to be analyzed so that it reaches to the bottom and the water 
is siphoned off into the flask for two or three minutes. After this 
the filled flask is closed by the rubber stopper with the tube R so 
that the bottom of the stopper reaches just to the mark again. 
The tube £# is raised (Fig. 64, b) and the spring-water within the 
tube is washed out by a stream of distilled water from a wash- 
bottle.* The flask is then connected with the bulb-tube P (of about 
40 c.c. capacity), which in turn js connected with the measuring- 
tube B. The latter is placed in a condenser through which a stream 
of ordinary water constantly flows. The reservoir N’ is now con- 
nected with the flask as shown in the figure and the screw-cock H is 
closed. All rubber connections must be tightly fastened with wire. 

The bulb P is exhausted by lowering N so that the air passes 
into B, from whence it is driven into the Orsat tube O by turning 
the stop-cock M and raising N. ‘This operation is repeated four 
times. The air is then removed from the Orsat tube by suction 
through the right-hand capillary and the stop-cock is changed to 
its original position as shown in the figure. 

The tube F# is now pressed into the flask so that the small 
opening reaches below the lower surface of the stopper. 





* With water containing much carbonic acid, the flask and its contents 
are cooled by ice in order to prevent the slass breakine. 


396 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Usually carbon dioxide is immediately evolved and the mercury 
in B at once begins to sink slowly. The evolution of the gas is 
hastened by gently heating the contents of the flask. As soon as 
the measuring-tube is almost entirely filled with gas, the flame 
is removed, M is closed, and the contents of B are brought under 
atmospheric pressure by raising N until the mercury in the two 
tubes is at the same height, after which its position in B is noted. 
The temperature of the water surrounding B is taken, the barometer 
is read,* and the gas is driven over into the Orsat tube and allowed 
to remain there. This boiling, measuring, and driving over of 
the gas is repeated until only a slight gas evolution can be made 
to take place. In this way all the free carbon dioxide and a part 
of that present as bicarbonate is driven off, while that present as 
normal carbonate together with the rest of the bicarbonate remains 
in the flask; the liquid in the latter is usually turbid at this point 
owing to the precipitation of alkaline-earth carbonates. The 
reservoir N’ is now filled with hydrochloric acid (1:2) and the 
air removed from the rubber tubing by raising N’ high and pinch- 
ing the tubing with the fingers. The levelling-tube N is placed in 
a low position, H is opened, and a small amount of acid is allowed 
to run into K, after which H is again closed. As soon as the acid 
reaches the contents of K, a lively evolution of carbon dioxide 
ensues, which is afterward hastened by gentle warming. When 
the measuring-tube B is nearly filled, its contents are read and 
driven over into the Orsat tube as before. The addition of the 
acid, etc., is repeated until finally the liquid in K clears up and 
the aluminium begins to evolve a steady stream of hydrogen, when 
the contents of the flask are heated to boiling; but care is taken that 
none of the liquid in the flask is carried over with the escaping 
gas. As soon as the aluminium has completely dissolved, WN is. 
lowered, H is opened, so that the flask is filled with the hydrochloric 
acid solution and the last portions of the gas are carried over 
into the measuring-tube B. As soon as the acid has reached 
the stop-cock M, this is closed, and after reading the volume of 
the gas as before it is led into the Orsat tube. After remaining 





* If this analysis is made at the spring, it is necessary to have a sensitive 
aneroid barometer at hand. 


DETERMINATION OF CARBONIC ACID, 397 


tiere three minutes the unabsorbed gas is once more transferred 
to B and its volume subtracted from the total amount of gas which 
has been expelled from the water that was analyzed. This differ- 
ence represents the volume of the carbon dioxide gas. By eor- 
rectly adjusting the current of water flowing through the condenser, 
the temperature at which the gas is measured will remain constant 
during the entire experiment. 

From the volume of the absorbed carbon dioxide the per cent. 
present is computed as was shown under the Pettersson method. 

Remark.—By this method the author has determined success- 
fully at the spring the amount of carbonate in a great many of the 
most important waters of Switzerland. For a single determination 
more than one hour is seldom required. The apparatus * can 
be readily transported. The author has travelled with an outfit 
during the last six years over the highest mountain passes under 
many disadvantages of weather, both in winter and summer, 
without its meeting with any accident. In order to maintain 
the tube N at any desired height it is well to fasten it to a ring- 
stand. 


Determination of Carbonic Acid in the Air, 


See Part II, Acidimetry. 


Determination of Carbonic Acid in the Presence of Other 
Volatile Substances. 


(a) Determination of Carbonic Acid in the Presence of Chlorine. 


If it is desired to determine the amount of carbonate present 
in commercial chloride of lime, chlorine will be evolved with the 
_ carbonic acid on treatment of the solid substance with hydrochloric 
acid, so that neither the direct nor the indirect method will give 
correct results. The determination can easily be effected by the 
following procedure: 

The chloride of lime:is decomposed with hydrochloric acid and 
the gases evolved (CO,+Cl,) are passed into an ammoniacal solu- 





* It can be purchased from Bender and Hobein of Zurich. 


398 GRAVIMETRIC DETERMINATION OF THE METALLOIDS., 


tion containing calcium chloride.* After standing several hours 
in a warm place, the precipitate is filtered off quickly, washed with 
water, and the carbonate determined in the precipitated calcium 
carbonate by one of the usual methods. 

Remarx.—On conducting the mixture of chlorine and carbon 
dioxide into the ammoniacal solution of calcium chloride, the 
chlorine is changed into ammonium chloride with evo-ution of 
nitrogen: ° 


4NH,+ 3Cl=3NH,CI+N, 


while the carbon dioxide is absorbed by the ammonia forming 
ammonium carbonate, and the latter is precipitated by the calcium 
chloride as calcium earbonate. 


(b) Determination of Carbon Dioxide in the Presence of Alkalé 
Sulphides, Sulphites, or Thiosulphates. 


The solution to be analyzed is treated with an excess of a solu- 
tion of hydrogen peroxide containing potassium hydroxide, but. 
free from carbonate. It is heated to boiling to destroy the excess 
of the hydrogen peroxide, concentrated, and the carbonate deter- 
mined preferably by the Fresenius-Classen method (p. 380). 


DETERMINATION OF CARBON. 


(1) In Iron and Steel. 
(2) In Organic Compounds. 


(1) DETERMINATION OF CARBON IN IRON AND STEEL. 


Carbon exists in two forms in iron and steel: 
(a) As Carbide Carbon. 
(b) As Graphite. 
On treating an iron containing carbide carbon with dilute hydro- 
chloric or sulphuric acid, only a part of it is evolved in the form of 





* One part of crystallized calcium chloride is dissolved in five parts of 
water, ten parts of ammonia (sp. gr. 0.96) are added, and the mixture allowed 
to stand at least four weeks before using. 


DETERMINATION OF CARBON. 399 


characteristic smelling hydrocarbons. This carbon is called by 
Ledebur * “hardening carbon” to distinguish it from ‘ordinary 
carbide carbon” which is left behind as a brown or gray mass when 
the iron is treated with dilute hydrochloric or sulphuric acid; but 
on boiling with strong hydrochloric acid the latter is also ntoesireck 
to volatile hydrocarbons. 

Graphite is unattacked by acids baer all circumstances. 

In the analysis of iron and steel it is customary to determine 
directly the total carbon and the graphite, in which case the differ- 
ence represents the carbide carbon. 


Determination of Total Carbon. 


Principle-—The carbon is oxidized to carbon dioxide, and the 
latter is either absorbed in a suitable apparatus or its volume 
is measured. 

For the oxidation of the carbon a great many methods have 
been proposed; they can be classified as follows: 

(a) Those in which the oxidation is effected with the un- 
changed substance itself. 

(8) Those in which the greater part of the iron is removed, 
and the residue subjected to combustion. 


The Chromic-Sulphuric Acid Method. 


In this method the borings, which should be as fine as possible 
and free from grease, are treated with a mixture of chromic and. 
sulphuric acids and ‘heated to boiling. Thereby, the iron goes 
into solution and the carbon is oxidized to carbon dioxide. In 
spite of a large excess of chromic acid, however, a considerable 
amount of the carbon is likely to escape in the form of hydro- 
carbons and carbon monoxide, unless precautions are taken. 
To prevent such losses, Sirnstrém + recommended leading the 


‘** Stahl und Eisen, 1888, p. 742. 

{ Sarnstrém, Berg- und Hiittenm. Ztg., 1885, 52, and Corleis, Stahl v. 
Hisen, 1894, 581. With ferromanganese the loss amounts to 22.5 per cent. 
of the total carbon, with steel 9 per cent. With ferromanganese the escaping 
gases contain, besides carbon dioxide and traces of heavy hydrocarbons, 18 
per cent. methane, 76 per cent. hydrogen, 3 per cent. oxygen, and 2 per 
cent. carbon monoxide, 





4990 GRAVIMETRIC DETERMINATION OF THE ME TALLOIDS. 


escaping vapors over copper oxide in a combustion tube,* 80 em. 
long, which is heated in a combustion furnace. Many experiments 
have shown that the method of Siarnstrém gives exact results, 
although objection has been raised to the long combustion tube 
that is required. 

Corleis has succeeded in simplifying the mothe by showing 
that a very short combustion tube, filled with copper oxide, heated 
by a single Bunsen flame, suffices # the sample is covered with a 
coating of copper during the treatment with chromic and sulphuric 














Fia. 65, 


acids. In fact the use of the combustion tube is unnecessary in an 
ordinary steel anlaysis, because only 2 per cent. of the total amount 
of carbon present is lost in this case. In the analysis of ferro- 
manganese, and similar alloys, however, the use of the hot tube is 
to be recommended. 

Ledebur f+ even found that the results obtained with irons rich 
in graphite were a little too high on account of the formation of 
some sulphur dioxide, but this error can be overcome by passing 
the gases through chromium trioxide just before they enter the 
combustion tube. 3 

The apparatus required is shown in Fig. 65 and consists of a — 
Corleis decomposition flask A with condenser. 





* A small platinum tube heated to glowing also suffices. 
{ Leitfaden fiir Eisenhiitten-Laborat. 


DETERMINATION OF CARBON. 401 


The flask is connected, as shown in the drawing, on one side 
with a soda-lime tower, W, at the bottom of which is placed .a 
little concentrated caustic potash solution, and on the other side 
with a system of tubes. The tube B is about 10 cm. long and 
contains chromium trioxide between two asbestos plugs. The 
tube C is 15 em. long, is made of difficuitly fusible glass, and 
filled with granular cupric oxide. It is placed in a little box made 
of asbestos paper. The tubes a, 0, c, d, e, and f are filled exactly 
as described on p. 380. Tubes a, b, and c are drying tubes, the 
first containing glass beads wet with concentrated sulphuric acid, 
the other two containing calcium chloride; d and e are glass- 
stoppered soda lime tubes, the upper third of the right-hand arm 
of each containing calcium chloride. The tube f is a safety tube 
which is not weighed, but is used to avoid any chance of carbon 
dioxide or moisture entering the weighed tubes from the air. 

Reagents.—1. A saturated solution of ordinary chromic acid 
containing some sulphate. It is not advisable to use chemically- 
pure chromic acid for this purpose, for the latter often contains 
organic substances. 

2. A solution of copper sulphate made by dissolving 200 gms. 
of the salt in 1 liter of water. 

Procedure.—The ground-glass stopper a is removed, and 
through the opening 25 c.c. of chromic acid solution, 150 c.c. of 
copper sulphate solution and 200 c.c. of concentrated sulphuric 
acid are poured into the flask, 4, and mixed. The mixture in the 
flask is heated to boiling and kept at this temperature for ten 
minutes. The flame is then removed and a current of air free 
from carbon dioxide is passed through the apparatus for ten 
minutes at the rate of about three bubbles per second. The flask is 
then connected with the tube B, the red-hot copper oxide tube, 
and with the U tubes,* while the current of air is continued for five 
minutes more. .The soda-lime tubes d and e are removed, closed, 
and allowed to stand ten minutes in the balance room. They are 
opened for a moment, quickly closed, rubbed off with a piece of 
chamois skin or a clean linen cloth, allowed to stand five minutes 
in the balance-case, and then weighed. 





* Corleis used phosphorus pentoxide for a drying agent, but calcium 
chloride is satisfactory. 


4°92 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


By means of this preliminary boiling, traces of organic matter 
contained in the apparatus are removed. 

After weighing the soda-lime tubes, they are connected with 
the apparatus again, the decomposition-flask is opened, and the 
weighed substance (from 0.5 to 5 gms. according to the amount of 
carbon present)* is introduced quickly from a glass-stoppered 
weighing tube, which is subsequently weighed again to determine 
the amount of sample. The flask*is immediately closed and the 
copper oxide tube heated to glowing, after which the contents of 
the flask are slowly heated so that after from 15-20 minutes the 
liquid begins to boil. The solution is kept boiling for one or two 
hours while aslow current of air is conducted through the apparatus, 
The flame is then removed, and about two liters more of air 
are passed through the apparatus, when the soda-lime tubes are 
removed and subsequently weighed as before. 

Since the use of the copper sulphate solution prevents the loss 
of more than about 2 per cent. of the total amount of carbon pres- 
ent, it is evident that the combustion-tube can be dispensed with 
for technical purposes. 


Combustion of Carbon in the Wet Way and Measuring the 
Volume of the Carbon Dioxide. 


This operation is best carried out by means of the Lunge- 
Marchlewski method. 

The apparatus necessary is shown in Fig. 61. In this ease, 
however, the decomposition-flask is larger and there should be 
a ground-glass connection between the flask and a condenser. 
Furthermore, a funnel-tube is fused into the neck of the flask, 
and runs along the side of the flask on the inside ending in a 
quite fine point nearits bottom. The upper end of the condenser is 
connected with the measuring-tube by means of a capillary tube 
about 36 cm. long, ground to fit the condenser-tube. 

Reagents.—1. A saturated, neutral solution of copper sulphate. 

2. A chromic acid solution (100 gms. CrO, in 100 c.c. of water). 

3. Sulphuric acid of specific gravity 1.65 and saturated with 
chromic acid. 





* For cast iron 0.5 gms. suffices but for steel from 1 to 2 gm. and for wrought 
iron 5 gms. should be used. : hy 


COMBUSTION OF CAFBON IN THE WET WAY. 403 


4, Sulphuric acid of specific gravity 1.71, also saturated with 
chromic acid. 

5. Pure sulphuric acid of specific gravity 1.10. 

6. Commercial hydrogen peroxide solution. 

Procedure.—The amount of iron or steel to be weighed out 
and the necessary quantities of the reagents are shown in the 
following table: 





c.c. 6.C. c.c. c.c. c.c. 








Per Cent. C. Weieh guiphate| Acid | SocGr. | Sp.Gr. | So. Gr. | Hsp, 
Grams. |Solution.| Solution.} 1.65. A472; 1h: 

Over 1.5 0.4-0.5 5 5 ptt Tes a mae 30 1 

0.8-1.5 ¥: 10 10 a 8 a a 25 2 

0.5-0.8 2 20 ae age Te! 5 2 

0.25-0.5 3 50 a BS pg 75 5 2 

Less than 0.25 5 7 te SO Sates +2570 5 2 


























The substance is treated with the copper sulphate solution in 
the decomposition-flask at the ordinary temperature. Malleable 
iron is allowed to stand for at least one hour, while pig iron re- 
quires at least six hours. The flask is then connected with the 
measuring-tube, which is filled with mercury, and the air in the 
flask is exhausted as was described on p. 390. After this is ac- 
complished, the levelling-tube is placed in a low position and the 
proper amount of the chromic acid solution is added through the 
funnel, followed first by the proper amount of the stronger acid and 
then by that of the weaker acid, after which the stop-cock in the 
funnel is quickly closed. The communication between the meas- 
uring-tube and the flask remains open. With the levelling-tube 
remaining in its low position,.the contents of the flask are heated 
to gentle boiling, which is continued for one hour, and the flame 
is then removed. Now, in order to remove the last traces of 
carbon dioxide from the solution, the prescribed amount of hydro- 
gen peroxide is added to the contents of the flask and the flask 
is afterwards completely filled with hot water until all of the gas is 
driven over into the measuring-tube. The stop-cock b is then 
closed, the gas is reduced to the volume corresponding to 0° C. 
and 760 mm. pressure as described on p. 388 and this volume 
read. It is then driven over into the Orsat tube and the volume 
of the unabsorbed gas is determined as before. The difference 
between the two readings represents the amount of carbon dioxide 


404 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


measured under the standard conditions of temperature and 
pressure. If this is multiplied by the factor 0.0005392 the amount 
of carbon present will be obtained. 

After the analysis has been completed, a blank determination 
must be made, using the same amounts of each reagent, in order 
to determine small amounts of organic matter which are invariably 
present in them. The amount of carbon dioxide found under 
these conditions must be subtracted from that obtained in the 
analysis proper. 


Method of Hempel.* 


Hempel objects to the above procedure on the ground that by 
dissolving the iron in the mixture of chromic and sulphurie acids 
some hydrocarbon is likely to escape oxidation. He found that 
by dissolving iron in chromic-sulphurie acid under diminished pres- 
sure in the presence of mercury all of the carbon would be readily 
oxidized to its dioxide. Fig. 66 represents the apparatus used. 


Reagents Required. 


1. Chromic acid solution. 100 gms. of chromic acid are dis- 
solved in 300 c.c. of water and 
30 gms. of sulphuric acid, sp. gr. 
1.704, are added. The resulting 
solution has a specific gravity of 
1.2. 

2. Sulphuric acid. 1000 ec.c. of 
concentrated sulphuric acid are 
mixed with 500 c.c. of water ana 
10 gms. of chromic acid and heated - 
for an hour in a large flask upon 
a sand-bath in order to completely 
destroy any dust, etc., that may 
be present. The flame is then 
taken away and a current of air is 
slowly conducted through the solu- 
tion in order to remove any carbon Fic. 66. 
dioxide that may have been formed. 

After cooling the solution, is diluted with water until it has a 
specific gravity of 1.704. 


=2" a 


= fmm meme mara am mmm crn se — Sh 
= SS ge 














* Verhandlg. d. Vereins z. Beférd. d. Gewerbefleisses, 1893. 


HEMPEL’S METHOD FOR THE COMBUSTION OF CARBON. 405 


Procedure. 


About 0.5 gm. of the iron or steel is placed in the decomposi- 
tion-flask B, about 2.3 gms. of mereury are added by means of 
a small pipette, and the apparatus is connected together as is 
shown in the drawing. 

By raising the levelling-bulb N, the inatenarinietesbis M is entirely 
filled with mercury, the stop-cock is closed, and the apparatus is 
connected at p with a suction-pump, by means of which the air in the 
flask B is exhausted as completely as possible. In order to make 
sure that the ground-glass connection between the flask and the 
condenser is perfectly air-tight, a little water is poured into the 
cup there. Into the funnel C are placed 30 c.c. of chromic 
acid solution, the stop-cock at p is closed, and by carefully lifting 
the latter a little the chromic acid is allowed to run into the flask, 
which is immediately heated to boiling over a small flame. After 
boiling for half an hour, 120 c¢.c. of sulphuric acid are added through 
C, the stop-cock at M is now opened for the first time and 
the contents of the flask boiled for half an hour longer. (At the 
start only carbon dioxide is generated, in proportion to the tem- 
perature of the solution, but toward the end of the operation there 
is a fairly lively evolution of oxygen.) The flame is removed, the 
gas in the flask is carried over into M by pouring water into C and 
lifting the tube p until the gas in the flask is entirely expelled. 
The total volume of the gas is read, after which the carbon dioxide 
is absorbed in a Hempel’s potash pipette and the volume of the 
unabsorbed gas is determined. The difference represents the 
amount of carbon dioxide formed by the oxidation. From this 
the amount of carbon present can be computed. 

The measuring of the gas in this apparatus will be described 
more in detail in Part III, Gas Analysis. 

Other methods for the determination of the volume of the car- 
bon dioxide formed from the carbon in iron or steel are those of J. 
Wiborgh,* Otto Pettersson and August Smitt.t 














* Zeit. f. anal. Chem., X XIX (1890), p. 198. 
{ Ibid., XXXII (1893), p. 385. 


406 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


The methods already described are suitable for the 
determination of carbon in wrought iron, cast iron, steel, 
etc., but not in alloys such as ferro-silicon, ferro-chrome or 
tungsten steel. For such materials, the following method is 
used. 


Wohler’s Chlorine Process.* 


Principle.—The sample of iron or steel is heated in a stream 
of pure chlorine gas whereby iron, silicon, phosphorus, and sulphur 
are volatilized while the carbon remains behind in the presence of 
small amounts of non-volatile chlorides. The silicon present 


as silica, due to inclosed slag, is not affected by the treatment. 


The residue is filtered through asbestos, the chlorides. washed out 
by water, and the carbon burned. to carbon dioxide either in the 
‘wet or in the dry way. 

The principal requisite for the success of the process is pure 
chlorine. This must not contain oxygen, water, nor carbon 
dioxide, because these substances all tend to convert a part of 
the carbon into carbon monoxide, whereby low results are ob- 
tained in the carbon determination. 

Procedure.—The specimen is subjected to the action of chlorine 
in an apparatus like that shown in Fig. 67. 

B is the liter flask in which the chlorine is generated; it 
contains about 200 gms. of pyrolusite and 500 c.c. of concentrated 
hydrochloric acid. The contents of the flask are heated over a 
very low flame and in this way a continuous stream of chlorine 
is evolved. When the current begins to slacken, more hydro- 
chloric acid is needed which is allowed to flow into the flask through 
a Bulk’s ¢ dropping funnel.{ To regulate the current of gas, the 





* Z. anal. Chem., 8, 401 (1869), ef. A. Ledebur, Leitfaden fiir Eisenhitten 
Laboratorien. 

+ Z. anal. Chem., 16, 467 (1892). 

t The flow of the acid is regulated by raising the tube S. Instead of S 
a glass rod covered with rubber tubing may be used. 


WOHLER’S CHLORINE PROCESS. 427 


flask is connected with the right-angled tube, h, which is provided 
with a stop-cock and leads to a cylinder, A, containing caustic. 
soda solution. If the stream of chlorine becomes too strong, the 
stop-cock is opened a little so that the excess of chlorine is absorbed 
by the sodium hydroxide. The chlorine is purified by means of 
the tubes a, b, c, C, and d; a contains water, b concentrated 
sulphuric acid, c glass beads, or pumice, moistened with sulphuric 
acid. C is a tube 40 cm. long and 1 cm. wide, made of difficultly- 





















































ty HU en anal 
i PAYOLPSEN FSW ENT TEL ROTH 
VT TTT wa! Ht 


It 
ll 
titty 


Fig. 67. 


fusible glass. It contains a layer, 15 cm. long, of coarse charcoal 
which has previously been well ignited and cooled in a desiccator. 
The charcoal is placed in the tube between two loose plugs of 
ignited asbestos. The tube is heated to dark redness in a small 
combustion furnace. If the chlorine gas contains small amounts 
of oxygen (air) or carbon dioxide, these are changed, on coming 
in contact with the hot charcoal, to carbon monoxide, which is 
unaffected by the carbon in the iron or steel. The last traces of 
moisture are removed by passing the gas through the tube d 
containing glass beads moistened with concentrated sulphuric 
acid. | 

‘The chlorine gas is next passed into the combustion tube D. 


408 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


_ This is about 40 em. long by 1.5 em. wide, is bent into a right 
angle and leads into concentrated sulphuric acid in the flask e. 
The sulphuric acid serves as a seal and prevents air from getting 
into the tube. | 

The substance, which should be as fine as possible, is sprinkled 
as a thin layer * upon a previously ignited porcelain boat. Of 
ferro-chrome about 0.5 gm. should be taken, and of ferro- 
silicon about 1 gm. The boat is shSved into the combustion tube 
and the evolution of chlorine is started as described above. The 
tube is not heated at all until after about twenty minutes, when 
the air will have all been expelled; then the heating is begun very 
gradually, lighting the burners one at a time from right to left. 
The formation and volatilization of the ferric chloride takes place 
at a relatively low temperature. | 

As soon as no more brown vapors escape from the tube, the 
heat is gradually raised until the tube begins to get red and then 
the residue in the tube is allowed to cool in the : 
stream of chlorine. 

The boat is removed from the combustion 
tube, and, in the case of ferro-silicon, the con- 
tents are rinsed with cold water into a beaker. 
From the beaker the insoluble residue is washed 
into an asbestos filter prepared as follows: In 
the funnel R, Fig. 68, which is about 1 cm. wide 
and 5 cm. long, is placed a little glass wool, 
and upon this a suspension of ignited asbestos 
fibers in water is poured until, with the aid of Fig. 68. 
light suction, the filtrate comes through perfectly 
free from asbestos fibers. The residue is washed, on such a filter, 
with cold water until no chloride can be detected in the 
filtrate. 

The carbonaceous residue can be oxidized in the apparatus 








* This is especially important in the case of ferro-chrome, because 
otherwise the metal will become covered with a coating of non-volatile 
chromic chloride which prevents it from being acted upon by the 
chlorine. | 


COMBUSTION OF CARBON IN THE DRY WAY. 409 


shown in Fig. 65, p. 400 but in this case the flask A should contain 
5 c.c. of a saturated, aqueous solution of chromic acid, and 60 c.c. 
of sulphuric acid, sp. gr. 1.71 which is likewise saturated with 
chromic acid. 

In the analysis of ferro-chrome, there is always some insoluble 
chromic chloride in the boat which cannot be removed by washing. 
In this case, therefore, the substance after being heated in a stream 
of chlorine, is heated in an atmosphere of hydrogen, whereby the 
insoluble chromic chloride is converted into soluble chromous 
chloride. The contents of the boat are then treated exactly as 
described above. 


Combustion of Carbon in the Dry Way.* 
(a) Solution of the Iron, 


A number of methods have been proposed for dissolving 
away the iron and leaving the carbon behind in the form of 
an insoluble residue. For this purpose a solution of potas- 
sium-cupric chloride containing 300 gms. of the double salt 
(2KCLCuCl2.2H2O) and 75 c.c. of concentrated hydrochloric 
acid to the liter has proved most satisfactory. Before using, 
the solution is filtered through ignited asbestos and preserved 
in a glass-stoppered bottle. The solution of the borings takes 
place very slowly unless the solution is stirred, which is best 
accomplished by means of a mechanical stirrer. Warming 





* The dry combustion methods are much used in the steel laboratories of 
the United States and by the Bureau of Standards at Washington, D.C., for 
analyzing special samples of iron and steel which are available for distribution 
and serve as standard samples of known chemical composition. Further- 
more, the Committee on Standard Methods of the American Foundrymen’s 
Association (Chemical Engineer, 5, 416 (1907) have recommended the use 
of a dry combustion method for settling all cases of dispute. 


410 GRAVIMETRIC DETERMINATION OF THE MéTALLOIDS. 


the solution also helps, but it should never be heated above 
60° to 70°. The following reactions take place: 


Fe+CuCle= FeCly + Cu, 
2Cu+2CuCls = 2CusC'o. 


The presence of potassium chloride aids the solution of 
the copper, probably on account of the formation of a double 
salt. 7 
The residue is filtered, dried and usually burned in a current 
of oxygen, the carbon dioxide being absorbed in a weighed bulb 
containing potassium hydroxide solution. To make sure that 
the oxygen used contains no carbon in any form, it is advisable to 
make use of a preheating tube, such as for example a short perve- _ 
lain tube filled with copper oxide; this serves to convert any 
carbon to carbon dioxide which is then absorbed in potassium 
hydroxide solution before coming in contact with any of the 
carbon from the sample. 

The combustion may take place in a porcelain or platinum 
tube, or in a special form of crucible, such as the jacketed crucible 
devised by Shimer, or the tubulated one of Gooch. These, how- 
ever, are made of platinum and are expensive but Ruppel * has 
shown that one of nickel answers the purpose equally well. 

The following directions are taken from the Report of the 
Committee on Standard Methods of the American Foundrymen’s 
Association and corresponds closely to the method used by the 
Bureau of Standards at Washington, D. C., in preparing standard 
samples for distribution. They apply equally well for the deter- 
mination of the total carbon in steel except that in the latter 
case usually 3 gms. of the sample and 200 c.c. of the potassium- 
cupric chloride solution are used. 





* J. Ind. Eng. Chem. 1, 415 (1909). 


DETERMINATION OF TOTAL CARBON IN CAST IRON. 445 


Determination of Total Carbon in Cast Iron. 


The train used consists of a pre-heating furnace containing 
copper oxide, followed by a bulb containing potassium hydroxide 
solution (sp. gr. 1.27), then one containing granular calcium 
chloride; the latter is connected with a ten-burner combustion 
furnace in which either a porcelain or platinum tube is placed. 
This combustion tube contains four or five inches of copper oxide 
between plugs of platinum gauze. The plug at the front end * 
of the furnace should be at about the point where the tube leaves 
the furnace. A roll of silver foil,t about two inches long, is placed 
in front of this plug. The combustion tube is connected at this 
end with a tube connecting anhydrous cupric sulphate, one of 
anhydrous cuprous chloride, and one of granular calcium chloride. 
A Geissler or Liebig bulb is connected with this last tube and 
contains potassium hydroxide solution (sp. gr. 1.27) with a pro- 
long of calcium chloride. A calcium chloride tube is used between 
this last tube and the aspirator bottle to prevent any moisture 
working backward. 

In filling the calcium chloride tubes, especially the prolong of 
the absorption bulb, care must be taken to press down the calcium 
chloride lumps well against one another, as when the tube is 
loosely filled, some moisture is likely to get by. A HEGATYP 
blank is often obtained by beginners for this reason. 

Before using the apparatus, the contents of the tube should 
be heated for half an hour in a stream of oxygen, then the weighed 
absorption bulb should be connected with the train and a blank 
determination made. The bulb should not gain in weight more 
than 0.5 mgm. 

One gram of the well-mixed drillings are meanwhile dissolved 
in 100 c.c. of the double chloride solution, and the solution event- 
ually filtered on ignited asbesots. The beaker in which the 





* In describing a combustion train it is customary to follow the direction 
of the flow of gas. The back or rear end is considered the end toward the gas 
reservoir, and the front. or forward end is that nearest to the weighed potash 
bulb. 

+ The silver foil is unnecessary if the carbonaceous residue is washed 
free from hydrochloric acid. 


412. GRAVIMETRIC DETERMINATION OF THE METALLOIDs. 


solution took place is washed thoroughly with 20 c.c. of dilute 
hydrochloric acid (4:1), and transferred to the filter by means of 
more of the same acid from a wash-bottle. Finally the residue is 
washed on the filter with hot water until free from chlorides. 
The filter is then dried at a temperature between 95° and 100°. 

Blair recommends the use of a perforated platinum boat for’ 
the filtering. This unquestionably works well, but where many 
determinations are being made it Mivolves considerable expense. 
An excellent substitute can be prepared from a funnel such as 
was described for use with a Gooch crucible, although it is well 
to shorten the sides somewhat. A tight coil of copper wire is 
placed in the bottom of the funnel with a long free end of wire 
reaching down below the bottom of the stem. Loose ignited 
asbestos is placed upon the coil of wire, followed by a suspension of 
the same asbestos in water. After applying suction, the asbestos 
is gently tamped down with the flattened end of a stirring rod. 
The finished pad is about 0.75 of an inch in diameter and 0.25 
of an inch thick. 

The dried asbestos, with the carbon upon it, is pushed into 
back end of the furnace and the funnel wiped out with dry, ignited 
asbestos. Care should be taken that the carbon on the asbestos 
reaches far enough into the tube to get the full heat from the 
furnace. The burners under the pre-heating furnace are now 
lighted, the oxygen turned on and allowed to pass through the 
absorption bulb at the rate of three bubbles per second, but no 
faster. The two forward burners under the combustion tube are 
lighted, at first turning them low and gradually raising the heat 
until the tube is hot. As soon as this end of the tube is hot, the 
third burner from the forward end is lighted and a few minutes 
later the fourth burner, which should be near the forward end 
of the carbon residue. Light each burner in succession until 
finally all are lighted and turned high enough to heat the tube red 
hot. After twenty minutes have elapsed from the time the last 
burner is turned on full, the oxygen is stopped and a current of 
air swept through the tube at the same rate for twenty minutes 
longer, gradually turning down the burners under the com- 
bustion tube. The potassium hydroxide bulb at the front of 
the train is then detached and weighed with the usual precautions. 


DETERMINATION OF TOTAL CARBON IN CAST IRON. 413 


When the Shimer or similar crucible is used for the combustion, 
it should be followed by a copper tube ;3; of an inch inside diameter 
and 10 inches long with its ends cooled by water jackets. In the 
center of this tube is placed a disc of platinum gauze and for three 
or four inches in the side toward the crucible a roll of silver 
foil, and for the same distance on the other side some copper oxide. 
The ends of this tube are plugged with glass wool and the tube 
heated with a fish-tail burner before the aspiration of air is 
started. : 


b. Direct Combustion of the Sample. 


In most cases it is possible to effect the combustion by heating 
the finely divided substance itself in a current of oxygen. In 
fact, according to Blair,* this is true not only of ordinary steels 
and pig iron, but experiments have shown that with chrome- 
tungsten steels the direct method is capable of giving exact 
results, whereas those obtained by dry combustion after solution 
of the iron in potassium-cupric chloride are from 10 to 40 per 
cent. too low. | 

It has been claimed, however, that there is difficulty in burn- 
ing all of the carbon on account of the sample becoming coated 
superficially with oxide, but according to Schneider f this may 
be overcome by mixing the finely divided sample with three parts 
of lead and one of copper. 

The combustion may be carried out in a platinum bbe, in 
one of the special forms of crucible,t in a porcelain tube, or in 
one of fused quartz. When platinum is used it is advisable to 
place the drillings in a little depression of sand, the layer of which 
being not less than 0.25 in. deep. 

According to Campbell and Gott,§ if a combustion boat con- 

taining the borings of sample is placed in a cold, platinum-lined 


iiieimitindits 





* A. A. Blair, The Chemical Analysis of Tro, 7th ed. (1908). 
+ Oesterr. Zeitschr., 1894, No. 21. 
t P. W. Shimer, J. fod Eng. Chem., 1, 738. 
§ J. Ind. Eng. Chem., 1, 739. 


414 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


porcelain tube and then heated at a temperature of about 900°, 
complete combustion will take place without endangering the 
platinum by any spattering of the oxides formed. 

It is very convenient to heat the tube by means of an electric 
furnace.* Such a furnace can be constructed at a cost not 
exceeding $30. 


Determination of Graphite. 


One gram of pig iron is treated with 35 c.c. of nitric acid 
(sp. gr. 1.13) in a 300-c.c. beaker and heated gently until there 
is no further evolution of gas. By this means the carbide carbon 
is dissolved while the graphite is not attacked. The solution is 
filtered through an ignited asbestos filter and the residue washed 
with hot water, then with a hot solution of potassium hydroxide 
(sp. gr. 1.1), followed by hot water, dilute hydrocohloric acid, 
and finally with hot water again until free from chloride. After 
drying at 110°, the asbestos and graphite are transferred to a 
combustion-tube and the carbon burned in a current of pure 
oxygen as described above. From the weight of carbon dioxide 
absorbed, the amount of graphite is calculated. 


Determination of Carbon and Hydrogen in Organic Substances, 
according to Liebig. 


(Elementary Analysis.) 


Principle.—The organic substance is burned in air or in oxygen 
and the products of the combustion are passed over glowing cop- 
per oxide, which oxidizes all of the carbon to carbon dioxide 
and the hydrogen to water. The latter is collected in a weighed 
calcium ‘chloride tube, the former in a weighed vessel which con- 
tains either caustic potash solution or dry soda-lime. 

The combustion is performed. 

(a) In an open tube; 
(b) In a closed tube. 





* J. Ind. Eng. Chem., 1, 741. Campbell and Gott, loc. cit. W.H. Keen, 
J. Ind. Eng. Chem., 1, 741. 


DETERMINATION OF GRAPHITE, ETC. 415 


(a) Combustion in an Open Tube. 


By far the greater majority of all combustions are carried gut 
in this way. , 

Requirements.—1. An open tube made of difficultly-fusible glas 
which is from 12-15 mm. wide. The length of the tube depends 
upon that of the combustion-furnace; it must be 10 cm. longer 
than the furnace. 

2. 350 gms. of coarse and 50 gms. of fine copper oxide. 

3. A drying apparatus (Fig. 69, on the left). 

4. A calcium chloride tube (Fig. 70). 

5. A Geissler potash bulb (Fig. 71) or two soda-lime tubes 
(see p. 381, d and e). 

6. A screw-cock. 

7. Dry rubber tubing. 

8. Two plates of asbestos board to protect the rubber stoppers 
in the two ends of the tube from the heat of the furnace. 








Fig. 69. 


Procedure for the Combustion of Organic Substances Free from 
_ Nitrogen, Halogen, Sulphur, and Metals. 


Preparation and Combustion. 


1. The calcium chloride tube (Fig. 70) is filled from the left 
side as was described on p. 377, closed with a plug of glass-wool 
and the end of the tube fused together, as shown in the figure.* 
It is more practical to use a calcium chloride tube fitted with 
ground-glass stoppers. After filling the tube, its contents are satu- 
rated with carbon dioxide, as described on p. 380, in the foot-note. 

The outside of the tube is rubbed with a piece of chamois skin 
or old linen, and the two ends are stoppered with short pieces 
of rubber tubing each containing a piece of stirring-rod. It is 





* Or the tube is stoppered and an air-tight seal made by covering it 
neatly with sealing-wax, 


416 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


allowed to stand in the balance-case for fifteen minutes and is 
then weighed without the stoppers. 

2. The Geissler bulb (Fig. 71) is filled with caustic potash solu- 
tion (two parts solid KOH in three parts of water) as follows: 
The small soda-lime tube d is replaced by a piece of rubber tubing, 
c is dipped into the solution of caustic potash, and the bulbs are 





Fic. 70. 


end of the tube c is then cleaned by means of a piece of filter- 
paper, the soda-lime tube d is again connected (its right half is 
filled with soda-lime and the outer half with calcium chloride), 
and the two ends are closed with pieces of rubber tubing each 
containing a piece of stirring-rod with rounded ends. The apparatus 
is wiped with wash-leather and weighed without the stoppers, 
after taking the same precautions as with the weighing of the 
large calcium chloride tube. 

3. The drying apparatus (Fig. 69, on the left), which serves to 
_ free the air and oxygen used for the combustion from carbon diox- 
ide and water vapor, consists of a wash-bottle, D, containing con- 
centrated caustic potash solution, the soda-lime tube a, and the 
two calcium chloride tubes b and c. 






a ad S k’ Eg SSS SSS aie 
Fig. 72. 


4. The Combustion-tube (Fig. 72), both ends of which are 
rounded by heating in the blast-lamp; after cooling, the tube is 
washed, dried, and filled as follows: First a short roll, k, of copper 
gauze is introduced into the right-hand end of the tube so that 


. COMBUSTION OF ORGANIC SUBSTANCES. 417 


5-6 en. of the latter are left empty. This roll serves as a plug and 
must, therefore, fit tightly in the tube. A layer of coarse copper 
oxide, K, about 45 em. long, is next added, and after this is placed 
another plug of copper gauze, k’. Finally another roll of copper 
gauze, d, about 10 em. long and large enough to fill the tube loosely, 
is placed so that a space of about 10 em. is left on the right and 
about 5 cm. on the loft. The tube is now placed in a combus- 
tion-furnace, so that about 5 cm. extend beyond the furnace 
at each end, as shown in Fig. 69. The left end of the tube is closed 
with a tightly fitting rubber stopper through which a glass tube 
passes, and is connected with the drying apparatus by means of 
a short piece of rubber tubing. (The tube should be provided 
with a glass stop-cock, as is shown in Fig. 72, a, but which is 
lacking in Fig. 69.) The right end of the tube is left open for the 
time being. 

A slow current of oxygen * is passed through the apparatus 
and the furnace is lighted. At first the flame is turned low and 
the whole tube is heated equally. Gradually the temperature 
is raised, until, with the tiles covering the tube, the copper oxide 
is at a dull-red heat. 

Usually some water condenses in the right-hand end of the 
tube; this is expelled by carefully holding a hot tile under the tube. 
When all the water is removed, and when the presence of oxygen 
can be detected at the right end of the tube (by its igniting a 
glowing splinter), this end of the tube is closed with a rubber 
stopper through which an open calcium chloride tube is placed. 
The burners are now turned down and the oxygen current dis- 
continued. All of the flames are extinguished after some time 
except those under the right half of the tube. 

While the tube is cooling, the calcium chloride tube and the 
potash bulb (or soda-lime tubes) are weighed (the stoppers being 
replaced immediately after the weighing) and from 0.15-0.2 gm. of 
the substance is weighed into a porcelain or platinum boat. 

If the substance is a difficultly-volatile oil it is weighed into a 





* The oxygen must be free from hydrogen. Commercial oxygen often 
contains the latter, in which case it is necessary to pass the gas through a 
“preheating”? furnace before using it. The gas should come from a gas- 
ometer, never from the bomb itself. 


418 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


small glass tube open at one end. If it is readily volatile, a bulb 
is blown into a small capillary tube; this is weighed, the bulb is 
warmed, and the capillary is introduced into the liquid to be ana~ 
lyzed, so that the liquid rises in the bulb as it cools. The bulb is 
then turned so that the capillary lies in a horizontal position, the 
latter is warmed slightly to expela little liquid that adheres to the 
sides of the tube, the end is melted together, and the tube is again 
weighed. Care must be taken that éhere is no liquid in the capil- 
lary. Everything is now ready for the combustion. The stopper 
is removed from the left (and now cold) end of the combustion- 
tube, the long copper roll is removed by means of a piece of wire 
with a hook in the end of it, the boat with the substance in it is 
~ placed in the tube and the copper roll right after it. Connection 
is made with the drying apparatus on the left and with the absorp- 
tion-tubes on the right, as is shown in Fig. 69. In case the sub- 
stance is a liquid, the tube containing it is placed so that its capillary 
is pointed towards the left, and in the case of a volatile liquid the 
end of the capillary is broken off with a file just before introducing 
it into the combustion-tube. The stop-cock between the tube and 
the drying apparatus is closed, the latter is connected with an air- 
gasometer, and the stop-cock in the drying apparatus is wholly 
opened, while that between the drying apparatus and the com- 
bustion-tube is opened just enough to permit two, or at the most 
three, bubbles of gas per second to pass through the apparatus. 
The two outer burners on the left are now lighted and the copper 
oxide spiral d (the copper was changed to the oxide by the ignition in 
oxygen) is slowly heated just to redness. The tube is now gradu- 
ally heated from right to left, taking care that the gas evolution is 
never greater than four bubbles per second through the potash 
bulb; this can be easily regulated by means of the stop-cock or. by 
turning the gas-burners. When the contents of the entire tube 
have been brought to redness, with the tiles in place, and the boat 
is empty, the combustion is usually complete. It is well, however, 
to pass oxygen through the hot tube until the gas can be detected 
at the right-hand end of the combustion train (a glowing splinter 
ignites at n).* The flames are then turned down and a current of | 





* To prevent moisture from getting into this tube from the air, it is well to 
connect it with an unweighed calcium chloride tube. 


COMBUSTION OF ORGANIC SUBSTANCES. 419 


air passed through the apparatus until the oxygen is completely 
expelled. A little water always collects in the front (right) end 
of the tube, and this must be driven over into the calcium chloride 
tube by holding a hot tile under it. The calcium chloride tube 
and the potash bulbs are now removed, wiped off with a piece of 
chamois skin or a clean linen cloth, allowed to stand in the balance- 
room for twenty minutes, after which time they are weighed | 
without the stoppers. ‘The gain in weight represents the amount 
of water and carbon dioxide respectively, and from this the amount 
of hydrogen and carbon can be calculated as follows: 

If a represents the weight of the substance, p that of the water, 
and p’ that of the carbon dioxide, then 


oi saa.’ and © OO. ps2 


18.02 :2.02= p:x A4:12=p':a’ 
e202 ee ae 
~ 18.027 na? Cn Tes 
and in per cent. 
Ok Be 300 p’ | 
O01 ae cent. H i ee cent, C 


Determination of Carbon and Hydrogen in Nitrogenous Organic 
Substances. 


By the combustion of many organic substances containing nitro- 
gen, especially nitroso- and nitro-compounds, oxides of nitrogen are 
formed which are partly absorbed in the calcium chloride tube and 
partly in the potash bulb, so that if such substances were analyzed 
according to the previous process, both the carbon and hydrogen 
results will be too high. If, however, an unreduced copper spiral 
is introduced in the front (right) end of the combustion-tube, 
this serves to reduce the oxides of nitrogen to nitrogen itself, and, 
as the latter is not absorbed, correct results will be obtained. 

The copper spiral is prepared by rolling together a piece of 
copper gauze about 10 cm. wide, making it as large as will con- 
veniently pass into the combustion-tube. The spiral is heated 
till it glows by holding it in a large gas flame, and while still hot 
it is thrown into a test-tube containing one or two cubic centi- 
meters of methyl alcohol. The alcohol quickly boils away, but 


420 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


some of it is oxidized to aldehyde by the hot copper oxide, and 
the latter is reduced completely to bright metallic copper. The 
spiral is taken out with a pair of crucible tongs and dried by 
_ quickly passing it through a flame a few times, and while still 
warm it is introduced into the front end of the combustion-tube, 
which has been previously burned out as described in the pre- 
vious analysis. 

To carry out the combustion, the stop-cock between the com- 
bustion-tube and the drying apparatus (Fig. 69) is closed, the 
substance introduced into the tube, and the copper oxide spiral at 











: Fig. 74. Fig. 75. 


d is first heated and then the reduced spiral at the other end of the 
tube. ‘Then ‘beginning at & (Fig. 72), one burner is lighted after 


‘COMBUSTION OF ORGANIC SUBSTANCES. 421 


another, until finally the entire contents of the tube are heated to 
dull redness and no more bubbles escape through the potash 
bulb. Now for the first time the stop-cock is opened somewhat 
and oxygen is passed through the tube until it can be detected at 
n, by atest with a glowing splinter. The flames are then gradually 
turned down, the oxygen replaced by air, and the analysis com- 
pleted as in the previous case. 

Substances hard to burn are treated somewhat differently. 
First of all the left end of the combustion tube (Fig. 69) is filled 
with the aid of the funnel 7 (Fig. 73), with finely granular, but 
not pulverized, copper oxide, and this is ignited in a stream of 
oxygen. The oxygen is then replaced by air and the tube allowed 
to cool until it can be held in the hand. The finely granular 
copper oxide is next transferred to the small flask K, Fig. 74, and 
the flask closed by inserting a tin-foil covered cork which is fitted 
with a calcium chloride tube. While the copper oxide in the 
flask is becoming perfectly cold, the substance to be analyzed is 
weighed into the glass-stoppered mixing tube M, Fig. 75. From 
one-sixth to one-fifth of the copper oxide in the flask is transferred 
to the mixing tube, which is then stoppered and its contents well 
shaken, whereby the substance becomes intimately mixed with 
the copper oxide to which it adheres. The mixture is then 
transferred to the combustion tube, and the mixing tube is 
shaken repeatedly with small portions of the remaining copper 
oxide in the flask until finally it can be assumed that all of the 
substance has been transferred to the combustion-tube. The 
combustion is then carried out in the usual manner.* 


Combustion of Organic Substances Containing Halogens. 


The analysis is conducted exactly the same as in the case of 
nitrogenous substances, except instead of a reduced copper spiral 
one of silver is used to keep back any halogen set free. The 
silver spiral should not be heated to redness, but only to about 
180-200° C. In case a silver spiral is not at hand, a long copper 
spiral is used, its end reaching outside the furnace. 





* For another method of conducting a combustion in an open tube, consult 
M. Dennstedt, Z. anal. Chem. 40, 611 (1903). 


422 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Combustion of Crganic Substances Containing Sulphur. 


Sulphur compounds cannot be burned in a tube containing 
copper oxide, for the sulphur dioxide escapes and is partly ab- 
sorbed by the water in the calcium chloride tube and partly in 
the potash bulb, so that absolutely worthless results are obtained. 
Instead of the long layer of coppemoxide, one of ignited lead chro- 
mate is used. The latter oxidizes the sulphur dioxide to sulphur 
trioxide, forming difficultly-volatile lead sulphate which remains 
in the tube. When lead chromate is used, the combustion must 
take place at a lower temperature than with copper oxide, for 
the former melts easily, and by adhering to the glass is likely to 
cause the tube to break. 


Combustion of Organic Substances Containing Metals. 


If the substance contains alkalies, alkaline earths, or cadmium, 
a part of the carbon will remain in the tube as carbonate. In 
this case the substance is mixed in the boat with a mixture of ten 
parts of powdered lead chromate and one part of potassium chro- 
mate, and the combustion is conducted as is the case when sul- 
phur is present. 


Dumas’ Method for Determining Nitrogen in Organic 
Substances. 


This determination should really be discussed under Part III, 
but it will be described here on account of its being an analysis 
by combustion. 

Principle.—The substance is burned in a combustion aaa 
free from air, which contains copper oxide and copper spirals 
exactly as in the determination of the hydrogen and carbon in 
substances containing nitrogen, but in this case the nitrogen 
evolved is measured, | | 

_ Procedure.—This determination may be carried out in either 
a closed or open tube. 7 


DUMAS’ METHOD FOR DE TERMINING NITROGEN. 423 


(a) Determination in a Closed Tube. 


The necessary apparatus is shown in Fig. 76. The combus- 
tion-tube is closed at one end and is about 75 cm. long. It 
contains at M a layer of magnesite 15 em. long, in pieces about the 
size of a pea, followed by a loose plug of ignited asbestos and 
then a 10-cm. layer of coarse copper oxide, S. The substance 
is added at a in a boat and mixed with powdered copper oxide 
by means of a spiral wire (cf. p. 421), after which a layer of 
coarse copper oxide * about 40 cm. long is added, and finally 
the reduced copper spiral (prepared as described on p. 419). The 
tube is then placed in a combustion-furnace and connected with 
an azotometer,{ as shown in the figure, which is filled with mer- 
cury to a little above the lower end of 7, and upon this rests a 
liberal amount of caustic potash solution (800 gms. KOH dis- 
solved in a liter of water). 

The experiment is begun (with the levelling-bulb low and the 
stop-cock of the azotometer open) by heating the left half of the 
magnesite layer, whereby the air in the tube is expelled by the 
carbon dioxide and passes through the azotometer. From time 
to time a test is made to see whether the air has all been expelled. 
The levelling-bulb is raised, and the stop-cock closed with the 
azotometer tube completely filled. If all the air has been replaced 
by carbon dioxide gas, the bubbles of gas will all be absorbed by the 
caustic alkali. When this is the case the flame is put out under J. 
The tube is heated first at & and the burners are lighted one after 
another toward the left until about three-quarters of the layer of 
coarse copper oxide is heated to a dull redness. The tube is then 
heated at S and the process is continued as in an ordinary com- 
bustion until the whole tube (with the exception of the part where 
the magnesite is found) is heated to a uniform temperature and 
finally no more nitrogen is evolved. 

The heating must be accomplished so that there will be a slow 
but steady evolution of nitrogen. When the combustion is com- 
plete, the magnesite layer is once more heated and the nitrogen 





* The copper oxide must be previously ignited, as described on p. 417. 
+ H. Schiff, Berichte, XIII, p. 885. 


424 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


remaining in the tube is completly driven over into the azotom- 
eter by the carbonic acid set free. As soon as the volume of 
the gas in the azotometer remains constant, the experiment is 
ended and it remains only to measure the nitrogen. 

For this purpose the azotometer together with the connecting 
piece of rubber tubing is removed from the combustion-tube and 





Fig. 76. 


the tubing closed by means of a pinch-cock. The apparatus is then 
set aside for at least thirty minutes at a place where a uniform 
temperature prevails, after which the gas levelling-tube is raised 
until the solution in it stands at exactly the same height as that 
in the tube, when the volume is read. At the same time the 
barometer and thermometer * readings are noted. 
. The weight of the nitrogen present is computed as follows: 
Let us assume that a gms. of the substance were used for the 





* An accurate thermometer should hang at the side of the azotometer. 


DUMAS’ METHOD FOR DETERMINING NITROGEN. 425 


analysis and V c.c. of nitrogen were obtained at (° C. and B mm. 
barometric pressure. In order to obtain the weight of the nitrogen, 
its volume must be first reduced to 0° C. and 760 mm. pressure. 
If the gas had been measured over pure water the formula _ 


_ V(Bo—w) +273 
~ 760 (273 +1) 





Vo 


would hold in which Bo represents the observed barometer 
reading reduced to a temperature of 0° and w is the tension of water 
vapor measured in millimeters of mercury. The nitrogen, how- 
ever, was not measured over pure water but over a solution of 
potassium hydroxide, and the vapor tension of this solution is 
less than that of pure water. In fact, with potassium hydroxide 
of the concentration used, the diminution of the vapor tension as 
compared with pure water almost exactly compensates the 
correction which would be applied in reducing the barometer 
reading to 0°. Consequently the following formula holds with 


sufficient accuracy: 


V (B—w) - 273 


Yo= 760 (273 +1) ° 





As 1 c.c. of nitrogen at 0° and 760 mm. has been found to weigh 
0.0012506 gm.,* then Vo c.c. of nitrogen will weigh 


0.0012506 X Vo gms. 


and in per cent., 
a=0.0012506- Vo = 100: x 


pw 012506: Vo 
aa 








* Cf. Nitrogen under Gas Analysis. 


420 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


If the value for Vo is inserted in this last equation, and the 
constant values are united, it becomes 


V(B-—w) 


x= 0.04493 Q73 40 -a 





=per cent. N. 


(b) Determination of Nitwogen in an Open Tube. 


The determination is carried out in practically the same way 
as before, except that in this case the carbon dioxide is generated 
outside of the tube. If the combustion-tube of Fig. 76 is imag- 
ined to be cut off at M and connected by means of the two-bulbed 
tube with a long test-tube, as shown in the upper part of the fig- 
ure, the apparatus necessary for this determination will be seen. 

The long test-tibe contains sodium bicarbonate, and it is cov- 
ered with a piece of copper gauze in order that it may be heated 
more uniformly. 

At S is a long copper oxide spiral, this is followed by a copper 
boat containing the substance mixed with powdered copper oxide, 
then the long layer of coarse copper oxide, and finally the reduced 
copper spiral. After the connection with the azotometer hes been 
made, the tube containing the sodium bicarbonate is heated and 
the air removed from the combustion-tube by means of the carbon 
dioxide evolved. The greater part of the water that is simul- 
taneously set free collects in the two-bulbed tube. Otherwise 
the procedure is exactly the same as in the former case. 

Remark.—The advantage of this method over the former lies 
in the fact that the combustion-tube can be used for a large num- 
ber of nitrogen determinations without refilling it each time. 

With difficultly-combustible substances the author prefers to 
work with the closed tube, for in this way it is possible to get 
a very intimate mixture of the substance with the powdered 
copper oxide. 


OXALIC ACID. ’ 427 


OXALIC ACID, H,C,O,. Mol. Wt. 90.02. 


Forms: Calcium Oxide, CaO, and Carbon Dioxide, 
CO,. 


Determination as Calcium Oxide. 


The neutral solution of an alkali oxalate is treated with a few 
drops of acetic acid, heated to boiling, and precipitated with boil- 
ing calcium chloride solution. After standing twelve hours the 
precipitate is filtered off, washed with hot water, ignited wet in 
a platinum crucible, and from the weight of the calcium oxide the 
amount of oxalic acid is calculated as follows: 


CaO: H,C,0,=p:2 


H,C,0, 
Cao 


Determination as Carbon Dioxide. 


Principle.—The method is based upon the fact that oxalic acid 
on being heated with manganese dioxide and dilute sulphuric 
acid is quantitatively oxidized to carbon dioxide: 


H,C,0,-+ MnO,+ H,SO,=MnSO,+ 2H,0-+ 2C0,,. 


Procedure.—A weighed amount of the oxalate is treated with 
one and a half times as much manganese dioxide (free from car- 
bonate) either in the apparatus shown on page 376 (Fig. 58), or 
in that of Fresenius-Classen (Fig. 59, p. 381). The procedure is 
exactly the same as was described for the determination of carbon 
dioxide. If pgm. of carbon dioxide were found, this corresponds to 


p:1.023 gm. = Oxalic Acid, H,C,O,. 


Remark.—Both methods give good results, but oxalic acid 
can be much more conveniently determined by a volumetric 
process (see Part II, Volumetric Analysis). 

The free acid crystallizes with two molecules of water and its 
molecular weight is then 126.05. This should be borne in mind in 
determining the purity of a commercial sample. 


428 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


BoRIC ACID, HBO,,* Mol. Wt. 44.01. 


Determination as Boron Trioxide, B,O,, by the Method of 
Rosenbladt-Gooch.} 


Principle-—Alkali and alkaline-earth borates, on being dis- 
tilled with absolute methyl alcohol (free from acetone) and acetic 
acid, give up all their boron in thesgform of methyl borate, a liquid 
which boils at 65° C. If the methyl borate is passed over a 
weighed amount of lime in the presence of water, it is completely 
saponified : Sate 

B(OCH,),+ 3H,0 =3CH,OH+ B(OH),. 


The boric acid set free combines with the lime to form calcium 
borate. if the paste of water and lime is evaporated to dryness, 
the gain in weight, therefore, represents the amount of B,O,. 

Procedure.—About 1 gm. of the purest lime tf obtainable is ig- 
nited to a constant weight over the blast-lamp, and as much of it as 
possible is transferred to the dry Erlenmeyer flask (Fig. 77) which 
serves as a receiver. The crucible, with some of the lime adhering 
to it, is placed in a desiccator and set aside for the present. 

The lime in the flask is slaked by the careful addition of about 
10 c.c. of water, and the flask is connected with the distillation- 
flask as shown in the figure. § | 

The aqueous solution of the alkali borate (containing not more 
than 0.2 gm. B,O,) is treated with a few drops of either litmus or 
lakmoid solution, and hydrochloric acid is added drop by drop 
until the solution turns red. A drop of dilute sodium hydroxide 
is added and then a few drops of acetic acid.|| The slightly acid 





* This is the formula of meta-boric acid. 

+ Zeit. f. anal. Chem., 27 (1887, pp. 18, 364). 

t Instead of lime, oooh and Tones use 4 to 7 gms. of sodium tungstate 
which is fused with about 0.5 gm. WO, in a platinum crucible to expel any 
carbonic acid. The fused mass is cagles and weighed. 

§ To permit the escape of air from the flask, a cut is made in the ‘aide of the 
cork stopper, at s. 

|| It is absolutely necessary to neutralize the greater part of the alkali 
with hydrochloric acid and then the last of it with acetic acid. If the alkali 
were all neutralized with acetic acid, little or none of the boric acid would 
pass over into the receiver during the subsequent distillation with alcohol. 


DETERMINATION OF BORIC ACID. 429 . 


solution, prepared in this way, is added by means of the funnel 7 to 
the pipette-shaped retort, R, of about 200 c.c. capacity. The funnel 
is washed three times by the addition of 2 or 3 cubic centimeters . 
of water and the stop-cock is closed: The liquid is distilled 
by placing R in a paraffin bath at not over 140° C., and the 
distillate collected in the Erlenmeyer flask containing the lime 
When all of the liquid has distilled over, the paraffin bath is lowered, 
and after R has cooled somewhat, 10 ¢.c. of methyl alcohol (free 
from acetone) are added through the funnel and the contents 
of FR are again distilled off. This process is repeated three times. 














Fia. 77. 


Then 2-3 c.c. of water are added to the retort, also a few drops 
of acetic acid until the liquid becomes distinctly red again,* 
and the distillation with 10 c.c. of methyl alcohol is repeated 


* By the repeated distillation, the contents of the retort become alkaline, 
as shown by the blue color of the solution. 





~ 430 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


three times more. At the end of this time all of the boric acid 
will be found in the receiver.* The stoppered flask is thoroughly - 
shaken and allowed to stand for an hour or two in order to make 
sure that all of the methyl borate is saponified. The contents 
of the receiver are then poured into a platinum dish of about 
200 c.c. capacity and evaporated on the water-bath to dryness at 
as low a temperature as possible. During this process the alcohol 
must not be allowed to boil undtr any circumstances. Then, 
in order to remove the small amount of lime that remained adher- 
ing to the sides of the flask, a few drops of dilute nitric acid are 
added to the receiver, and, by carefully inclining the flask, its entire 
inner surface is wet by the acid, after which the contents are washed 
into the platinum dish and evaporated to dryness again. This 
time the water in the bath may boil, as there is now no danger of 
losing the boric acid, the alcohol being all removed by the first 
evaporation. The residue in the dish is then gently ignited over 
a small flame in order to destroy the calcium acetate t present; 
it is allowed to cool and is transferred by means of a little water 
to the crucible in which it was originally weighed. The dark-colored 
lime and carbon remaining on the sides of the dish are dissolved 
in a little nitric or acetic acid and washed into the crucible. 
The contents of the latter are evaporated to dryness on the water- 
bath, and, with the cover upon it, the crucible is ignited at first 
gently and finally more strongly until a constant weight is obtained. 
The increase in weight represents the amount of B,O,. 
Remark.—This method affords faultless results, even in the 
presence of considerable amounts of other salts. Free halogen 
hydride or sulphuric acid must not be present, for these acids 
form compound ethers with the methyl alcohol and distil over 
with the boric acid, with which they would be weighed. Instead 
of using lime in the receiver, the methyl borate can be distilled 
into a dilute solution of ammonium carbonate, and the latter 
evaporated with slaked lime in a platinum dish immediately after 
the distillation. The author, however, prefers the above method. 





* When the distillation is over, the retort should be removed from the. 
paraffin bath, by lowering the latter. If this is not done, the retort is likely 
to break when the paraffin solidifies. 

+ Due to the excess of the acetic acid added. 


DETERMINATION OF BORIC ACID IN SILICATES, ETC. 431 


If one possesses a large platinum crucible (with a capacity of 
from 80 to 100 c.c.), the first evaporation can take place in this 
and it is then advisable to place the crucible within a ring-shaped 
copper or tin tube through which steam passes (Fig. 17, page 32). 
In this way the calcium acetate does not creep up over the sides 
of the dish, and there is no danger of any bumping. 


Determination of Boric Acid in Silicates, Enamel, etc. 


The finely-powdered substance is fused with four times as 
much sodium carbonate, the melt is extracted with water, and 
the aqueous solution containing the boric acid* is evaporated 
to a small volume, acidified with acetic acid, and, without regard 
to any separation of silica, the solution is transferred to the Gooch 
retort and analyzed as above directed. 

Remark.—This determination can be performed in the presence 
of fluorine provided acetic and not nitric acid is used to set free 
the boric acid; but, for that matter, it is in no case advisable 
to use nitric acid and it is not permissible when chlorides are 
present. 


Determination of Boric Acid in Mineral Waters. 


If the water contains considerable boric acid (0.1 gm. or more 
of B,O, in a liter), a weighed amount (from 200 to 300 ¢.c.) is evap- 
orated to a small volume,f the precipitated calcium and magnesium 
carbonates are filtered off, the filtrate concentrated, slightly acidi- 
fied with acetic acid, and analyzed as described on page 428. 

If the water contains only a little boric acid, as is true in the 
great majority of cases, a large amount must be taken for the 
determination. From 10 to 15 liters are evaporated in a large 
porcelain dish to about 1 liter,* the deposited salts are filtered 
off (these never contain any borate), washed thoroughly with 
hot water, and the filtrate and washings are evaporated on the 
water-bath until a moist residue is obtained. If this residue does 








* Sometimes the insoluble residue contains appreciable amounts of borie 
acid, In the method given under Volumetric Analysis, ‘this fact will be 
- taken into consideration. 
} If the water reacts alkaline, it is at once evaporated; otherwise enough 
sodium carbonate solution is added to make it so, 


432 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


not amount to more than 5 or 6 gms. it is redissolved, acidified 
with acetic acid, transferred to the Gooch retort, and distilled as . 
described on page 428. Usually a larger residue is obtained, such 
that it cannot be conveniently analyzed directly, in which case the 
boric acid is extracted from it. For this purpose the residue is 
acidified with a little hydrochloric acid, thoroughly stirred with 
abso!ute alcohol, and by means of more of the latter it is trans- 
ferred to a flask, corked up, and allowed to stand twelve hours with 
frequent shaking. The boric acid will then be found in the alco- 
holic solution. The residue is filtered off, washed with 96 per cent. 
alechol, diluted largely with water, 1 gm. of sodium hydroxide is 
added, the alcohol distilled off (see Remark), and the liquid evap- 
orated until a moist residue is obtained. This is again acidified 
with hydrochloric acid and the above extraction with alcohol, and 
subsequent distillation of the alcohol, after the addition of water 
and 1 em. of sodium hydroxide, is repeated. If the residue now ob- 
tained is not too large, it is gently ignited in order to destroy the or- 
ganic matter; after extracting with water, the carbonaceous residue 
filtered off, and the filtrate is acidified with nydrochloric acid. It is 
then made slightly alkaline with sodium hydroxide, after which just 
enough acetic acid is added to make the solution react acid again. 
The solution thus prepared is analyzed as described on page 428. 

_ Remark.—Unless a large amount of water and the sodium 
hydroxide are added, some of the boric acid will be volatilized 
with the alcohol. It is always best to test the alcoholic distillate 
for boric acid as follows: A few pieces of turmeric root are extracted. 
with alcohol, 2-3 drops of the yellow solution are placed in a porce- 
lain dish, the alcoholic solution to be tested for boric acid and a few 
drops of acetic acid are added, after which the solution is diluted 
with water and evaporated to dryness on the water-bath. Accord- 
ing to F. Henz, if as much as ;54;7 mgm. of boric acid is present, 
a faint but distinct coloration will be evident, while the presence 
of ;2; mgm. will cause a strong reddish-brown coloration, which 
cn being treated with sodium hydroxide is turned to the charac- 
teristic blue-black color. 

If boric acid is found in the alcoholic distillate, it must be 
again treated with water and sodium hydroxide, and the alcohol 
once more distilled off. 


MOLYBDIC ACID, TARTARIC ACID, IODIC ACID, ETC. 433 


Motyspic AciID, H,MoO,4. Mol. Wt. 162.02. 


The determination of molybdic acid has already been con- 
sidered on page 284. 


TARTARIC ACID, H,C,H,O,. Mol. Wt. 150.05. 


The composition of free tartaric acid as well as that of the tar. 
trates is determined by an elementary analysis, see page 414 et seq. 


META- AND PYROPHOSPHORIC ACIDS. 


These acids are changed to phosphoric acid and determined 
as described on page 434. : 


Iopic AcID, HIO,. Mol. Wt. 175.93. 
Form: Silver Iodide, AgI. 


For the determination of iodic acid as silver iodide, the solu- 
tion of the alkali iodate is acidified with sulphuric acid, and sul- 
phurous acid is added until the solution, which at first becomes 
yellow on account of the separation of iodine, is again colorless. 
After this an excess of silver nitrate and a considerable amount of 
nitric acid are added. ‘The solution is heated to boiling and the 
precipitated silver iodide determined as described on page 330. 

It is not permissible to change the iodate to iodide by ignition, 
for the decomposition takes place at a temperature above that 
at which the iodide itself begins to volatilize. The transforma- 
tion is therefore not quantitative. This is especially true of 
sodium iodate, which is only changed to iodide upon heating to a. 
white heat. Potassium and silver iodates are much more readily 
decomposed, but even then some iodide is lost. Both iodic and 
periodic acids may be more accurately determined by a volumetric 
process (see Part II, Iodimetry). 

For the determination of the metal present in an iodate it is 
first changed to the chloride by repeated evaporation with con- 
centrated hydrochloric acid: 


KIO,+ 6HCl=KCl+ 3H,0+ 2Cl,+ICl. 


434. = GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


GROUP IV. 


PHOSPHORIC, ARSENIC, ARSENIOUS, THIOSULPHURIC, 
CHROMIC, VANADIC, AND PERIODIC ACIDS. 


PHOSPHORIC ACID, H,PO, Mol. Wt. 98.06. 


Forms: Magnesium Pyrophosphate, Mg,P,0,; Ammonium 
Phosphomolybdate, (NH,),PO,-12M00,; Phosphomolybdic 
Anhydride, P,O,-24Mo00.. 


1. Determinat‘on as Magnesium Pyrophosphate, according to 
B. Schmitz. 


Until recently, it was the usual practice to precipitate phos- 
phoric acid in the cold with “ magnesia mixture ’’ and ammonia, 
but according to the experiments of Neubauer * and of Gooch + 
it is evident that it is very difficult to obtain a pure precipitate of 
magnesium ammonium phosphate in this way; sometimes it is con- 
taminated with Mg3(PO4)2 and sometimes with Mg(NH4)4(PO4)o. 
If, however, the precipitation takes place in a hot solution, as 
recommended by Schmitz,t Jarvinen,§ and Jérgensen,|| a very 
pure, coarsely crystalline precipitate of Mg(NH4)PO4-6H20 is 
obtained. | 

Procedure.—The solution of alkali phosphate is treated with 
a little hydrochloric acid, a considerable excess of ‘‘ magnesia 
mixture,” {| and 10-20 ¢.c. of a saturated solution of ammonium 
chloride. After heating the mixture to boiling, some 2.5 per cent. 
ammonia is added very slowly, while constantly stirring, until a 
precipitate begins to form, and then the addition of the ammonia 
is regulated so that about four drops are added in a minute. If 
a milky turbidity appears, it must be redissolved in hydrochloric 





* H. Neubauer, Z. angew. Chem., 1896, 439. 

{ F. A. Gooch, Z. anorg. Chem., 20, 135. 

{ B. Schmitz, Z. anal. Chem.; 45, 512 (1906). 

§ K. K. Jarvinen, Z. anal: Chem., 48, 279 (1904), 44, 333 (1905). 

|| G. Jorgensen, Z. anal. Chem. 45, 278 (1906). 

] The “magnesia mixture” is prepared, according to Schmitz, by dissolv- 
ing 55 gms. of crystallized magnesium chloride and 105 gms. of ammonium 
chloride in water adding a little hydrochloric acid and diluting to a volume 
of one liter. 


PHOSPHORIC ACID. 435 


acid. It is important that the precipitate which first forms 
shall be crystalline. As the precipitate increases in amount, the 
addition of the ammonia may be quickened, until finally the 
liquid smells of ammonia, after blowing away the vapors on top 
of the liquid. The solution is then allowed to cool, one-fifth of 
its volume of concentrated ammonia is added, and at the end of 
ten minutes more it is ready to filter. The precipitate is washed 
three times by decantation with 2.5 per cent. ammonia, then 
transferred to a filter and washed free from chloride. It is dried, 
_ ignited and weighed as described on p. 67-8. It is best to use a 
Munroe crucible and an electric oven. 
If the weight of the precipitate is p gms., then the amount of 
PO,, s, can be computed according to the proportion. 
Mg2P207 ‘ 2PO0,4 =p:s 
_ 2PO0,4 
~ MgeP207 io 


Solution and Reprecipitation of the Ignited Magnesium 
Pyrophosphate. 


If it is desired to dissolve the ignited precipitate and to repre- 
cipitate the phosphoric acid, the crucible together with its cover, 
is placed in a beaker, enough water is added to cover the crucible, 
and then an excess of concentrated hydrochloric acid. The beaker 
is covered with a watch-glass and its contents are heated on the 
water-bath, the liquid in the beaker being occasionally rotated. 
When the precipitate has dissolved, the heating is continued for 
three or four hours longer in order to make sure that the pyro- 
phosphoric acid is completely changed to orthophosphoric acid. 
_ This change is always complete at the end of this time if the weight 
of the magnesium pyrophosphate was not over 0.2 gm. The time 
necessary to effect this transformation is proportional to the 
amount of nitric acid used. | 

After the liquid has been sufficiently heated, the crucible and 
its cover are removed, washed off, from 2 to 5 c.c. of magnesia 
mixture are added, and the solution is treated, as described above, 
with 24 per cent. ammonia, etc. 

The method deser.sced on page 434 for the precipitation of 
pho sphoric ac.d 13 not app icable when the substance contains 
alkaline eartas or heav, met.ls. In such cases the phosphoric 


436 GRAVIMETRIC DETERMINATION OF THE METALLOIDS., 


acid should be precipitated first as ammonium phosphomolybdate 
and the phosphoric acid in this Paes determined by one of 
the following methods. 


1. Determination of Phosphoric Acid as Magnesium Pyro- 
phosphate after Previous Precipitation as Ammonium 
Phosphomolybdate. 


This method, first proposed by*Sonnenschein, has experienced, 
in the course of time, a great many modifications, and of these, that 
of Woy * will be described, for it is one of the quickest and most 
accurate. It may be mentioned that the molybdate method is 
always applicable when the phosphoric acid is present as ortho- 
phosphate, irrespective of what metals are in solution. 

Principle.—If a solution containing phosphoric acid, in the pres- 
ence of ammonium nitrate and sufficient nitric acid, is treated with 
a slight excess of ammonium molybdate and heated just to the 
boiling-point, all of the phosphoric acid is immediately precipitated 
as yellow ammonium phosphomolybdate. According to Hunde- 
shagen, the precipitate possesses the following composition: 


(NH,),PO,-12Mo0,-2HNO,-H,0, 


and always contains, when sufficient molybdic acid is present 24 
mols. of MoO, to 1 mol. P,O,;. It never contains more molybdie acid 
than corresponds to the above formula, but-is always some what 
contaminated with small amounts of the bases in solution, even 
when only alkalies are present. If, however, after decanting off 
the supernatant liquid, the precipitate is dissolved in ammonia, a 
little more ammonium molybdate added, and the boiling solution re- 
precipitated by the addition of nitric acid, it is then obtained pure. 

It must also be noted that the solution may contain neither 
silicic acid nor organic substances} and only a small amount of 





* Chem. Zeit., 21, p. 442. -— 

+ According to Hundeshagen, (Zeit. f. anal. Chem., 28, p. 164) and Eggertz 
(Jour. f. prak. Chem., 79, p. 496) the presence of tartaric and oxalic acids hin- 
ders the formation of the yellow precipitate, and in some cases prevents 
it entirely. According to Hans v. Jiiptner (Oesterr. Zeit. fur Berg- u 
Hiittenw., 1894, p. 471) this is not the case; he even recommends that 
tartaric acid be added for the determination of phosphorus in iron, on the 
ground that it prevents the precipitate being contaminated with molybdic 


acid and ferric oxide. - 


PHOSPHORIC ACID. 437 


chloride (best none at all), but there must be considerable free 
nitric acid present; 1 gm. of P,O,; requires 11.6 gms. of HNO,, 
but as much as 35.5 gm. of the latter acid does no harm.* The 
precipitate will dissolve somewhat if more nitric acid than the 
above quantity is used, but the addition of ammonium molyb- 
date decreases the solubility of the precipitate in nitric acid; 1 gm. 
of ammonium molybdate makes 55.7 gms. of nitric acid inactive. 
The presence of ammonium nitrate not only facilitates the forma- 
tion of the precipitate, but its presence is absolutely necessary, 
although about 5 per cent. is sufficient. 


Solutions Required. 

1. A 3 per cent. solution of ammonium molybdate obtained 
by the solution of 120 gms. commercial ammonium molybdate, 
(NH,),Mo,0,,+4H,O, in 4 liters of water (1 ¢.c. of this solution 
will precipitate 0.001 gm. P,O,). 

2. A solution of ammonium nitrate, obtained by dissolving 
340 gms. of ammonium nitrate in 1 liter of water. 

3. Nitric acid, sp. gr. 1.153 (containing 25 per cent. HNO,). 

4. As wash liquid, 200 gms. ammonium nitrate and 160 c.c. 
of nitric acid dissolved in 4 liters of water. 


Woy’s Method of Precipitation. 


In all cases 50 c.c. of the solution are taken, containing at 
the most 0.1 gm. P205. If the solution contains more than this 
amount of phosphoric acid, an aliquot part is used for the 
analysis. 

This amount of the neutral or slightly acid (HNO3) solution 
is placed in a 400-c.c. beaker and to precipitate 0.1 gm. of P2Os 
30 c.c. of ammonium nitrate solution and 10-20 c.c. of nitric acid 
are added and the solution is heated until bubbles begin to rise. 
At the same time the required amount of ammonium molybdate 





* These figures are taken from experimental data furnished by Hun- 
deshagen. They do not refer to the formula on p. 436 given the yellow 
precipitate.—[Translator. | 


438 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


solution (in this case 120 c.c.*) is likewise heated until it begins 
to boil, and then transferred to a separatory funnel and allowed 
to run in a thin stream into the middle of the phosphate 
solution, which is rotated while the molybdate solution is being 
added. The yellow ammonium phosphomolybdate is at once 
thrown down and the separation is quantitative. The contents 
of the beaker are kept in motion for about one minute more and 
then allowed to stand for fifteen*minutes, when the clear liquid 
is poured through a filter, the precipitate is washed once by decanta- 
tion with 50 c.c. of the wash liquid and then dissolved in 10 e.c. 
_ of 8 per cent.ammonia. To this solution 20 c.c. of the ammonium 
nitrate solution, 30 c.c. of weter, and 1 c.c. of ammonium molybdate 
are added. It is heated, as before, until bubbles begin to rise, 
when the phosphoric acid is reprecipitated by the addition of 20 c.c. 
of hot nitric acid, added drop by drop through the same funnel 
that was used for the molybdate solution, the solution being rotated 
as before. The precipitate is immediately formed and is now 
pure. After standing ten minutes it is filtered off and dissolved 
in warm 24 per cent. ammonia, after which hydrochloric acid is 
added until the yellow precipitate produced dissolves only slowly 
on being mixed with the solution. Now, according to Schmitz,f 
an excess of an acid solution of ‘‘ magnesia mixture ”’ is added, 
and the solution heated to boiling. A few drops of phenol- 
phthalein are added, and an approximately 2.5 per cent. ammonia 
solution introduced as quickly as possible from a burette, while 
stirring the solution, until the liquid becomes slightly red in color. 
It is allowed to cool and then one-fifth of its volume of concen- 
trated ammonia is added. After ten minutes, the precipitate 
of magnesium ammonium phosphate is ready to filter. | 





*AMOUNTS OF REAGENTS REQUIRED. 


Amount of P20; Ammonium Ammonium Nitric 

Present in Grams. Molybdate. Nitrate. Acid. 
0.1 120 c.¢ 30 c.¢ 19 c.c 
0.01 15% my 70> 
0 005 15:2 20°** 10:4 
0 902 1.0 i Fi 
0.001 8 a tae see 5% 


t Loc. cit. 


PHOSPHORIC ACID. 439 


2. Direct Determination of Phosphoric Acid as Ammonium 
Phosphomolybdate (Finkener).* 


The precipitate produced as described under 1, having the 
following composition, 


(NH,),PO,-12Mo0,-2HNO,-H,0, 


is transformed by heating for a long time at 160-180° C. into pure 
ammonium phosphomolybdate of the composition 


(NH,);PO,-12Mo00,. 


Theoretically this substance contains 3.782 per cent. of P,O,. 

If, therefore, the amount of yellow precipitate (dried until its 
weight is constant) is multiplied by 0.0378, the actual amount of 
P.O, present should be obtained. The results obtained by Finkener, 
however, were accurate only when the factor 0.03794} was used. 
Hundeshagen,t on the other hand, found that the factor 0.03753 
should be used, and this has been confirmed by experiments per- 
formed in the author’s laboratory.§ _ 

Procedure.—The phosphoric acid is precipitated twice, accord- 
ing to the directions of Woy (p. 437), with ammonium molyb- 
date; the precipitate is filtered through a Gooch crucible, washed 
with the prescribed mixture until no further brown coloration 
is produced by K,Fe(CN),, and dried in a current of air at 160° 
C. in a Paul’s drying oven, until a constant weight is obtained. 
If the precipitate should become slightly greenish, a small crystal 
of ammonium nitrate and one of ammonium carbonate are added 
and the contents of the crucible again heated, whereby the pre- 
cipitate will at once assume a homogeneous yellow color. 

Remark.—The results of Hundeshagen and Steffan show that 
this method gives very exact results. Steffan worked precisely 
according to the directions of Finkener, precipitating the phos- 
phoric acid in the cold with a 334 per cent. solution of ammonium 





* Berichte, 11 (1878), p. 1640. 
f Loc. cit. 
t Zeit. f. anal. Chem., XXXII (1893), p. 144. 
- § A. Steffan, using 50 ¢.c. of a potassium phosphate solution containing 
0.0989 gm. P,O;, in four experiments found 0.0994, 0.0994, 0.0995, 0.0992 


gm. P,O,. 


440 GRAVIMETRIC DETERMINATION OF THE ME TALLOIDS. 


molybdate and filtering after standing twenty-four hours. It is, 
however, not necessary, as Hundeshagen has shown, to work with 
such a concentrated solution of ammonium molybdate; the pre- 
cipitation from a hot solution with a 3 per cent. molybdate solu- 
tion yields just as accuraté results and the solution does not have 
to stand so long before filtering. Even when iron is present this 
method gives good results, so that it is to be recommended for the 
determination of phosphorus in iron and steel. 


3. Determination of Phosphoric Acid as Phosphomolybdic 
Anhydride (Woy). 

The precipitate, produced in the same way as before, is gently 
ignited, whereby a greenish-black residue remains of the com- 
position 24Mo0,-P,0,;, with 3.946 per cent. of P,O;. The pre- 
cipitate is ignited as follows: Upon the bottom of a nickel cru- 
cible is placed a disk of ignited asbestos paper about 2 mm. thick, 
or the porcelain plate of a Gooch crucible may be used. Upon 
this is placed the Gooch crucible containing the precipitate, which 
is covered with a watch-glass and heated at first gently and finally 
until the bottom of the nickel crucible is at a dull-red heat. When 
the precipitate has become of a homogeneous, bluish-black color, — 
it is allowed to cool in a desiccator, after which the covered cru- 
cible is weighed. 

This method is rapid and gives good results in the presence 
of iron and aluminium.* 


Determination of Phosphorus and Silicon in Iron and Steel. 


The determination of these two elements is often effected in. 
the same sample, and in all cases it is best to remove the silicic — 
acid before precipitating the phosphoric acid. 

Since phosphorus: and silicon are present in the iron as phos- 
phide and silicide, a too dilute nitric acid must not be used for dis- 
solving the sample or there will be a loss of volatile phosphides 
and silicides. 





* Steffan found, in the analysis of 50c.c. of a potassium phosphate solu- 
tion containing 0.0989 gm. P20;, 0.0988, 0.0992, 0.0986 gm. P,O,; and in 
a solution of 5 gms. of iron in the form of its nitrate, 0.0099 gm. P,O,, this 
method gave 0.0099 gm. and the same result was obtained by the method of 
Finkener. 


PHOSPHORUS AND SILICON IN IRON AND STEEL. 44. 


Determination of the Silicon. 


About 5 gms. of the iron borings, after having been washed with 
ether (cf. p. 236, foot-note), are placed in a 500-c.c. beaker under a 
good hood, covered with 60 c.c. of nitric acid (1 vol. concentrated 
acid, sp. gr. 1.4, and 1 vol. of water) and a watch-glass placed 
upon the beaker. A violent reaction at once takes place and 
brown vapors are evolved. As soon as the: action slackens, 
the beaker is placed upon wire gauze and its contents 
boiled gently until all the iron is dissolved and no more brown 
vapors are evolved. The contents of the beaker are then washed 
into a 250-c.c. porcelain casserole, evaporated on the water- 
bath to a syrupy consistency, and then heated over a free flame 
to dryness, constantly stirring with a glass rod. Care is taken 
during this operation that a cake of basic ferric nitrate does not 
adhere to the bottom of the dish, as in this case the latter will 
surely break during the subsequent ignition. The dry mass 
should at the end be reduced to a loose powder. When this point 
is reached, the contents of the dish are ignited until all of the ferric 
nitrate is changed to oxide, which is accomplished when no more 
brown fumes are expelled. By this procedure all organic matter 
formed by the oxidation of hydrocarbons is destroyed and the 
silicic acid is dehydrated. After cooling, the residue is covered 
with 50 c.c. of concentrated hydrochloric acid and heated with 
constant stirring almost to the boiling-point. This dissolves the | 
ferric oxide and phosphate, while the silicic acid remains behind.* 

When all of the iron oxide has dissolved, the solution is evap- 
orated to dryness, moistened with 2-3 c.c. of hydrochloric acid, 
allowed to stand for twenty minutes, after which water is added. 
After heating the liquid to boiling the silicic acid is filtered off 
through a small filter, washed with water containing hydrochloric 
acid and finally with pure hot water. The silica is ignited wet in a 
platinum crucible and weighed. The silica thus obtained usually 
contains ferric oxide, so that its purity must be tested in all cases. 
For this purpose it is covered with 1 c.c. of water, a drop of dilute 
sulphuric acid and 2 c.c. of pure hydrofluoric acid are added, and 
after evaporating on the water-bath as far as possible, the excess of 





* If graphite were originally present it remains with the silica. 


442 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


sulphuric acid is removed by placing the crucible on a triangle in 
an inclined position and carefully heating by means of a moving 
flame. As soon as no more vapors of sulphuric acid are given off, 
the contents of the crucible are more strongly ignited and the 
residue of ferric oxide is weighed. This amount deducted from 
the weight of impure silica gives the amount of pure silica, p, 
from which the amount of silicon, x, can be calculated as follows: 


> 
Si0,:Si=p:z 
pp 
BS ORS 


and in per cent., where a is the amount of iron taken for the analysis, 


x’ =S0, @ Per cent. Si. 
2 


Remark.—lf the impure silica was grayish colored (as is always 
the case when graphite is present) it is not weighed, but a little 
pure sodium carbonate and potassium nitrate are added to the 
contents of the crucible and, by fusing, the graphite is completely 
oxidized. The melt is placed in a small porcelain dish and dissolved 
in water. The solution is acidified with hydrochloric acid, evap- 
orated to dryness on the water-bath, moistened with a little con- 
centrated hydrochloric acid, diluted with water and filtered. The 
residual silica is ignited wet; a further purification of the silica 
_ Is unnecessary. : 


Drown Method for Determining Silicon in Iron and Steel. 


This method has come into very general use, and is much more 
rapid than the above method, though quite as exact. It is 
recommended by the American Foundrymen’s Association for thé 
analysis of cast iron and has been used by the Bureau of Standards 
at Washington, D. C., for analyzing samples of steel. 

One gram of borings is treated in a platinum or porcelain dish 
with 20 c.c. of nitric acid, sp. gr. 1.2. When all action has ceased 


PHOSPHORUS AND SILICON IN IRON AND STEEL. 443 


20 c.c. of 50 per cent. sulphuric acid are added and the solution 
evaporated until copious fumes are evolved. The liquid is then 
cooled, diluted with 150 c.c. of water, and heated until all the 
ferric sulphate has dissolved. The hot solution is at once filtered, 
washed with dilute hydrochloric acid, sp. gr. 1.1, and then with 
hot water. The residue is placed in a platinum crucible without 
drying, ignited and weighed. The contents of the crucible are 
then treated with 4 or 5 c.c. of hydrofluoric acid and a few drops of 
sulphuric aicd, evaporated to dryness, and the crucible again 
ignited and weighed. The difference in the two weights is the 
silica. : 
Determination of Phosphorus. 

In the hydrochloric acid filtrate from the silicon determina- 
tion (p. 441) all the phosphorus is present in the form of phosphoric 
acid. The latter is determined according to 

(a) The Acetate Method or 
_ (b) The Molybdate Method. 

Both methods give equally good results, judging from experiments 
performed in the author’s laboratory. 


(a) The Acetate Method. 


The filtrate from the silicic acid is diluted in a beaker to a volume 
of about 400 c.c. and ammonia is added until a permanent precipi- 
tate of ferric hydroxide is produced. The liquid is then treated 
with 200 c.c. of a saturated, aqueous solution of sulphurous acid 
and slowly heated to boiling. The precipitate of ferric hydrox- 
ide soon dissolves and the liquid assumes a dark reddish-brown 
color, which on further heating becomes a light green, or almost 
colorless. As soon as this point is reached, 10-20-c.c. of concen- 
trated hydrochloric acid are added and a current of carbon dioxide 
is conducted into the colorless solution until the excess of sul- 
phurous acidis removed. The solution is now cooled by placing the 
beaker in cold water, after which 1 or 2 c.c. of chlorine or bromine 
water is added to oxidize a part of the iron. To this solution 
ammonia is added very slowly with constant stirring until the 
greenish precipitate of ferrous-ferric hydroxide dissolves with 
difficulty. The addition is then continued drop by drop until a 
distinct red or brown precipitate is formed and then, on adding 
another drop of ammonia, the whole precipitate appears green. 


444. GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


If before this occurs the precipitate does not appear decidedly 
red in color, it is dissolved in a drop of two or hydrochloric 
acid and 1 or 2 c.c. more of chlorine or bromine water is added, 
and the addition of ammonia is repeated until the red and then 
the permanent green precipitate is obtained. Acetic acid (sp. 
gr. 1.04) is now added drop by drop until the green precipitate 
is dissolved and the remaining precipitate is quite red in color. 
About 1 c.c. of acetic acid in ex¢ess is added and the solution 
is boiled for one minute. All the phosphoric acid is then pre- 
cipitated as white ferric phosphate and the excess of ferric iron 
as red basic acetate. The greater part of the iron remains 
in solution as ferrous salt. The solution is filtered promptly 
through a large filter and washed once with hot water. The pre- 
cipitate filters readily and the filtrate is at first clear, but becomes 
turbid on standing in the air. 

The precipitate adhering. to the sides of the beaker is dissolved 
by warming with a mixture of hydrochloric acid (1:1) and 10 c.c, 
of bromine water. Should this not be sufficient to effect com- 
plete solution (as is usually the case) enough concentrated hydro- 
chloric acid is added to accomplish this. The solution is then 
poured upon the filter containing the precipitate and the filtrate 
received in a small beaker. The filter is washed well with hot 
water and the solution is evaporated nearly to dryness to get 
rid of the excess of hydrochloric acid, 5 c.c. of a 50 per cent. citric 
acid solution are added, an equal amount of magnesia mixture 
and enough ammonia to make the solution faintly alkaline. When 
perfectly cold, one-half of the liquid’s volume of strong ammonia 
is added and’ the mixture well stirred. After standing twelve 
hours, the precipitate is filtered off and washed with 2} per cent. 
ammonia containing 2.5 gms. of ammonium nitrate in each 100 c.e. 
This precipitate of magnesium ammonium phosphate always con- 
tains a small amount of iron and silicic acid (the latter from the 
glass) so that it is dissolved in hydrochloric acid, the solution evapo- 
rated to dryness, the residue moistened with concentrated hydro- 
chloric acid, taken up in a little water, filtered through a small 
filter and the residual silica washed with hot water. ‘The filtrate, 
amounting to not over 20 c.c. at the most, is treated with 1 c.c. 
of the citric acid solution and two drops of magnesia mixture 
and the precipitation with ammonia is repeated as above. In 


DETERMINATION OF PHOSPHORIC ACID IN SILICATES. 445 


this way a precipitate is obtained which yields pure magnesium 
pyrophosphate on ignition. 

Remark.—A. A. Blair* recommends the use of ammonium 
bisulphite (NH4HSO3) instead of sulphurous acid for the reduction 
of the ferric salt. Much of the ammonium bisulphite of commerce, 
however, contains phosphoric acid, so that it seems safer to use 
sulphurous acid for this purpose. Again, Blair suggests that 
hydrogen sulphide be passed into the solution after the excess of 
the sulphurous acid has been removed,in order to precipitate any 
arsenic as the trisulphide. The filtrate from the arsenic precipitate © 
is heated to boiling, the excess of hydrogen sulphide expelled by 
means of a current of carbon dioxide, and the solution then partly 
oxidized as above described. 


(b) The Molybdate Method. 


The filtrate from the silica (see p. 441) is evaporated to dry- 
ness in a porcelain dish, the dry residue is dissolved in as little 
nitric acid as possible, 30 c.c. of ammonium nitrate solution and 
10 c.c. of nitric acid are added, and the phosphoric acid is precipi- 
tated according to the procedure of Woy, p. 437, by the addition 
of 75 c.c. of ammonium molybdate. After decanting off the clear 
bquid, the precipitate is washed once by decantation with 10-20 
c.c. of the prescribed wash liquid and redissolved in a little ammo- 
nia. To this solution 6 ¢.c. of molybdate solution and 30 c.c. 
of water are added; it is heated just to the boiling-point and re- 
precipitated by the addition of 20 c.c. of hot nitric acid. The 
precipitate is then analyzed by the method of Finkener (p. 439) 
or by that of Woy (p. 440). 


1 gm. Mg,P,0, , = (0.27848 em, P 
3 (NH,),PO,- 12M00,= 0.01639 ‘ * 
"  P,05-24MoO, =0.01723 “ « 


Remark.—According to the above directions, some difficulty 
is likely to be encountered at the stage where the dry residue is 
taken up in nitric acid. If the residue is overheated at all, it 
dissolves very slowly in the nitric acid owing to the formation of 
basic ferric salts. For this reason many chemists prefer to carry 
out the analysis in accordance with the directions of the American 
Foundrymen’s Association, which are as follows: 





* The Chemical Analysis of Iron, 7th edition, 1908. 


446 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


A 2-gm. sample of the borings is dissolved in 50 c.e. of nitric 
acid, sp. gr..1.13, and 10 e.e. of hydrochloric, sp. gr. 1.2. In case 
- the sample contains a fairly high percentage of phosphorus, it is 
advisable to use half the above quantities of sample and reagents. 
The solution is evaporated to dryness and the residue baked until 
free from acid, at a temperature of about 200°. This baking 
serves to oxidize carbonaceous matter which otherwise interferes 
with the precipitation of the phosphorus. The residue is dis- 
solved by heating it with 25-30 c.c. of concentrated hydrochloric 
acid; the solution-is diluted to about 60 ¢c.c. and filtered. The 
filtrate is evaporated to about 25 c.c., 20 c.c. of concentrated nitric 
acid are added, and the evaporation is repeated until a film begins 
to form. At this point 30 c.c. of nitric acid, sp. gr. 1.2, are added 
and once more the solution is evaporated until a film forms. It is 
then diluted with hot water to a volume of about 150 c.c. and 
allowed to cool somewhat. When at a temperature between 70° 
and 80° C., 50 c.c. of ammonium molybdate solution are added 
and the solution agitated for a few minutes. The precipitate is 
then filtered on a tared Gooch crucible which has a paper dise — 
at the bottom. The precipitate is washed three times with 3 per 
cent. nitric acid and twice with alcohol. It is then dried at a 
temperature between 100° and 105° to constant weight. The 
precipitate contains 1.63 per cent. of phosphorus. 

The ammonium molybdate solution used in these last directions 
is prepared by dissolving 100 gms. of molybdic acid in 250 c.e. 
of water and 150 c.c. of concentrated ammonia, stirring until all 
is dissolved, whereupon 65 c.c. of nitric acid, sp. gr. 1.42, are added. 
Another solution is prepared containing 400 c.c. of the concentrated 
nitric acid and 1100 c¢.c. of water. When the two solutions are 
cold, the first is poured slowly into the second with constant stirring 
and a few drops of ammonium phosphate solution are added. 
After a little ammonium phosphomolybdate precipitate has settled 
out, the reagent is decanted off and is ready for use. The solution 
does not keep very well, so that the analysis should always be 
carried out with a reagent that has not stood very long. 

The phosphorus in iron and steel is very conveniently analyzed 
by a volumetric method. See Volumetric Analysis. 


SEPARATION OF PHOSPHORIC ACID FROM THE METALS. 447 


Determination of Phosphoric Acid in Silicates. 


In the analysis of silicates (see p. 491) the phosphoric acid is 
found in the precipitate produced by ammonia in the filtrate 
from the silica together with iron and aluminium hydroxides. 
It is analyzed according to p. 111. 


Determination of Phosphoric Acid in Mineral Waters. 


The contents of a 5-6 liter flask is acidified with hydrochloric 
acid and evaporated to dryness, the residue is moistened with 
concentrated hydrochloric acid, taken up with water, and the 
silicic acid filtered off. The filtrate is precipitated with ammonia, 
by which means the phosphoric acid is usually completely thrown 
down in the form of phosphate of iron, aluminium, or alkaline 
earth. The filtered and washed precipitate is dissolved in nitric 
acid and the phosphoric acid present determined according to one 
of the molybdate methods (pp. 486-440). 

Remark.—lf the mineral water does not contain much iron, 
aluminium, or alkaline-earth metal, but is rich in phosphoric acid 
and the alkalies, the precipitate produced by ammonia will not 
contain all of the phosphoric acid. In such a case the hydrochloric 
acid solution from the silica is evaporated several times to dryness 
with nitric acid, the residue is dissolved in as little nitric acid 
as possible, and the phosphoric acid determined by one of the 
molybdate methods. 


Recovery of Molybdenum Residues (H. Borntriger).* 


In practice the great majority of phosphoric acid determina- 
tions are carried out according to p. 436. The acid and am- 
moniacal filtrates containing molybdenum are saved, and the 
molybdenum is recovered as follows: Into a large, wide-mouthed 
flask 250 c.c. of strong ammonia are placed and the molyb- 
denum filtrates are added to this. Either immediately or after 
standing some time a crystalline deposit of almost pure molybdic 
acid is formed. When the flask is nearly full, the solution is 
made almost neutral, the precipitate allowed to settle, and the 





* Zeit. f. anal. Chem., XX XTII (1894), p. 341. 


448 GRAVIMETRIC DETERMINATION OF THE METALLCIDS, 


upper liquid containing only a small amount of molybdenum is 
poured off. The residue is poured upon a suction plate, washed 
once with water (not more, or the molybdic acid will dissolve) 
and sucked as dry as possible. The precipitate is dissolved by 
warming with as little ammonia as possible, leaving behind a 
residue of iron and aluminium hydroxides, magnesia, and silicic 
acid. These are filtered off and the solution diluted with dis- 
tilled water until at 17° C. it has a specific gravity of 1.11=14° Bé, 
It then contains 150 gms. of ammonium molybdate in a liter. If 
this solution is diluted with four times as much water, a 34 per 
cent. solution will be obtained. 


Determination of Phosphorus in Organic Substances. | 


The substance is decomposed by the method of Carius. By 
the action of the nitric acid in the closed tube the phosphorus is 
oxidized to phosphoric acid and this is determined as usual. 


SEPARATION OF PHOSPHORIC ACID FROM THE METALS. 


1. Separation from the Metals of Groups I and II. 


Hydrogen sulphide is conducted into the hydrochloric acid 
solution,* by which means all the members of these groups are 
precipitated as sulphides while the phosphoric acid remains in 
solution. 


2. Separation from the Metals of Group III. 


(a) The phosphoric acid is first precipitated as ammonium 
phosphomolybdate according to p. 436. In order to determine 
the metals, the solution containing molybdenum, but free from 
phosphoric acid, is evaporated with the addition of sulphuric 
acid to a syrupy consistency, and carefully heated over a free 
flame until the nitric acid is expelled. After cooling, the residue is 
moistened with hydrochloric acid and taken up in water. The 
solution is placed in a pressure-flask, saturated with hydrogen 
sulphide, the flask stoppered and heated for some time on the 
water-bath, when the molybdenum is precipitated in large flocks. 
After cooling, the pressure-flask is slowly opened and the molyb- 





* When silver is present it is precipitated as silver chloride, filtered off, 
and the filtrate treated with hydrogen sulphide. 


SEPARATION OF PHOSPHORIC ACID FROM THE METALS. 449 


denum sulphide is filtered off. The filtrate, now free from phos- 
phoric acid and molybdenum, is analyzed for the metals as 
described on pages 82 to 167. 

(b) The phosphoric acid is separated as before, the filtrate 
is made slightly ammoniacal and saturated with hydrogen sul- 
phide. After standing for some time the solution becomes red- 
dish yellow in color, when the precipitate is filtered off: The 
metals of this group will be found in the precipitate while the 
molybdenum is in the filtrate in the form of its sulpho-salt. 

Remark.—lf nickel is present, some of it will remain in the 
filtrate with the molybdenum on account of the solubility of 
nickel sulphide in ammonium sulphide, so that method (a) will 
then give more accurate results. 


3. Separation of Phosphoric Acid from Iron, Cobalt, 
Manganese, and Zinc. 


In case the solution contains iron in the ferric form, it is acidi- 
fied with hydrochloric acid, saturated with hydrogen sulphide, 
and for each gram of the mixed oxides 3 gms. of tartaric acid are 
added; the solution is made slightly ammoniacal and allowed 
to stand overnight in a stoppered flask. The precipitate con- 
tains the metals as sulphides free from phosphoric acid. It is 
filtered, washed with water containing ammonium sulphide, dis- 
solved in acids, and analyzed according to pp. 150 and 156. 


4. Separation from Chromic Acid. 


If the solution contains free alkali or alkali carbonate it is acidi- 
fied with nitric acid, then made slightly alkaline with ammonia 
and the phosphoric acid precipitated with ‘‘ magnesia mixture” as 
described on, page 434. 


5. Separation from Calcium, Strontium, Barium, Magnesium, 
and the Alkalies. 


Ammonium carbonate is added to the hydrochloric acid solu- 
tion until a slight permanent turbidity * is produced, which is 





* Tf only alkalies are present there will be no turbidity, and the ammo- 
nium carbonate is added until the solution is neutral. 


45° GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


redissolved by a few drops of hydrochloric acid. Ferric chloride 
is then added drop by drop until the liquid above the yellowish- 
white precipitate of ferric phosphate becomes distinctly brown 
in color. The solution is diluted with water to a volume of 300 
to 400 c.c., boiled for one minute, filtered and washed with water 
containing ammonium acetate. In the filtrate are now found 
the alkaline earths and alkalies, which, after expelling the am- 
monium salts by igniting the residue gbtained after evaporating to 
dryness, is analyzed in the usual way (see pages 43 and 76 ff.). 


THIOSULPHURIC ACID, H,S,0,. Mol. Wt. 114.16. 
Form: Barium Sulphate, BaSO,. 


The aqueous solution of the alkali thiosulphate is treated 
with an ammouilacal solution of hydrogen peroxide, or with am- 
moniacal percarbonate solution, heated for some time on the water- 
bath, and then boiled to destroy the excess of the reagent. This 
solution is acidified with hydrochloric acid and the sulphuric 
acid formed by the above treatment is precipitated as barium sul- 
phate. Two mols. BaSO, correspond to 1 mol. H,8,0,. 

A much better procedure for the estimation of thiosulphuric 
acid will be discussed under Jodimetry, Part II. 

The remaining acids of this group, arsenious, arsenic, vanadic, 
and chromic, have been discussed under the respective metals, while 
periodic acid is analyzed precisely in the same way as iodic acid. 


‘DETERMINATION OF NITRIC ACID AS NITRON NITRATE. 451 


GROUP V. 
NITRIC, CHLORIC, AND PERCHLORIC ACIDS. 
NITRIC ACID, HNO3. Mol. Wt. 63.02. 


Forms: Nitron Nitrate, CooH,.N,-HNO3, Nitrogen Pentoxide, N20;; 
Ammonia, NH3; Nitric Oxide, NO, and Volumetrically. 


1. Determination of Nitric Acid as Nitron Nitrate.* 


The base diphenyl-endo-anilo-hydro-triazole, CopHigN4, or 











called “ nitron ” for the sake of brevity, forms a fairly insoluble, 
_erystalline nitrate, CopHigN4-HNO3, which can be used for the 
separation and quantitative estimation of this acid. 
Procedure.—Enough of the substance is taken to furnish about 
0.1 gm: of nitric acid, and dissolved in 80-100 c.c. of water with 
the addition of 10 drops of dilute sulphuric acid. The solution is 
heated nearly to boiling and treated with 10-12 c.c. of nitron 
acetate solution}, which is added all at one time. The beaker 
containing the solution and precipitate is kept surrounded by ice- 
water for about two hours. The precipitate is then transferred to 
a Munroe crucible and drained as completely as possible from the 





* M. Busch, Ber. 38, 861 (1905). A. Gutbier, Z. angew. Chem., 1905, 494. 

+ The reagent is prepared by dissolving 10 gm. of nitron (which can be 
obtained of Merck) in 100 c.c. of 5 per cent. acetic acid. The solution usually 
has a reddish color, but can be kept for a longtime in a dark-colored bottle 
without its undergoing any change. 


452 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


pale yellow mother-liquor. It is washed with 10 or 12 c.c. of 
ice-water, added in small portions, and the precipitate drained. 
well after each washing. The precipitate is dried at 110° to 
constant weight. It contains 16.53 per cent. of NOs. 

Remarks.—The following acids interfere with the determination 
of nitric acid by the nitron method,—hydrobromic, hydriodic, 
nitrous, chromic, chloric, perchloric and the less common thio- 
cyanic, hydroferrocyanic, hydrofegricyanic, picric and oxalic 
acids. All of the above acids form salts with nitron which are not 
very soluble; these acids must, therefore, be removed from the 
solution before precipitating the nitric acid. 

Hydrobromic acid is decomposed by adding chlorine water 
drop by drop to the neutral solution and boiling, until the yellow 
coloration entirely disappears. 

Hydriodic acid is removed by adding an excess of potassium 
iodate to the neutral solution, and boiling until the iodine is all 
expelled. 

Nitrous acid is removed by dropping finely powdered hydrazine 
sulphate into the concentrated solution (0.2 gm. of substance in 
5 or 6 c.c. of water). 

Chromic acid is reduced by ide sulphate. 

Some idea as to the relative solubilities of the various salts of 
nitron is obtained from the following table: 

100 c.c. of slightly acid water dissolve at ordinary temperatures 
about 


0.0099 gm. of nitron nitrate, corresponding to 0.0017 em. HNO; 


0.61 Re bromide, 0.125 HBr 
OL: os" We iodide, 23 0.005. °°" 

tess 2" de nitrite, ee 0.022 ‘‘ HNO, 
0.06.5 2)" af chromate, = 0.011 ‘‘ H,Cr,0, 
0.422475 f: chlorate, ag 0.022 ‘‘ HClO, 
0.008 ‘‘ v perchlorate, sf 0.002 ‘‘ HClO, 


D086 0 3% = thiocyanate, =e 0.007 ‘‘ HCNS 


These values: are only approximate. The solubility of the 
nitrate is given a little too high and that of the other salts a little 
too low. ; 


DETERMINATION OF NITRIC ACID. 453 


On account of the apprec able solubi ity of the nitrate, it was to 
be expected that the results wou'd be a little low. This is not the 
case, however, as Busch and Gutbier have proved. It is probable 
that the precipitate occludes a little nitron acetate and in his way 
the error caused by amount left in solution is compensated. 


2. Determination of Nitric Acid as Nitrogen Pentoxide.* 


This method is based upon the fact that when an intimate 
mixture of a dry nitrate is heated with an excess of silica, nitrogen 
pentoxide is evolved and the amount is determined by the loss in 
weight. 


2NaNO3 + S102 = NaoSiOs + NoOs. 


This method cannot be used when there is any other volatile 
substance present, which is usually the case. 


3. Determination of Nitric Acid as Ammonia. 


The usual method for the determination of nitric acid is to 
_ reduce it in alkaline solution to ammonia by means of aluminium, 
zine, or, best, Devarda’s alloy (ef. Vol. I, page 6): 


NaNO3+8H =2H20+ Na0H+NHs3. 


After the reduction, the solution is distilled into a known 
amount of acid and the excess of the acid is found by titration, 
or the ammonia is determined as ammonium chloroplatinate or 
as platinum (cf. page 58, b and c). 





* Reich. Z. Chem., 1, 86 (1862). 


454 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Procedure of Devarda.* 


About 0.5 gm. of the nitrate is placed in a 600-800-c.c. 
Erlenmeyer flask (Fig. 78) and dissolved in 110 ¢.c. of water. To 
this solution 5 ¢.c. of alcohol, 50 ¢.c. of caustic potash (sp. gr. 1.3), 
and 2 to 24 gms. of powdered Devarda’s alloy are added. After 

















this the flask is immediately connected with the distilla- 
tion apparatus as shown in the figure. The Péligot tube, A, 
of about 250 c.c. capacity, is constructed as proposed by F. Pan- 
nertz.} Its left arm is connected by a curved tube with the mid- 
dle bulb, so that a spurting back of the liquid is avoided. The 
delivery-tube (of potash glass) connecting the flask K with the 
tube A is about 1 cm. in diameter and is provided with a small 
opening at 0, inside the flask, to prevent any of the alkaline solution 
being carried over with the ammonia. Twenty cubic centimeters 





* Zeit. f. anal. Chem., XX XIII (1894), p. 113. 
+ Ibid., XXXIX (1900), p. 318. 


DETERMINATION OF NITRIC ACID AS AMMONIA. 455 


of half-normal sulphuric acid are added to the tube A * and diluted 
so that the solution just reaches to each of the bulbs 011 the side, 
while 5 c.c. of the acid are placed in B, with a few drops of methyl 
orange, and diluted in the same way. The tubes A and B are 
connected by means of a T tube, of which the upper end is closed 
by a pinch-cock upon a piece of rubber tubing, so that a piece of red 
litmus paper may be introduced here. | 

When all is ready, the contents of the flask K are gently heated 
in order to start the reaction, then the flame is removed and the 
reaction allowed to proceed by itself. After an hour this will be 
shown to be complete py the cessation of the hydrogen evolution. 
The liquid in K is then slowly heated to boiling, and kept at this 
temperature until about half of the liquid has distilled over into A; 
this requires about half an hour. Durine the last ten minutes 
a slow current of air is passed through the tube r. 

If the distillation has been correctly performed, all of the 
ammonia will now-be found in A; no trace should reach B, and the 
red litmus paper in the T-tube should show no tinge of blue. 

When the distillation is finished, the pinch-cock at r is opened 
and the flame removed. A little methyl orange is added to A where- 
by the liquid is colored red, the contents of B are poured in, the 
latter tube is washed with water that is added to A, and the excess 
of the sulphuric acid is titrated with half-normal caustic potash 
solution until the solution is changed to yellow. The amount of 
nitric acid is computed as follows: 

The tubes originally contained 25 c¢.c. of half-normal acid, 
and ¢ c.c. of half-normal caustic potash solution were used up 
in the titration; consequently the ammonia formed from 0.5 gm. - 
of the nitrate was neutralized by 25—¢ c.c. of half-normal sul- 
phuric acid. 

Since 1 mol. of HNO3 (63.02 gms.) on being reduced yields 
1 mol. of NHg3, and one liter of half-normal sulphuric acid con- 
tains enough sulphuric acid to neutralize 4 mol. of NHs3, it is evi- 

63.02 
5000 = 0.03151 gm. 
* The tube A has a capacity of about 250 cubic centimeters. 


+ Ostwald has proposed that the molecular weight in grams of a sub- 
stance be designated by the word “mol.” 


dent that 1 c.c. of the acid is equivalent to 





456 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


of nitric acid so that 25—t ¢.c.=(25 —t) X0.03151 gm. of HNO or 
(25—1)-03101 gm, NOs, and the per cent. of NO3 present is 


0.5: (25-2) -0.03101=100:2 
x=6.202-(25—t)=per cent. NOs. 


Determination of Nitric Acid as Nitric Oxide. 


Method of Schlésing and Grandeat, modified by Tiemann and 
Schulze.* 


Principle.—If{ a nitrate is heated with ferrous chloride and 
hydrochloric acid, the nitric acid is reduced to nitric oxide: 


NaNO,+ 3FeCl,+ 4HCl=NaCl+ 3FeCl,+ 2H,O+ NO. 


From the volume of the nitric oxide its weight is calculated. 

The method of Schlésing in its original formt was not much 
used on account of the apparatus required; but after being modi- 
fied by Grandeau tf it has become one of the best methods for the 
determination of nitric acid. 

The apparatus necessary is shown in Fig. 79 and consists of 
a 150-cc. flask K fitted with a double-bored rubber stopper. 
Through one of the holes is passed the tube b, which reaches into 
the flask just to the lower surface of the stopper; through the 
other hole passes the tube a,§ ending in a restriction about 1 mm, 
wide and reaching 14 cm. below the stopper. The tube b is con- 
nected by means of a piece of rubber tubing 5 em. long, which is 
wired on to the tube, and is provided with a pinch-cock, with a 
second tube whose lower end reaches up into the measuring-tube 
and is covered with rubber tubing as is shown in the figure. In 
the same way the tube a is connected with a straight tube. _ 

Solutions required —1. A nitrate solution of known strength, 
prepared by dissolving in one liter of water 2.0222 gems. of recrystal- 
lized potassium nitrate that has been dried at 160°C. Fifty c.c. 





* Zeit. f. anal. Chem., [IX (1870), p. 401, and Berichte, VI (1873), p. 1041. 

+ Annales de chim. et de phys., [3], 40 (1853), 479. 

t Grandeau, Analyse chimique appliquée A I’agriculture. 

§ Grandeau used a separatory funne! instead of the tube a; the lattes 
was proposed by Tiemann and Schulze. 


DETERMINATION OF NITRIC ACID AS NITRIC OXIDE. 457 


of this solution evolve at 0° C. and 760 mm. pressure 22.41 c.c. 
of NO. | ; 
2. A ferrous chloride solution obtained by dissolving 20 gm. 
of iron (nails) in 100 ¢.c. of concentrated hydrochloric acid. 
3. Hydrochloric acid, of specific gravity 1.1. 
Procedure.—First of all, 10 ¢.c. of water are poured into K 
and its upper level is marked on the outside of the flask by means of 











Fig. 79. 


a colored pencil, then 40 c.c. more are added and its position is 
also marked. : 

The water is now poured out and exactly 50 c.c. of the standard 
nitrate solution is added to K, the stopper fitted with the two 
tubes is placed in the flask, and the pinch-cocks h’ and h” are 
opened. The contents of the flask are heated to boiling with a 
free flame (a wire gauze is not used) until finally no more bubbles 
of air escape from the lower end of b into the bath containing 
boiled water. To make sure that the air is all expelled from 
the apparatus, the rubber tubing at h’ is pinched with the thumb 
and finger, when, if no air is present, the liquid will quickly rise 
in b, exerting a noticeable pressure. The pinch-cock h’ is then 
closed and the boiling is continued until the 50 c.c. has been re- 


458 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


duced to a volume of 10 c.c., when the flame is removed and the 
pinch-cock h’’ is immediately closed. The lower end of a, which 
dips into distilled water, is immediately filled with the latter up 
to the pinch-cock. The vapors in the flask condense, forming a 
vacuum, as shown by the closing together of the rubber tubing 
at h’ and h’’. 

30 c.c. of the ferrous chloride solution are poured into a beaker 
and the upper level is marked on the outside with a colored pencil, 
then 20 c.c. more are added and the position in the beaker is again 
marked. The lower end of the tube a is placed in the ferrous 
chloride solution so that it reaches below the lower mark on the. 
beaker, and, by opening h’’, 20 c.c. of the solution are allowed 
to pass into the flask K. The beaker containing the ferrous chloride 
is then replaced by one containing boiled water. The tube a 
should not extend vertically into the water, but should be inclined 
as much as possible. The specifically heavier ferrous chloride - 
solution in the tube passes into the water, while the latter takes 
its place. When the lower end of a has become filled with pure 
water in this way, it is dipped into a beaker containing hydrochloric 
acid (sp. gr. 1.1) and about 20c.c. of the acid are allowed to flow 
into K,and finally 3-4 ¢.c. of water are added to replace the acid 
ina. <A 50-c.c. measuring-cylinder is now filled with boiled water, 
placed over the lower end of b as shown in the figure, and the con- 
-.tents of the flask K are heated fifteen minutes on the water-bath,* 
then heated to boiling with a free flame. As soon as the com- 
pressed rubber tubing begins to expand h’ is opened, but the rubber 
tubing is at the same time pinched between the thumb and finger. 
As soon as the liquid no longer rises in b, the hand is removed from 
the rubber tubing and the nitric oxide begins to collect slowly in 
the measuring-tube. After half of the liquid has evaporated 
there is no further evolution of nitric oxide to be noticed, although 
the brown color of the solution shows that the gas has not been 
completely expelled. In order to accomplish this, the flame is re- 
moved, h’ is closed, and the liquid in K allowed to cool. By means 
of the vacuum thus produced the remainder of the nitric oxide 
is expelled from the solution and the: boiling is once more repeated, 
with the same precautions as before, until the lower mark is 


* The heating on the water-bath is necessary, as otherwise a little nitric 
will distil over and not be reduced. A. Wegelin, Inaug. Dissert. Ziirich, 1907. 





DETERMINATION OF NITRIC ACID AS NITRIC OXIDE. 459 


reached. The flame is removed, h’ is closed, and the measuring- 
tube containing the nitric oxide is placed in a cylinder containing 
pure water at the temperature of the room. To prevent the tube 
containing the gas from sinking, its upper end is encased in a large 
cork so that it floats on the water. After standing fifteen to 
twenty minutes the tube is raised by means of the cork until 
the level of the liquid within stands at the same height as that 
in the cylinder without, and the volume of the gas is read. At 
the same time the temperature of the water i is taken and the barom- 
eter reading is noted. 

The volume thus obtained is reduced to 0° C. and 760 mm. 
pressure. If the temperature was {°, the barometer reading B. 
millimeters, and w the tension of aqueous vapor at 7°, then the 
reduced volume is 
V(B—w)273 * 

760(273+0)° 





Vo= 


Now 50 c.c. of the standard potassium nitrate solution con- 
tain 0.1011 gm. of KNOs corresponding to 0.06201 gm. of NO3, 
so that the vo'ume Vo of the nitric oxide corresponds to 0. 06201 
gm. NQ;. 

The same procedure is now followed with 50 c.c. of the solu- 
tion of the unknown nitrate, which should be prepared so that 
the amount of nitric oxide evolved will be about the same as that 
from 50 c.ec. of the standard solution. If at ¢/° C. and B, mm. 
pressure the volume V’ of nitric oxide is obtained, and w, is the 
tension of aqueous vapor at ¢°, then the reduced volume of the 
nitrogen will be as before 


V’(B,—w,)273 
760(273+10’°) © 





Vo= 
The following proportion now holds: 
| V,:0.06201= Vi :a 


_ V%0.06201 


7 =em. NO, in 50 c.c. of solution. 
0 








* Three or four experiments are performed with the standard solution, 
and the mean value is used. 


460 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Remark.—It is not permissible to compute directly the weight 
of NO, which corresponds to the volume of nitric oxide obtained, 
for some nitric oxide always remains in the flask, so that low 
values would result. This error is eliminated by the above pro- 
cedure. | 

L. L. de Koninck * has devised an apparatus which prevents 
the liquid from sucking back into the decomposition-flask and 
at the same time permits the carrying out of a number of deter- 
minations one after the other without cleaning the apparatus 
or boiling it free from air in the meantime. 


Determination of Nitric Acid in a Drinking-water. 


Irom 100 to 300 c.c. of the water are evaporated to 40-50 c.e. 
in a porcelain dish, a few drops of methyl orange are added, fol- 
lowed by dilute hydrochlorie acid, free from nitrate, until the solu- 
tion is pink in color. Sodium carbonate solution is now added 
until the liquid is barely alkaline (it becomes yellow) and the con- 
tents of the flask are washed into the decomposition-flask K, 
Fig. 79, and analyzed as described on page 457 with the 
difference that, instead of collecting the gas over water, a 10 per 
cent. solution of sodium hydroxide is used, to make sure that the 
carbonic acid which is set free is completely absorbed. 

After the experiment has been performed with the water to 
be analyzed, it is repeated with an amount of the standard solu- 
tion sufficient to evolve about the same quantity of nitric oxide. 
The analysis is then computed as before. 

Remark.—In drinking-water the neutralization of the evap- 
orated sample is not absolutely necessary, except in the case of 
alkaline mineral waters; in that case the introduction of the hydro- 
chloric acid would otherwise cause such a violent evolution of 
carbon dioxide that the flask might crack. 

CHLORIC ACID, HCIO;. Mol. Wt. 84.47. 
Forms: Silver Chloride, AgCl, besides volumetric and gasometric 
methods. | 

In order to determine chloric acid as silver chloride it must 
previously be reduced to chloride by means of ferrous sulphate or 
zinc. 


* 7. anal. Chem., 38, 300 (1894). See also Liechti and Ritter, ibid., 42 
205, (1903). Eire 





CHLORIC ACID. 461 


Reduction by means of Ferrous Sulphate. 


About 0.3 gm. of the salt is dissolved in 100 c.c. of water, 
treated with 50 c.c. of a 10 per cent. solution of crystallized ferrous 
sulphate, heated with constant stirring till it begins to boil, and 
kept at this temperature for fifteen minutes. After cooling, 
nitric acid is added until the deposited basic ferric salt is dis- 
solved, when the chloride is precipitated by means of siiver nitrate 
and weighed after the usual treatment. 

One gram of silver chloride corresponds to 0.8550 gm. KCIO,. 


Reduction with Zinc. 


Although chlorates are reduced in neutral solution by means 
of zine or Devarda’s alloy, it is not: advisable to effect the reduc- 
tion in this way for quantitative purposes. The same end is 
reached more expeditiously by adding zinc-dust to the acetic 
acid solution. The dilute chlorate solution is treated with acetic 
acid until it reacts distinctly acid, an excess of powdered zinc is 
added, and the solution boiled for one hour. After cooling, nitric 
acid is added in sufficient quantity to dissolve all of the excess of 
zine, after which the solution is filtered if necessary and the chloride 
precipitated and determined as silver chloride. 

Remark.—Both methods afford exact results, but the former 
is to be preferred, for it is accomplished in less time. 

Chlorates are not quantitatively decomposed into chlorides 
by ignition in open vessels or in a current of carbon dioxide. Some 
chlorine and a little alkali is always lost, so that even when the 
residue is evaporated with hydrochloric acid, too low results are 
obtained. L. Blangey, working in the author’s laboratory, ob- 
tained results which were from 0.3 to 1.1 per cent. below the theo- 
retical value. 

According to the two following methods, the decomposition 
of alkali chlorate into chloride is quantitative. 


(a) By Evaporation with Hydrochloric Acid. 


The chlorate contained in a weighed porcelain crucible is covered 
‘with hydrochloric acid (1:3). A watch-glass is placed upon the 
crucible, and the contents of the latter are heated on the water- 


462 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


bath until the evolution of chlorine ceases. The liquid on the lower 
surface of the watch-glass is then washed into the crucible, and 
its contents are evaporated to dryness on the water-bath. The 
cover is placed upon it and it is then gently ignited over a free 
flame until the decrepitation ceases. After cooling in a desic- 
cator, the crucible is again weighed. In this way L. Blangey 
obtained, as a mean of four experiments, 100.02 per cent. of the 
theoretical value. - . 


(b) By Ignition with Ammonium Chloride. 


The alkali chlorate is mixed in a porcelain crucible with three 
times as much pure ammonium chloride, covered with a watch- 
glass, and heated over a free flame, kept in constant motion, until 
the ammonium chloride is completely removed. The crucible 
is then weighed. As a mean of two experiments, L. Blangey 
obtained 100.06 per cent. of the theoretical value. 


PERCHLORIC ACID, HCl1O,. Mol. Wt. 100.47. 
Form: Silver Chloride, AgCl. 


Perchlorates cannot be reduced to chloride by means of ferrous 
sulphate, zinc, or by repeated evaporation with concentrated 
hydrochloric acid.* On ignition, some chlorine and alkali chloride 
are lost, so that an error amounting to as much as 1 per cent. may 
be expected. On the other hand, Winteler has shown that per- 
chlorates may be changed to chlorides by heating with concen- 
trated nitric acid and silver nitrate in a closed tube (see Carius’ 
method for determining chlorine in organic substances, page 325), 
while L. Blangey found that ignition with ammonium chloride | 
would accomplish the same result. 


Decomposition of Perchlorates by Ignition with Ammonium 
Chloride. 

By twice igniting an intimate mixture of 0.5 gm. potassium 

perchlorate with 13 to 2 gms. of ammonium chloride f in a platinum 





* On evaporating with hydrochloric acid there is a loss without any evo- 
lution of chlorine; it must be due to the volatilization of smali amounts of 
perchloric acid. 

+ When 2 gms. of NH Cl are used, one and one-Lalf to two hours are nec 
essary. 


PERCHLORIC, CHLORIC, AND HYDROCHLORIC ACIDS. 463 


crucible covered with a watch-glass, the former is completely 
changed to chloride. Care should be taken not to melt the residual 
chloride, for in that case the platinum is attacked, although the 
accuracy of the results is not affected. Blangey obtained in two 
experiments 100.06 and 100.08 per cent. of the theoretical values. 

It is worth mentioning that complete decomposition could 
not be effected by igniting three times in a porcelain * crucible; 
the platinum evidently plays the part of a catalyser, as was proved 
by the following experiment: 0.4767 gm. of KClO, was mixed 
in a porcelain crucible with 14 gm. of NH,Cl, and 1 c.c. of hydro- 
chlorplatinic acid (containing 0.0918 gm. Pt) was added. After 
evaporating to dryness on the water-bath, the ammonium chloride 
was completely expelled and the residue was ignited twice more 
with the same amount of the latter. The residue of potassium 
chloride then weighed 0.2572 gm., corresponding to 100.24 per 
cent. of the theoretical amount. t 


Determination of Perchloric Together with Chloric Acid. 


In one portion the chlorate is reduced, as described on page 461, 
with ferrous sulphate, and the chloride formed determined as 
silver chloride. A second portion is ignited in an old platinum 
crucible (or in one of porcelain) with the addition of 1 ¢.c. of hydro- 
chlorplatinic acid and three times as much ammonium chloride 
(as described above). In this way the total amount of chlorine 
is obtained and from these data the amount of each acid can be 
calculated. 


Determination of Perchloric, Chloric, and Hydrochloric Acids 
in the Presence of One Another. 


The three acids are assumed to be present,in the form of their 
alkali salts. 








* Thus on igniting 0.4395 gm. KClO, with 2 gms. NH,Cl a residue of 
0.3205 gm. was obtained instead of one weighing 0.2365 gm. 

+ There is often a slight deposit of alkali chloride upon the cover-glass, 
To determine this, the glass together with the deposit is weighed, then the 
glass i3 washed, cried, and again weighed: the difference between the two 
weights represents the amount of alkali chloride. This rarely amounts to 
_ more than a fraction of a milligram, and if the ignition was performed with 
care, there will be no deposit at all upon the glass. 


464 GRAVIMETRIC DETERMINATION OF THE METALLOIDs. 


In one portion the chloride-chlorine is determined by precipita- 
tion with silver nitrate. In a second sample the chlorate and 
chloride-chlorine are determined after the former has been reduced 
to chloride by means of ferrous sulphate. The total amount 
of chlorine present is determined in a third portion after ignition 
with ammonium chloride. 


GROUP VI. 
SULPHURIC, HYDROFLUORIC, AND HYDROFLUOSILICIC ACIDS. 


SULPHURIC ACID, H,SO,.. Mol. Wt. 98.09. 
Form: Barium Sulphate, BaSO,. 


Theoretically the gravimetric determination of sulphuric acid 
is extremely simple, it being only necessary to precipitate with 
barium chloride, filter and weigh the barium sulphate. Prac- 
tically, howeyer, it is a process connected with many difficulties. 

According to the manner of precipitating barium sulphate, 
the composition of the precipitate varies in such a way that some- 
times the results are too high and sometimes too low. 


Errors which may Occur in the Precipitation of Barium Sulphate.* 
I. In the Precipitation of Barium Chloride with Pure Sulphuric Acid. 


If a dilute, slightly acid solution of barium chloride is treated — 
at the boiling temperature with an excess of dilute sulphuric 
acid, the precipitate contains all of the barium except a very small, 
negligible amount. If, however, the precipitate is weighed, the 
result is invariably too low; and this is true even when the solution 
is evaporated to dryness in order to recover the last traces of 
barium. The precipitate always contains barium chloride in a 
form which cannot be removed by washing. A mixture, there- 
fore, of barium sulphate and barium chloride is weighed, and as 
the molecular weight of the latter is less than that of the former. 
the result must be too low. In order to obtain accurate results the 
chlorine combined with barium in the precipitate must be replaced 





* See the interesting article by M. J. van’t Kruys on the determination of 
sulphuric acid in the presence of various salts which affect the result. Z. anal, 
Chem., 1910 ,393. 


SULPHURIC ACID. 405 


by S04; and this is easily accomplished by moistening the precip- 
itate with concentrated sulphuric acid, and heating until the 
excess of the latter is removed by volatilization. 

Not only is barium chloride carried down with barium sulphate, 
but all barium salts as well, especially the chlorate and nitrate. 
These are, however, readily changed to sulphate by the above 
treatment with concentrated sulphuric acid. It is immaterial in 
the estimation of barium how the precipitation is effected; whether 
the sulphuric acid is added quickly, or drop by drop, the results 
are always the same. 


IT. In the Precipitation of Pure Sulphuric acid with Barium Chloride. 


This is the reverse process, but in this case it is not a matter 
of indifference whether the barium chloride is added slowly, drop 
by drop, or rapidly all at one time. In the first instance, the 
results are very near the truth without applying any correction 
whatsoever; in the latter instance, too high results are obtained, 
because by the rapid addition of the reagent much more barium 
chloride is carried down with the precipitate than when the 
reagent is added very slowly. 

In order to obtain the true weight of barium sulphate, it is 
necessary to make a deduction for the amount of barium chloride 
contained in the precipitate and to add the weight of barium 
sulphate remaining in solution. 

The chlorine contained in the precipitate can be determined in 
several different ways. 

1. The precipitate is fused with four times as much pure 
sodium carbonate, the melt extracted with hot water, the solution 
filtered, acidified with nitric acid, and the chlorine precipitated 
with silver nitrate which is filtered off and weighed. (Cf. p. 320.) 

2. Still more accurate is the process of Hulett and Duschack * 
The greater part of the ignited precipitate of barium sulphate is 
placed in a U-tube of which one arm is drawn out into a thin, 
right-angled, gas delivery tube. Concentrated sulphuric acid is 
added to the precipitate and the mixture is heated by placing the 
U-tube in hot water. The barium sulphate dissolves readily in 





* Z. anorg. Chem., 40, 196 (1904). 


466 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


the hot concentrated sulphuric acid and the barium chloride 
present is decomposed. In order to determine the amount of 
hydrochloric acid set free, a slow stream of air, which has pre-~ 
viously passed through caustic potash solution, is led through the 
tube, which has the drawn-out end of the ‘atter dipping into a 
stout test-tube containing 0.01 N. silver nitrate solution. After 
two or two and one-half hours all of the hydrochloric acid will have 
been expelled from the sulphuric &cid. 

The decomposition apparatus is then removed, the gas delivery 
tube washed out with a little water, and the silver remaining in 
solution is determined volumetrically (cf. pp. 702-05). 

For the determination of the dissolved barium sulphate the 
filtrate from the first precipitation is evaporated to dryness,* 
the residue moistened with a few drops of concentrated hydro- 
chloric acid, taken up with water and the slight precipitate of 
barium sulphate filtered off and weighed. 

Calculation of the true weight of Barium Sulphate.—If the weight 
of the first precipitate of crude barium sulphate is a, the weight 
of the barium chloride contained in this precipitate, as deter- 
mined by titration of the amount of chlorine, is b, and the amount 
of barium sulphate in solution is c, then a—b+c represents the 
weight of pure barium sulphate. 

Experience has shown, however, that when pure sulphuric 
acid is precipitated by means of dilute barium chloride solution 
added drop by drop, the errors b and c are approximately equal 
and counterbalance one another so that the weight a is very close 
to that of the pure barium sulphate. 


III. In the Precipitation of Sulphates with Barium Chloride. 


Here the relations are far more complicated than in the pre- 
cipitation of pure sulphuric acid, partly because the barium 
sulphate is much more soluble in salt solutions than in water 
containing a little acid, and partly because of the tendency of 
barium sulphate to occlude not only barium chloride but many 
other salts as well. Solutions of chromium sulphate are either 





* During all such work care should be taken to prevent sulphuric acid 
contamination from the air in the laboratory. The evaporation should 
therefore take place on the steam bath or steam table. 


SULPHURIC ACID. | 467 


violet or green. From the boiling-hot green solution only one- 
third of the sulphurie acid is precipitated, the remainder probably 
being present in the form of a complex chromium sulphate cation; * 
on cooling there is a tendency for the green solution to become 
violet and after some time all of the sulphuric acid is precipitated. 
The precipitation of barium sulphate in the presence of ferric iron 
has been much studied. In the boiling hot solution all of the 
sulphuric acid is not precipitated and considerable iron is thrown 
down with the barium sulphate and furthermore the precipitate 
then loses SO3 on ignition. Since ferric oxide weighs much less 
than an equivalent weight of barium sulphate sometimes the results 
are as much as 10 per cent. too low. On the other hand, Kiister 
and Thiel,t were able to get satisfactory results (1) by precipitating 
the sulphuric acid from such a solution in the cold, or (2) by 
slowly adding the ferric chloride and sulphuric acid solution to the 
hot solution of barium chloride, or (8) by precipitating the iron 
by an excess of ammonia, heating, and adding barium chloride to 
the solution without filtering off the ferric hydroxide, and finally 
dissolving the latter in dilute hydrochloric acid. 

Most chemists, however, deem it advisable to remove trivalent 
metals before attempting to determine the sulphuric acid. This 
is accomplished in the case of ferric iron by adding a liberal 
excess of ammonia to the dilute slightly acid solution which is at a. 
temperature of about 70°. If from 5-7 c.c. of concentrated 
ammonia (sp. gr. 0.90) is added in excess of the amount required 
for neutralization,{ the precipitate is not likely to contain any 
basic ferric sulphate. If, on the other hand, the solution is barely 
neutralized with ammonia, the precipitate produced will invariably 
contain some sulphate. 

The bivalent metals are occluded to a much less extent, so 
that it is not, as a rule, necessary to remove them. On the other 
hand, in the presence of considerable amounts of bivalent metal 
with relatively small amounts of sulphuric acid, the error arising 
from occlusion is likely-to be large, so that it is better to remove 
the bivalent metals in all such cases. 

* Recoura, Comptes rendus, 113, 857; 114, 477. 


+ Z. anorg. Chem., 22, 424. 
{ Pattinson, J. Soc. Chem. Ind., 24, 7. 





468 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


‘In the presence of nitric or chloric acid the barium sulphate 
precipitate will contain considerable quantities of barium chlorate 
and nitrate which it is impossible to remove by washing with hot 
water. These acids, therefore, must be decomposed by evapora- 
tion with hydrochloric acid before attempting to precipitate the 
sulphuric acid. 

In ordinary chemical practice it is usually a question of deter- 
mining sulphuric acid in a solwtion containing considerable 
amounts of ammonium or alkali chloride, ammonium or alkali 
sulphate, and some free hydrochloric acid. Now ammonium and 
alkali sulphates are also occluded by barium sulphate, and the 
amount of occulsion increases as the solution is more concentrated 
with respect to these substances. For this reason it is evident 
that barium sulphate should always be precipitated in a very 
dilute solution. Furthermore, a small amount of free hydrochloric 
acid is indispensable, but larger amounts have a solvent effect upon 
the precipitate. 

For an amount of sulphuric acid corresponding to between 
1 and 2 gms. of barium sulphate, the precipitation should take 
place in a volume of between 350 and 400 c.c. and in the presence 
of hydrochloric acid amounting to 1 c.c. of sp. gr. 1.17. 

If a neutral solution is at hand, it is diluted to a volume of 
350 c.c. and 1 ¢.c. of concentrated hydrochloric acid is added. 

An alkaline solution is carefully neutralized with hydrochloric 
acid, using methyl orange as indicator, 1 c.c. of concentrated 
hydrochloric acid is added in excess, and the solution is diluted 
to 350 c.c. 

Finally, in the case of an acid solution, it is either evaporated 
to dryness, the residue moistened with 1 c.c. of concentrated 
hydrochloric acid and 350 c.c. of water added, or, with methyl 
orange as indicator, the solution is neutralized with ammonia,) 
treated with 1 c.c. of concentrated hydrochloric acid, and diluted) 
to 350 c.c. 

After the solution has been prepared in accordance with the 
above directions it is ready for the 


SULPHURIC ACID. 469 


Precipitation of Sulphuric Acid in the Presence of Ammonium or 
Alkali Salts according to E. Hintz and H. Weber. 


The solution is heated to boiling, and then for each gram of 
barium sulphate precipitate 10 ¢.c. of normal barium chloride 
solution are taken, diluted to 100 c¢.c., the solution heated to — 
boiling, and added all at one time to the hot sulphate solution 
which is being stirred continuously. After the solution has stood 
for half an hour, best in a warm place, it is filtered, washed with 
hot water and ignited (cf. p.74). The use of a Gooch or Munroe 
crucible is to be recommended. 

Remarks.—In the presence of ammonium salts the pre- 
cipitation of the barium sulphate should not be effected, as is 
otherwise desirable, by the cautious addition of the barium 
chloride, for, as Hintz and Weber have shown, this leads to low 
results whereas the occlusion caused by the rapid addition of the 
barium chloride counterbalances this error. 

Under no circumstances should a precipitate of barium 
sulphate be heated over a blast lamp, for in that case sulphuric 
anhydride would be evolved from the barium sulphate itself. 

To explain the occlusion of barium chloride by barium sulphate, 
Hulett and Duschak * have suggested that perhaps the precipitate 
may contain salts such as BaCl. HSOq, (BaCl) 2804, and Ba(HS0O,4)> 
and Folin ¢ believes that such is this case because some of his 
precipitates have lost SO3 on ignition while others have lost 
HCl. He also suggests the possibility of salts such as Ba( KSO«4)2 
being precipitated. 


Determination of Sulphuric Acid in Insoluble Sulphates. 


Calcium and strontium sulphates are decomposed by long 
digestion with ammonium carbonate solution, but barium sulphate 
is not. The latter is mixed with four times as much sodium car- 
bonate, fused in a platinum crucible, the melt extracted with 
water, and the barium carbonate residue washed with sodium 
carbonate solution. After acidifying the filtrate with hydro- 





* Z. Anorg. Chem., 40, 196 (1904). 
¢ J. Biol. Chem., 1, 131 (1905). 


470 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


chloric acid and boiling off the carbon dioxide, the sulphurie acid 
is precipitated as usual. 

Lead sulphate is boiled with sodium carbonate solution; af er 
cooling, the solution is saturated with carbon dioxide and filtered. 
The lead remains behind as carbonate, while the filtrate contains 
all of the sulphuric acid. 

For the determination of sulphuric acid in silicates, the finely 
powdered substance is fused with stx times as much sodium car- 
bonate, the melt is extracted with water, the filtrate acidified 
with hydrochloric acid and evaporated to dryness in order to 
dehydrate the silica. The residue is moistened with a little 
concentrated hydrochloric acid, taken up in hot water, and the 
silicic acid filtered off; the sulphuric acid is determined in this 
filtrate. 


Determination of Sulphuric Acid in the Presence of Soluble 
Sulphides. 


The substance is placed in a flask, the air replaced by carbon 
dioxide, dilute hydrochloric acid is added, and the sclution boiled 
while carbon dioxide is passed through it until all of the sulphide 
has been expelled. The sulphuric acid is then precipitated from 
the solution. 

This determination is used for the analysis of cements. In 
this case, however, the hydrochloric acid solution will contain 
much calcium as well as iron and aluminiun, so that these metals 
are precipitated by the addition of ammonia and ammonium 
carbonate and the sulphuric acid determined in the filtrate. 

If it is desired to determine the amount of sulphide-sulphur, 
the substance is covered with bromine water until the color of 
the bromine is permanent, hydrochloric acid is added, and the 
solution boiled to expel the excess of the bromine. The iron, alu- 
minium, and calcium are precipitated by ammonia and ammonium 
carbonate, and the total sulphur is determined in the filtrate. The 
difference between the two results represents the amount of sul- 
phur present as sulphide. For the volumetric determination 
of sulphuric acid consult Part II. 


HYDROFLUORIC ACID. 471 


HYDROFLUORIC ACID, HF. Mol. Wt. 20.01. 


Forms: Calcium Fluoride, CaF,; Silicon Fluoride, SiF,, besides 
volumetric and gasometric methods. 


1. Determination as Calcium Fluoride. 


If the,solution contains free hydrofluoric acid or an acid fluoride, 
sodium carbonate is added until the reaction is alkaline and from 
one-fourth to one-fifth as much more in excess.* To solutions of neu- 
tral fluorides about 1 ¢.c. of double-normal sodium carbonate solution 
is added. The alkaline solution is heated to boiling, precipitated 
by means of an excess of calcium chloride solution, filtered, and 
thoroughly v-ashed with hot water. The precipitate consisting of the 
fluoride and carbonate of calcium is dried, as much of it as possible 
is transferred to a platinum crucible, the ash of the filter is added, 
and the contents of the crucible are ignited.t After cooling, the 
mass is covered with an excess of dilute acetic acid, by which the 
lime is changed to the soluble acetate, while the fluoride is unaf- 
fected. After evaporating to dryness on the water-bath, the mass 
is taken up in water, filtered, washed, and dried.{ After transfer- 
ring as much of the dried precipitate to the crucible as possible, the 
dilter-paper is burned, its ash added, and after ignition the crucible 
is again weighed. To confirm the result the substance is treated 
with a little concentrated sulphuric acid (added cautiously), and 
after evaporating off the excess of the latter and once more igniting, 
the contents of the crucible are weighed as calcium sulphate. 

1 gm. CaF? yields 1.7436 gms. CaSO. | 

Remark.—The results are usually a little low on account of 
the solubility of calcium fluoride; 100 c.c. water dissolves 0.0016 
gm., and 100 ¢.c. 1.5 N. acetic acid dissolves 0.011 gm. CaF2 at 
the temperature of the water bath. 





* By the addition of the excess of sodium carbonate the precipitate of 
calcium fluoride will contain calcium carbonate, and presence of the latter 
renders the precipitate easy to filter. A pure precipitate of calcium fluoride 
is so slimy that the pores of the filter become so clogged that it is almost 
impossible to complete the filtration. 

+ The ignition makes the CaF, denser and hence easier to filter. 

¢{ The calcium fluoride is not volatilized in an open platinum crucible 
heated over a Bunsen burner. Heated over the blast, there is appreciable 
volatilization. 


472 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Example: Determination of Fluorine in Calcium Fluoride.— 
As was stated in Vol. i, page 391,* calcium fluoride is not com- 
pletely decomposed by fusing with sodium carbonate; but if the 
fluoride is mixed with 24 times as much silicic acid and then fused 
with 6 times as much sodium-potassium carbonate, the greater 
part of the silicic acid and all of the fluorine will be changed to sol- 
uble alkali salts, while the calcium will be left as insoluble calcium 
carbonate. The mixture must be*heated gradually, as otherwise 
the evolution of carbon dioxide may cause the contents of the 
crucible to run over its sides. The thin liquid fusion soon changes 
to a thick paste or only sinters somewhat. On raising the tempera- 
ture, it is almost impossible to further melt this mass, and it is 
not necessary. In fact too high a temperature is to be avoided on 
account of the danger of losing some alkali fluoride by volatilization. 
The reaction is complete when there is no further evolution of car- 
bon dioxide. After cooling, the melt is treated with water, the 
insoluble residue is filtered off and thoroughly washed. The 
alkaline solution containing all the fluorine and considerable 
silicic acid is freed from the latter by the addition of considerable 
ammonium carbonate f (about 4 gms. of the solid salt). The liquid 
is heated for some time at about 40° C., allowed to stand over- 
night, and in the morning the cbiastinenth precipitate is filtered 
off and washed with ammonium carbonate water. The filtrate 
now contains only a small amount of silicic acid. It is evaporated 
almost to dryness on the water-bath,{ diluted with a little waver 
and a few drops of phenolphthalein are added. The liquid is colored 
pink by the indicator and enough hydrochloric acid is now added 
to make it colorless. The solution is heated to boiling, and this 
causes the reappearance of the pink color. After cooling the color 
is again discharged with hydrochloric acid, and this operation 
is repeated until finally the addition of 1-1} c.c. of double- 
normal hydrochloric acid is sufficient to effect the decolorization. 
It is best to perform the operation in a platinum dish, but if this 
is lacking one of porcelain may be used. 





* Second edition. 

+ Before adding the ammonium carbonate, the greater part of the alkali 
carbonate should be neutralized with dilute hydrochloric acid, but. care 
should be taken not to make the solution acid. 

+ The liquid foams during the evaporation owing to the desaroaietia 
of the excess of ammonium carbonate; the evaporating-dish is covered with 
a watch-glass until the evolution of carbon dioxide ceases. 


HYDROFLUORIC ACID. 473 


The solution still contains traces of silicic acid, which are re- 
moved, as recommended by Berzelius, as follows: The solution is 
treated with 1 or 2 c.c. of ammoniacal zine oxide solution,* 
boiled until the ammonia is completely expelled and the precipi- 
tate of zinc silicate and oxide is filtered and washed with water. 
An excess of calcium chloride is added to the filtrate and the re- 
sulting precipitate, consisting of calcium carbonate and fluoride, is 
treated as described on page 471. | 

The calcium fluoride finally obtained should be tested for 
fluorine, for the addition of calcium chloride will almost always 
cause a precipitation (cf. page 471). which may consist of calcium 
fluoride and phosphate, or the latter only. After weighing the 
precipitate, it is treated with a few drops of concentrated sulphuric 
acid and covered with a watch-glass whose convex surface is 
coated with a thin layer of beeswax with a few lines scratched 
in the latter. The crucible is allowed to stand this way for 
twelve hours at the ordinary temperature. A little water is then 
poured upon the watch-glass and the crucible is heated over a tiny 
flame until the vapors of sulphuric acid begin to be evolved. If 
fluorine is present there will be a distinct etching of the glass 
where the wax coating was removed. 

The weight of the calcium fluoride obtained should stand in the 
same relation to that of calcium sulphate obtained after treatment 
with concentrated sulphuric acid, as 


CaF (78.09) :CaSO4(136.16). 


This relation does not hold exactly in practice, for it is almost 
impossible to obtain a precipitate of calcium fluoride absolutely 
free from silica. 

Remark.—By this method the fluorine present in all fluorides 
can be determined, e.g., in topaz, lepidolite, cryolite, etc. With 
a silicate containing much silica, the addition of silicic acid is 





* Moist zinc oxide is dissolved in ammonia water. The oxide is best 
prepare by dissolving chemically pure zine in hydrochloric acid, and pre- 


cipitating the zinc with potassium hydroxide; the precipitate is filtered and 
washed. 


474. GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


unnecessary, and the substance is at once fused with 4-5 times 
as much sodium-potassium carbonate; with silicates containing 
little silica, from 4-1 part of silicic acid is added. 

If the substance contains phosphoric acid, the fluorine cannot 
be determined by the above method, because the calcium fluoride 
precipitate is then contaminated with calcium | PADapi It is 


' then necessary to effect a 
> 


Separation of Phosphoric and Hydrofluoric Acids. 


The following method is that of Rose as modified by the author 
and A. A. Koch.* It is based upon the fact that silver phosphate 
is insoluble in water whereas silver fluoride is soluble. 

Procedure.—The alkaline solution of the two acidsf is carefully 
neutralized with nitric acid and then transferred to a 250 c.c. 
calibrated flask. A slight excess of silver nitrate is added, the 
solution diluted to the mark, thoroughly mixed and the precipitate 
allowed to settle completely. The solution is then filtered through 
a dry filter, but the first 10 c.c. of the filtrate are rejected, and the 
rest allowed to run into a dry beaker. Of this filtrate, exactly 
200 c.c. are transferred to a 250-c.c. flask again, the excess of the 
silver nitrate precipitated by the addition of some dissolved 
sodium chloride, the solution made up to the mark, well shaken 
and the precipitate allowed to settle completely. This solution is 
filtered, using the same precautions as before and the fluorine is 
determined in 200 c.c. of the filtrate as calcium fluoride according 
to the directions on p. 471. 

If the weight of the calcium fluoride dieripitate is p, that of 
the original substance is a, and z is the per cent. of fluorine origi- 
nally present, then 

76.03 p 


a na 





7 





* Z. anal. Chem., 48, 469 (1904). 
+ It is usually a matter of analyzing an aqueous solution of a sodium 
carbonate fusion. 


DETERMINATION OF FLUORINE AS SILICON FLUORIDE. 475 


Determination as Silicon Fluoride. 


This method, proposed by Fresenius, depends upon the fact 
that many fluorides are decomposed by the action of concentrated 
sulphuric acid and silica, while the fluorine escapes as silicon 
fluoride, which can be absorbed and weighed. . 

Procedure.—The same reagents and a very similar apparatus 
to that described on p. 477 is required for this determination, 
except that in place of the Péligot tubes (Fig. 80) two weighed 
U-tubes are used,* of which the first is filled with moistened pieces 
of pumice, and the second has one arm filled with soda lime and 
the other with calcium chloride. The analysis is carried out in 
exactly the same way as is described for the Penfield method 
(see below) but at the end of the experiment the two U-tubes are 
weighed. The increase in weight represents the amount of SiF4, 
and from this the amount of fluorine present is calculated as 
follows: Assume that a gms. of calcium fluoride yielded p gms. 
of Sify. The treatment with the concentrated sulphuric acid 
caused the following reaction to take place: 


2CaF,,+ 2H,SO,+ SiO, =2CaSO,+ 2H,0+SiF,, 


consequently the following proportion holds: 








SiF,:4F=p:s 
| s= Si, -p=gins. fluorine 
and in per cent. 
4F 
“SF, p=100:2% 
is 400F p 
Nay) eae 


or 


x=72.87- c =per cent. fluorine. 





* It is best to use U-tubes for the absorption which are scagreanil with 
ground-glass stoppers (see Fig. 59, p. 381). ny 


479 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Remark.—This method is suitable for the determination of 
fluorine in all fluorides which are decomposed by sulphuric acid. 
The analysis can be carried out in the presence of phosphates, but 
if carbonates are present they should be decomposed by ignition 
before the treatment with sulphuric acid. According to K. 
Daniel,* exact results are obtained only when the decomposition 
of the fluoride takes place at the temperature at which sulphuric 
acid boils. Pe 

For the determination of fluorine in topaz and micas, the 
method is not suitable. 


Determination .of. Fluorine as Hydrofluosilicic Acid, according 
| ito berate §. L. Penfield.} 


Modified byiTreadwell and Koch.t 


vil 


Principle.—Penfield expels the fluorine as silicon fluoride in 
exactly the same way as in the method of Fresenius (page 475), 
but the gas is absorbed in 50 per cent. alcoholic potassium chloride 
solution. By contact with water the silicon fluoride is decomposed 
into hydrofluosilicic and silicic acids. The former unites with the 
potassium chloride, forming potassium silicofluoride, insoluble in 
50 per cent. alcohol: 


H,SiF,+ 2KCl=K,SiF,+ 2HCl, 


and sets free an equivalent amount of hydrochloric acid; the 
latter is titrated with N/5 sodium hydroxide solution, with 
cochineal is an indicator. For the calculation the following pro- 
portion holds: ; 


1000 c.c. N/5 HCl= {’, mol. CaF, =F. 
*. lec. N/5 NaOH=0.0234 gm. CaF, or 0.0114 gm. F. 


rm; 





*Z,. anorg. Chem., 38, 257 (1904). 
+ Chem. News, 39, p. 179; also Am. Chem. Jour., 1, p. 27. 
»7. anal. Chem., 48, 469 (1994). 


' DETERMINATION OF FLUORINE AS HYDROFLUOSILICIC ACID. 477 


_ Requirements.—1. Pure Quartz Powder. Pieces of pure rock 
crystal are placed in a platinum crucible, heated strongly over the 
blast lamp, and then thrown into cold water. After this treatment 
it is very easy to reduce the quartz to a fine powder by grinding in 
an agate mortar. The powder is ignited, and while still warm is 
transferred to a flask, fitted with ground-glass stopper. The open 
flask and its contents are allowed to cool in a desiccator, after 
which the flask is stoppered and set aside. 

2. Sea Sand. The purest sea sand is treated with boiling, 
concentrated sulphuric acid, washed, dried, ignited, and cooled 
in a desiccator. 

3. Anhydrous Sulphuric Acid. Chemically pure concentrated 
sulphuric acid is heated in a porcelain crucible until it has fumed 
strongly for some time and then allowed to cool in a desiccator. 
over phosphorus pentoxide. 

Procedure.—The weighed sample of the fluoride is intimately 
mixed in an agate mortar, which is placed upon black glazed paper, 
with 1.5-2 gms. of the quartz powder and then transferred through 
the cylindrical arm A of the perfectly dry decomposition apparatus 
to the pear-shaped compartment B shown in Fig. 80. Then 
1.5-2 gms. of the sea sand are added, and mixed with the rest of 
the material by shaking the apparatus, which is then connected 
with the dry U-tube containing glass beads. The two Péligot 
tubes P and P; each contain between 10 and 15 e.c. of alcohol 
which is saturated with potassium chloride. When the apparatus 
is all connected as shown in the drawing, a dry current of air,+ 
free from carbon dioxide, is allowed to enter at h, and pass through 
_ the apparatus at the rate of 2 or 3 bubbles per second. Then 
without stopping the air current, about 20 c.c. of anhydrous sul- 
phuric acid is allowed to enter the decomposition apparatus 
through the funnel 7. By introducing the sulphuric acid in this 
way, while maintaining the air current, the sulphuric acid and the ~ 





* This tube serves to keep back any sulphuric acid that is carried over 
mechanically. 

{+ The air is passed through caustic potash solution, then over calcium 
chloride, and finally through concentrated sulphuric acid before entering 
the apparatus. 


478. GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


greater part of the silica and fluoride mixture is made to pass 
directly into the compartment B. After adding the sulphuric acid, 
the decomposition vessel is placed in a paraffin bath which is 
slowly heated to a temperature. of 130° to 140° C. The evolution 
of silicon tetrafluoride at once begins to take place, as is evident 
from the formation of foam. The passage of the air and heating 
of the bath is allowed to continue for five hours, at the end of 
which time the flame under the bath is turned down and air is 
passed through the apparatus for half an hour longer at the rate 





Laos ABA 
SILT ODEPTEOLTTEE PETE EE EEE | 


N ‘ 
\ N 
‘ i. 
N : 
; 
: q 
: ' 
J 


Vie SS 
it nittiit 
| 


SXLESSDSS 


UN a OM 








4 

RN) 
yj he NCCE NAYY 
OSELESEE Ls 





Fia. 80. 


of 3 to 4 bubbles per second. During the heating the apparatus 
should be frequently shaken in order that the sulphuric acid is 
brought into contact with all portions of the solid mixture. It is 
not necessary, however, with this arrangement of the apparatus, 
to shake as frequently as in the forms of apparatus described by 
Penfield and by Fresenius, because the air in its passage through 
the narrow connecting tube between A and B of the decomposition 
apparatus serves of itself to effect a good mixing. In order to 
accomplish this end, however, it is necessary to construct the — 
apparatus exactly as shown in Fig. 80; the connecting tube - 


DETERMINATION OF FLUORINE IN HYDROFLUOSILICIC ACID. 479 


between A and B must be so narrow that it is completely filled 
with the bubbles of air passing, and furthermore the parts marked 
c eb must form an inclined plane upon which the substance can 
readily pass back and forth. If there is a hollow in the apparatus 
at c eb, in which some of the substance can collect, the sulphuric 
acid may not come in contact with some of the fluoride so that the 
decomposition will be incomplete. Similarly it is necessary to 
guard against making the connecting tube c e too narrow, as other- 
wise the air will not pass in a uniform stream, but in spurts, so 
that in spite of the long tube D some of the sulphuric acid fumes 
are likely to reach the Péligot tubes and thereby give rise to high 
results. | 

If not more than 0.1 gm. of the fluoride was present, the action 
is over at the end of five and one-half hours, and this is evident, as 
Daniel * was the first to discover, from the fact that the foaming 
in the apparatus ceases; the hydrochloric acid which has been set 
free in the Péligot tubes can now be titrated. To this end a few 
drops of fresh cochineal ¢ solution are added to each tube and the 
contents are titrated with fifth-normal potassium hydroxide 
solution with frequent shaking, until the indicator changes from 
yellow to red. This is, however, by no means the correct end- 
point, because as Penfield observed, the gelatinous silicic acid 
encloses very appreciable amounts of hydrochloric acid. The 
silicic acid, therefore, must be thoroughly worked over with a 
stirring rod and the addition of the alkali continued until the 
color change is permanent. 

The results obtained by this method, using 0.1 gm. of substance, 
appear to be about 0.4 per cent. too high. The method can be 
~ used in the presence of phosphoric acid, but carbonates are iirst 
removed by ignition before the treatment with sulphuric 
acid. 





* Z. anorg. Chem., 88, 257 (1904). 

+ Instead of eoshisinal: methyl orange may be used, although it is neces- 
sary then to add an equal volume of alcohol before Pins the hydro- 
chloric acid. 


480 GRAVIMETRIC DE TERMINATION OF THE METALLOIDS. 


Determination of Fluorine in Mineral Waters. 


From 1 to 10 liters of the water (according to the amount of 
salts present) are evaporated in a large platinum or porcelain 
dish to a small volume, with the addition of enough sodium 
carbonate to keep the solution slighgly alkaline. Then an excess 
of calcium chloride is added, the liquid boiled, and the precipitate 
filtered and washed with hot water until free from chlorides. The 
precipitate is dried, transferred as completely as possible to a . 
platinum dish, and the ash of the filter added to the main precipitate 
which is then gently ignited. This residue contains all of the 
fluorine as calcium fluoride; besides considerable calcium (possible 
strontium) and magnesium carbonates; iron, aluminium and 
manganese oxides; often barium sulphate; and almost invariably 
some calcium phosphate. It is treated with an excess of d lute 
acetic acid, allowed to stand for some time with frequent stirring, 
and then evaporated to dryness on the water bath. This residue 
is treated with water, filtered, and washed with hot water. As 
much of it as possible is transferred to a platinum crucible, the 
ash of the filter added, and the contents of the crucible gently 
ignited. From 4-2 gms. of ignited quartz powder are then 
intimately mixed with the residue in an agate mortar. The 
mixture is transferred to the decomposition vessel A, Fig. 80, 
and treated with concentrated sulphuric acid exactly as described 
on p. 477 by the method of Penfield. As only very little fluorine 


is present in this case, two small U-tubes are used instead of the. . 


large Peligot tubes shown in Fig. 80. 

Remark.—The formation of a precipitate in the first U-tube 
at the place marked aa in Fig. 80 indicates the presence of fluorine. 
It is well to confirm it by the etching-test. After carrying out the 
titration of the hydrochloric acid set free, the contents of the U-tube 
are transferred to a platinum dish, a few drops of double normal 
sodium carbonate added, and the solution evaporated to dryness. 
Ammoniacal zine oxide is then added (ef. p. 473), the liquid again 
removed by evaporation, the residue taken up in water, and the 


\ 


DETERMINATION OF FLUORINE. 481 


zine oxide and silicate filtered off. The filtrate is treated with 
calcium chloride as described on p. 473 and the etching tes 
applied. ; 
In the former editions of this book it was recommended to 
determine the fluorine in mineral waters by evaporating a very 
much targer volume of the water nearly to dryness, filtering, and 
then examining the insoluble residue alone for fluorine. In this 
way the fluorine content of many mineral waters was entirely 
overlooked. 


Gas-Volumetric Determination of Fluorine according to Hempel 
and Oettel. 


See Part III, Gas Analysis. 


Separation of Fluorine. 


(a) From the Metals. 


For the determination of the metals present, the fluorine usually 
can be removed by heating with concentrated sulphuric acid; 
in the case of many silicates containing fluorine, however, e.g., 
topaz, lepidolite. and other micas, this treatment will not accom- 
plish the desired result. In such cases the mineral is fused with 4 to 
6 times as much sodium-potassium carbonate, the melt is extracted 
with water, the silica and aluminium precipitated from the solu- 
tion obtained by means of ammonium carbonate (see page 472). 
and these two substances determined in the residue, while the filtrate 
is used for the fluorine analysis. The metals and the remainder of 
the silicic acid are determined in the residue obtained on extracting 
the melt with water (cf. p. 493). The estimation of the alkalies 
must be undertaken in a separate portion of the substance (pp. 
497-501). 


482 © GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


(6b) Separation of Fluorine from the Acids. 


“4. Determination of Hydrochloric and Hydrofluoric Acids in the 
Presence of One Another. 


In the case of soluble alkali«salts, the fluorine is first pre< 
-cipitated from the solution by means of a little sodium car- 
bonate and an excess of calcium nitrate solution, as described on 
p. 471. The filtrate is acidified with nitric acid and the chlorine 
determined by precipitation with silver nitrate, according to 
~p. 320. 

It is simpler to treat the solution containing hydrochloric and 
hydrofluoric acids in a platinum evaporating dish with nitric acid 
and silver nitrate. Silver chloride is alone precipitated and can 
be filtered off, using a funnel of hard rubber, or a glass one coated 
over with wax. The precinitate is washed and weighed as 
described on p. 320. When phosphoric acid also is present, this 
is precipitated with the hydrochloric acid by the addition of 
silver nitrate to the slightly alkaline solution, the precipitate 
is filtered off, washed with as little cold water as possible, and 
the precipitate treated with dilute nitric acid. By this means 
the silver phosphate goes into solution, while the silver chloride 
is unaffected. In order to determine the amount of phosphoric 
acid present, the silver is removed from the solution by the 
addition of hydrochloric acid, and the phosphoric acid is 
precipitated in the filtrate by addition of magnesia mixture and 
ammonia (cf. p. 434). 

In the filtrate from the silver phosphate and silver chloride 
precipitate, the excess of silver nitrate is removed by the addi- 
tion of sodium chloride and the fluorine is determined as calcium 
fluoride. 

In the case of an insoluble compound containing chlorine 
and fluorine, the melt obtained after fusing with sodium- 
potassium carbonate is extracted with water, the silica is removed — 
with ammonium carbonate and zinc-ammonium hydroxide as 


H YDROFLUOSILICIC ACID. 483 


described on pp. 472-3, and the chlorine and fluorine determined 
as above. | 

In a majority of cases it is more convenient to determine the 
two acids in separate portions of the substance. 


2. Determination of Boric and Hydrofluoric Acids. 


The solution containing the alkali salts of these two acids is 
precipitated at the boiling temperature by means of an excess of 
calcium chloride ; the precipitate is filtered off and washed with 
hot water. | 

The precipitate, consisting of calcium carbonate, calcium 
fluoride, and some calcium borate, is gently ignited, treated with 
dilute acetic acid, evaporated to dryness, and more acetic acid and 
water are added. By this means the calcium acetate and cal- 
cium borate go into solution, while the calcium fluoride is left 
behind and is determined as described on p. 471. For the boric 
acid determination a second portion of the solution is taken, 
made barely acid with acetic acid, and treated with a slight 
excess of calcium acetate solution in order to precipitate the 
fluorine. The solution, together with the calcium fluoride, is 
placed in the Gooch retort and subjected to distillation as 
described on p. 428. 


HYDROFLUOSILICIC ACID, H,SiF,. Mol. Wt. 144.32. 


Forms: Calcium Fluoride, CaF,; Potassium Silicofiuoride; 
or volumetrically. 


1. Determination as Calcium Fluoride. 


Principle.—Alkali fluosilicates are decomposed on heating ‘ 
with sodium carbonate solution into fluoride and silicic acid: 


Na. SiF, +2Na,CO,+ H,O=6NaF + H,SiO, + 2CO,,. 


If a solution is to be analyzed containing free hydrofluosilicic 
acid or its sodium salt, it is treated with sodium carbonate solu- 


484. GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


tion until the reaction is alkaline, a considerabie amount of ammo- 
nium carbonate is added, the solution heated to about 40° C., » 
and, after standing twelve hours, the precipitated silicic acid is 
filtered off. 

The solution now contains all the fluorine as sodium fluoride, 
in the presence of small amounts of silicic acid, which are pre- 
cipitated by the addition of zinc-ammonia hydroxide (see p. 473). 
In the filtrate the fluorine is determined as calcium fluoride, as 
described on p. 471. 

An insoluble fluosilicate is fused with four times as much 
sodium-potassium carbonate, the melt extracted with water, and 
the solution subjected to the above treatment. 


2. Determination as Potassium Silicofluoride. 


This analysis is only applicable for the determination of free 
hydrofluosilicie acid in aqueous solution. 

Procedure.—The solution is treated with potassium chloride 
and an equal volume of absolute alcohol. The barely-visible 
potassium silicofluoride is filtered through a tared filter which 
has been dried at 100°C. After washing with 50 per cent. alcohol 
the precipitate is dried at 100° C. and weighed as K,SiF,. 

The volumetric determination of hydrofluosilicic acid will be 
discussed in Part II. 


Analysis of Salts of Hydrofluosilicic Acid. 


For the determination of the metal present, the salt is treated 
with concentrated sulphuric acid in a platinum dish and heated until 
dense fumes of sulphuric anhydride are given off; silicon fluoride 
and hydrofluoric acid volatilize, while the metals are left behind 
as sulphates (cf. Vol. DI). 


Determination of Water Present in Fluosilicates, (Rose-Jannasch).* 


The water cannot be determined by ignition, because all fluo- 
silicates, even topaz, evolve. silicon fluoride when subjected to 
this treatment (cf. Vol. I, p. 355). If, as proposed by Rose, the 





* Rose-Finkener: Lehrbuch der analyt. Ch., Bd. II; and Jannasch, Prak- 
tischer Leitfaden der Gewichtsanalyse, Leipzig, 1897, p. 243. 


SILICIC ACID. 485 


substance is fused with six or eight times as much lead oxide, all 
the water is evolved, while the fluorine remains behind: 


R,SiF,+3PbO=2RF+2PbF, + PbSid,. 


The analysis is best performed according to the directions of 
Jannasch: A bulb with a capacity of about 25 c.c. is blown near 
one end of a tube of difficultly fusible glass which is 26 cm. long 
and 1 cm. wide. Near the middle of the longer side of the tube 
is placed, between asbestos plugs, a layer 3 to 5 cm. long of pulver- 
ized, anhydrous lead oxide, and this end of the tube is connected 
with two weighed calcium chloride tubes. The substance is placed 
in the bulb, after which six or eight times as much lead oxide is 
added and mixed with the substance by carefully revolving the tube. 
A dry current of air is now conducted through the apparatus and 
the contents of the bulb are slowly melted. All of the water and 
often some of the fluorine is thereby expelled, and the latter is 
absorbed by the layer of lead oxide. At the end of the operation 
this layer is cautiously heated with a moving flame until no more 
water condenses in the cooler part of the tube. When all of the 
water has been driven over into the calcium chloride tubes the 
latter are weighed with the customary precautions. 


GROUP VII. 


SILICIC ACID (ALSO TITANIC, ZIRCONIC, TANTALIC, AND 
- NIOBIC ACIDS). 


SILicic AcID, H,SiO,. Mol. Wt. 78.32. 
Form: Silicon Dioxide, SiO,. 


- Two cases must be considered: 
(a) The silicate is decomposed by acids. 
(b) The silicate is not decomposed by acids. 


(a) Silicates Decomposed by Acids. 


These are treated with hydrochloric acid in a porcelain dish 
and evaporated upon the water-bath with frequent stirring until 
the residue is obtained in the form of a dry powder. In many 
cases the decomposition is shown to be complete by the fact that 


486 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


no gritty particles can be felt with the stirrinz-rod on the bottom 
of the dish. If, however, the substance contained quartz or some 
silicate that is not decomposed by hydrochloric acid, this is not 
the case and the procedure described on p. 507 may be followed. 

The dry powder is moistened with concentrated hydrochloric . 
acid and the covered dish is allowed to stand for 10 or at the most 
20 minutes at the ordinary temperature, in order that basic salts 
and oxides formed during the eVaporation and drying may be 
once more changed to chlorides. Then 100 c.c. of water are 
added, it is heated to boiling, and after the silicic acid has been 
allowed to settle, the clear liquid is decanted through a filter sup- 
ported upon a platinum cone placed in the apex of the funnel. The 
residue is washed 3 or 4 times with hot water by decantation, then 
transferred to the filter and washed with hot water until free from 
chloride.* The precipitate is then dried by means of suction, 
placed in a platinum crucible, and set aside for the time being. 
The separation of the silicic acid is now by no means quantitative; 
as much as 5 per cent. of the total amount may remain in the 
filtrate. In order to remove this, the solution is once more evap- 
orated to dryness on the water-bath, kept at this temperature 
for one or two hours (or more), moistened with a few cubic cen- 
timeters of concentrated hydrochloric acid, and allowed to stand 
not more than fifteen minutes.t Hot water is then added, the 
residue is filtered through a new and correspondingly small filter, 
and washed with hot water. The amount of silicic acid now 
remaining in the filtrate amounts to not more than 0.15 per cent. 
of the total amount, and for most purposes can be neglected. It 
can be removed, however, by a third evaporation to dryness. 
The filters containing the silica are ignited wet in a platinum 
crucible and finally over the blast-lamp, and weighed.{ The 
silica obtained is only slightly hygroscopic. 








* If the precipitate is not perfectly white, but somewhat brownish owing 
to the presence of a basic ferric salt, concentrated hydrochloric acid is allowed 
to run around the upper edge of the filter and is immediately washed down 
through the funnel by means of a stream of hot water. This is repeated 
until the filtrate comes through perfectly colorless. 

+ By being kept in contact with the acid for too long a time some silicic 
acid will go into solution. 

¢ With regard to the temperature at which silica is completely dehydrated, 


SILICIC ACID. a as 


Testing the Purity of the Silica. 

The silica thus obtained is never absolutely pure, except in 
the analysis of a water-glass. Its purity must always be tested. 
For this purpose it is covered with 2 or 3 c.c. of water,* a drop of 
concentrated sulphuric acid is added, and 3 to 5 ¢.c. of pure hydro- 
fluoric acid (distilled from a platinum retort). The crucible is then 
placed in a platinum-lined cone (Fig. 16, p. 31) on the water-bath 
and evaporated under a good hood until no more vapors are 
expelled. The excess of sulphuric acid is then removed by heat- 
ing over a free flame. The temperature is raised and the crucible 
is finally heated over a blast-lamp, after which it is again weighed. 
This process is repeated until the contents of the crucible (usually 
AlsOs and Fe2O3) are at a constant weight, and this amount is 
deducted from the weight of impure silica. 

Remark.—In order to make the separation of silicic acid more 
nearly quantitative it has been proposed to heat the residue 
obtained by evaporation at 110°-120°C.t+ This hastens the 
dehydration, but the temperature of 120° should not be 
exceeded on account of the danger of the silicic acid being 
contaminated with basic salts, and because of the tendency for 


there is a difference of opinion. Lunge and Millberg (Zeit. f. angew. Chem., 
(1897), p. 425) state that the temperature of the Bunsen burner is sufficient, 
but they operated with silica obtained by the hydrolysis of silicon tetra- 
chloride, in order to obtain a product absolutely free from alkalies. Hille- 
brand (Am: Chem. Soc., XXIV (1902), p. 362) confirmed the results 
of Lunge and Millberg with regard to the ignition of a silica obtained 
in this way, but positively asserts that silicic acid when obtained by the decom- 
position of an alkali silicate with acid must be ignited over the blast-lamp 
in order to dehydrate it completely... The results of Hillebrand have been 
confirmed in the author’s laboratory by A. Schréter. Jordis and Kanter 
(Z. anorg. Chem., 35, 20 (1903) ) state that hydrated silica forms a small 
amount of a chlorine compound with hydrochloric acid. This compound 
is decomposed on heating only with great difficulty, unless it is treated with 
water and evaporated to dryness several times. This treatment is recom- 
mended by Jordis and Kanter in all accurate silica determinations. 

*If the water is not added, the mass will effervesce so strongly that 
there is danger of losing some of the impure silica. 

+ James P. Gilbert, Tech. Quarterly, III, p. 61, and Zeit. fiir anal. 
Chem., X XTX (1890), 688. 





488 _ GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


it to recombine with these.* It is, therefore, not advisable to 
attempt to dehydrate the silica at a temperature higher than that 
of the water-bath. 


(b) Silicates Not Decomposed by Acids. 
These must be fused; this can be effected by 


(a) The Sodium Carbonate Method. 


One gram of the very finely powdered substance is placed in 
a spacious platinum crucible together with 4 to 6 parts of calcined 
sodium carbonate (or a mixture of equal parts sodium and potas- 
sium carbonates) and fused. The powdered silicate should be 
intimately mixed with the flux and a little sodium carbonate 
sprinkled on top, the crucible covered and heated for some time 
over a small flame in order to drive out any moisture present. 
The temperature is raised gradually until finally the highest heat 
of a good Teclu burner is obtained; or, lacking the latter, a blast- 
lamp should be used. As soon as the mass melts quietly and 
there is no further evolution of carbon dioxide, the decompo- 
sition is complete. ‘The crucible is seized with a pair of crucible 
tongs having platinum points and placed in cold water, but so 
that the water does not enter the crucible. By means of this . 
rapid cooling the ‘melt is usually detached from the sides of the 
crucible and can be removed by simply turning the crucible up- 
side down and gently tapping its sides.t| The melt is received 
in a good-sized beaker, covered with water, a sufficient quantity 
of strong hydrochloric acid is added, and the beaker covered with 
a watch-glass. A lively evolution of carbon dioxide at once takes 





* When considerable magnesium was present, more silica was found in 
the filtrate after igniting at 280° than when dried on the water-bath. This 
is due to the fact that magnesia formed by hydrolysis reunites with the silica 
to form magnesium silicate, and the latter is decomposed by hydrochloric 
acid with the formation of soluble silicic acid. 

+ Instead of the cold water a blast of air may be used. Then, when the 
melt is still warm but not hot enough to cause spattering, the addition of 
water enough to cover it will usually help to loosen the melt from the sides 
of the crucible. Hillebrand recommends the rotation of the contents of the 
crucible while still molten so that a relatively thin layer solidifes on the © 
sides and bottom of te crucible. 


SILICATES NOT DECOMPOSED BY ACIDS. 489 


place, but in proportion as silicic acid separates out, the inner 
part of the cake gradually becomes coated with a film of silicic 
acid which protects it from the further action of the acid. Con- 
sequently it is necessary to break up the cake from time to time 
by means of a glass rod until finally there is no further evolution 
of a gas and no more hard lumps remain. When manganese 
is present the melt is colored green and the solution is pink. The 
latter is heated until this pink color disappears and is then 
transferred to a platinum dish (or lacking this, one of porcelain 
may be used). The small amount of the melt adhering to the 
sides of the crucible is transferred to the contents of the dish by 
means of water and hydrochloric acid. The solution is then 
analyzed as described on page 485. 

Remark.—lf the fusion cannot be removed from the crucible, 
it is placed, together with its cover, in the beaker and treated as 
above. 

In this case, if the melt was very green-colored, it should not 
be decomposed with hydrochloric acid, but with nitric acid, for 
the chlorine evolved by the action of the hydrochloric acid upon 
the manganate would attack the platinum. 

Substances containing considerable fluorine cannot be treated 
as above, for silicon fluoride will be lost by volatilization. In 
this case it is necessary to use the old method of Berzelius. The 
melt from the sodium carbonate fusion is extracted with water, 
as in the determination of fluorine (p. 472), and the greater part 
of the silica removed by means of ammonium carbonate. The 
precipitate is filtered off, ignited, and weighed. 

_ The silicic acid remaining in the filtrate is precipitated by 
means of ammoniacal zine hydroxide. The precipitate thus ob- 
tained, consisting of zinc oxide and zinc silicate, is decomposed 
with hydrochloric acid and the silica obtained by evaporation 
on the water-bath as usual. As a rule, the insoluble part of the 
melt contains silicic acid, and this must also be removed by evapo- 
ration with hydrochloric acid. All three silica precipitates are 
ignited together and the purity of the silica tested. . 

Eesides the sodium carbonate method for the analysis of 
silicates not decomposable with acids a great number of other 
methods have been proposed, but of these only the following will 
be mentioned here. 


49° GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


(G) Lead Oxide Method of Jannasch.* 


This analysis is interesting because it permits of an exact 
determination of the alkalies and of silicic acid in the same sample. 

Inasmuch as commercial lead oxide (litharge) is not free from 
impurities, it is prepared for the analysis by the ignition of pure 
lead carbonate. 

The lead carbonate is prepared by adding the theoretical 
amount of ammonium carbonate to a boiling solution of lead 
acetate. The precipitate is washed several times by decantation 
with hot water, then transferred to a hardened filter, and com- 
pletely washed, using suction. The mass is finally carefully re- 
moved from the filter-paper and dried on the water-bath. 

Procedure.—For each gram of the silicate 10-12 gms. of lead 
carbonate are used. First of all a little lead carbonate is placed 
in the crucible, then the very finely powdered substance, and after 
mixing thoroughly with a platinum spatula the covered crucible 
is heated for fifteen to twenty minutes over a flame which is 
not more than 3-4 cm. high, by which means the greater part 
of the carbon dioxide is expelled. The contents of the crucible 
are then more strongly heated until fusion is effected, taking care 
that the flame used is strictly non-luminous; the lower third of 
the crucible, and no more, may be heated to redness. 

After fusing for ten to fifteen minutes the decomposition is 
complete, and the covered crucible is quickly touched into cold 
water, but so that.its contents remain dry. The melt is placed in 
a platinum dish, covered with hot water and a sufficient quantity 
of concentrated nitric acid and evaporated on the water-bath, 
breaking up the melt with a stirring-rod as much as_ possible. 
When the cake is completely disintegrated, as is shown by there 
remaining no more hard yellow pieces and only slightly colored 
flocks of silicic acid floating in the liquid, the latter is evaporated 
on the water-bath until a dry powder is obtained; this is moist- 
ened with concentrated nitric acid and once more evaporated 





* Gaston Bong, Zeit. fiir anal. Chem., XVIII (1879), p. 270, first pro- 
posed that silicates be decomposed by fusion with red lead (Pb,O,), but 
Jannasch in his Praktisches Leitfaden der Gewichtsanalyse has greatly 
improved the method. 


ANALYSIS OF SILICATES—ORTHOCLASE. ‘491 


to complete dryness. The dry residue is moistened with 20 e.c. of 
concentrated nitric acid, and allowed to stand fifteen minutes; 100 
c.c. of water are added, and the liquid is heated for twenty 
minutes on the water-bath. The residue of silicic acid is filtered 
off, washed first with hot water containing nitric acid, then with 
pure water, and weighed after the usual ignition. 

Remark.—In the analysis of minerals containing fluorine, e:g. 
topaz, Jannasch finds that the results obtained are about 0.5-1 
per cent. lower than when the Berzelius method is used. In such 
a case this method of decomposition is used only for the deter- 
mination of the metals and of the alkalies, after introduction of 
hydrogen sulphide and removal of the lead. 


ANALYSIS OF SILICATES, 
_ Orthoclase. 


Constituents: silicic acid (63-70 per cent.); aluminium oxide 
(16-20 per cent.); ferric oxide (0.3 per cent.); potassium oxide 
(8-16 per cent.); sodium oxide (1-6 per cent.); and often small 
amounts of calcium oxide, magnesium oxide, and in rare cases 
_barium and ferrous oxides. 


Preparation of the Substance for Analysis. 


The substance is placed upon a thick steel plate within a steel 
ring (about 2 cm. high and 6 cm. in diameter) and broken into 
small pieces by means of a hardened steel hammer; the pieces are 
then reduced to a coarse powder. The latter is placed in an 
agate mortar in small portions and ground as fine as possible 
and preserved in a glass-stoppered bottle. In this way from 5-6 
gms. of powder are obtained. 

By this means, as proposed by Hillebrand, there is less danger 
of contaminating the substance with small particles of iron than 
when a so-called steel mortar is used, especially after the latter 
has been worn rough on its inner surface. Further, the practice 
of passing the powder through bolting-cloth is to be «voided when 
possible, as in this way the substance becomes contaminated with 


492 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


fibres of cloth and too large an amount of ferrous iron will be 
found. 


Weighing the Substance. 


It is customary to dry the powder before weighing at 100- 
110° C. until a constant weight is,obtained. If there is danger of 
losing combined water by this procedure, it has been recommended 
to dry the powder in a vacuum over concentrated sulphuric acid. 
The practice of drying the substance in either of the above ways 
is, however, to be discountenanced. It is far better to use the air- 
dried substance for the analysis, and to determine the moisture in a. 
separate sample. This is more accurate, because the dry silicate 
powder is hygroscopic, so that a portion weighed out to-day is 
likely to contain a different amount of moisture than one taken 
to-morrow, and this is not the case when the air-dried powder is 
taken for the analysis. Further, as Hillebrand has conclusively 
shown, chemically combined water is not only likely to be ex- 
pelled by heating at 100° C., but also by drying in a vacuum 
over sulphuric acid. This is particularly true of the zeolites. In 
the case of orthoclase, however, only about 0.1 per cent. of moist- 
ure is present, so that in this particular case accurate results will 
be obtained by either method. 

For the analysis two portions must be taken, each amounting 
to about 1 gm. in weight. The first serves for the determination 
of SiO,, Al,O,+ Fe,O,, CaO; and MgO; the second for that of the 
alkalies. 


Determination of Silica, Aluminium, etc. 


About 1 gm. of the air-dried substance is placed in a spacious 
platinum crucible, dried for one hour at 105°-7° C., cooled in a 
desiccator, and weighed. The difference in weight represents the 
amount. of hygroscopic moisture. 

The dry substance is mixed with 4 to 5 gms. of calcined sodium 
carbonate by means of a platinum spatula, and the silicie acid is 


DETERMINATION OF ALUMINIUM AND FERRIC OXIDES. 493 


determined exactly as described on p. 488.* The silica obtained 
is treated with sulphuric and hydrofluoric acids, as described on 
p. 487, and the residue of Al,O3 in the crucible is placed at one 
side Gi: the present. 


Determination of Aluminium and Ferric Oxides. 


The filtrate from the silicic acid contains, besides the chlorides 
of aluminium, iron, calcium, and magnesium, weighable amounts 
of platinum, partly coming from the crucible in which the fusion 
was made, and partly from the action of the ferrie chloride and | 
hydrochloric acid upon the platinum dish in which the evapora- 
tion took place (cf. p. 110, foot-note). 

To remove the platinum, the solution is heated to boiling and 
hydrogen sulphide is passed into it. The mixture of platinum 
sulphide and sulphur is filtered off and the solution is boiled to 
expel the excess of hydrogen sulphide. The iron is then completely 
oxidized back to the ferric state by the addition of bromine water 
and boiling until the excess of the latter is expelled. After this 
about 10 ¢c.c. of double-normal ammonium chloride solution are 
added and the boiling-hot solution is precipitated by the addition of 
a slight excess of ammonia, free from carbonate (ef. p. 149, Remark). 





_* Formerly a single evaporation of the melt with hydrochloric acid was 
made, and it was assumed that the silica remaining in solution was quan- 
titatively precipitated with the iron and aluminium by the addition of ammo- 
nia. After obtaining the weight of the ignited ammonia precipitate it was 
fused with potassium pyrosulphate and the melt taken up in the dilute sul- 
phurie acid; the residual silica was filtered off and weighed. The filtrate 
was analyzed as above described. Hillebrand has recently shown that this 
procedure is inaccurate. In the first place, the silica remaining in solution 
is not completely thrown down with the iron and aluminium precipitate, 
‘and in the second place the silicic acid is not absolutely insoluble in dilute 
sulphuric acid. Hillebrand found that from a solution containing 0.20 
gm. Al,O, and 0.0101 gm. SiO, as much as 0.0007 gm. SiO, could be detected 
in the filtrate from the ammonia precipitate. From the potassium pyro- 
sulphate melt he succeeded in obtaining, according to the old method, only 
0.0033 gm. SiO,, while he obtained, by evaporating the solution until fumes 
of sulphuric acid came off and subsequently diluting with water, as much 
‘as 0.0060 gm. SiO,, or about twice as much as was at first insoluble. 


494. GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


The precipitate is allowed to settle, after which it is filtered, and 
washed twice by decantation with hot water. It is then dissolved 
by running hot dilute hydrochloric acid through the filter into the 
beaker containing the greater part of the precipitate. The precipi- 
tation with ammonia is repeated as before, and after filtering and 
washing by decantation, the precipitate is transferred to the filter 
and washed until free from chloride with water containing am- 
monium nitrate. The precipitate i allowed to drain as completely 
as possible, and is ignited wet in the crucible containing the residue 
obtained from the treatment of the impure silica with sulphuric 
- and hydrofluoric acids. After igniting strongly over a good Teclu 
burner (or the blast-lamp) the crucible is weighed; its contents 
represents the sum of the aluminium and ferric oxides, 

For the determination of the ferric oxide, the mixed oxides 
are fused with potassium pyrosulphate as described on p. 109. 
The decomposition is complete after two to four hours. The melt 
is dissolved in water containing a little sulphuric acid and the 
iron is determined, after previous reduction with hydrogen sulphide, 
by titration with potassium permanganate (cf. p. 99). If the 
weight of the Fe,O, is deducted from the weight of Fe,O,+Al,O,, 
the weight of Al,O, is obtained.* 


Determination of Calcium. 


The combined filtrates from the ammonia precipitate are 
evaporated to a small volume, heated to boiling, and precipitated 
by means of a boiling solution of ammonium oxalate. After 
standing twelve hours the calcium oxalate is filtered off, and with 
small amounts of calcium this precipitate is ignited wet in a platinum 
erucible and weighed. If, however, considerable calcium is present, f 





* The amount of iron and aluminium can be determined more quickly, 
though less accurately, as follows: The moist ammonia precipitate is dis- 
solved in hot dilute sulphuric acid and diluted to a volume of exactly 250 
c.c After thoroughly mixing, 100 c.c. are removed by means of a pipette 
into a beaker and a second portion of the same volume is placed in a 200- 
c.c. flask. In the first portion the sum of Fe,O,+Al,0,; is determined by 
precipitating with ammonia, filtering, igniting, and weighing; in the other 
portion the iron is reduced by hydrogen sulphide and then titrated with 
permanganate 

+ Cf. pp. 76-78. 


DETERMINATION OF MAGNESIUM. 495 


the moist precipitate is redissolved in hydrochloric acid, and again 
precipitated by the addition of ammonia and a little more am- 
monium oxalate. The precipitate is ignited strongly, and weighed 
as CaO. (Cf. p. 70.) 


Testing of the Calcium Oxide Precipitate for Barium. 


Although it is usually unnecessary to make either a qualitative 
or quantitative test for barium in a sample of orthoclase, yet it 
is likely to be present in traces so that it may be well to show 
how this can be done. As far as the author knows strontium 
has never been found in orthoclase. On account of the solubility 
of barium oxalate in a solution of ammonium oxalate, the barium 
will rarely be found in the calcium precipitate when a double pre- 
cipitation is made, except when it is present to an extent of more 
than 3 or 4 mgms.* 

To test the calcium precipitate for barium, it is dissolved in 
nitric acid, evaporated to dryness, and heated for some time at 140° 
C. The calcium nitrate is dissolved out by ether-alcohol (p. 79, a), 
and any residue remaining behind is tested in the spectroscope for 
barium. If an appreciable amount of the latter is found, the 
calcium must be determined in the ether-alcohol extract. It is 
carefully evaporated to dryness, the residue dissolved in a little 
water and precipitated as before by the addition of ammonium 
oxalate. After standing twelve hours the precipitate is filtered 
off, washed, ignited, and weighed. . If no barium is found with 
the lime, it is by no means safe to conclude that the former is 
absent; it can very well have gone into the filtrate from the 
double precipitation of calcium. This amount will be precipitated 
with the magnesium as barium phosphate unless it is removed as 
indicated below. ! 

For the quantitative determination of barium a separate 
portion of the substance is taken (see below). 


Determination of Magnesium. 


The combined filtrates from the calcium oxalate are evapo- 
rated to dryness, ignited in a porcelain dish, and the residue dis- 





* W. F. Hillebrand, Journ. Am. Chem. Soc., 16 (1894), p. 83. 


496 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


solved in water to which a few drops of hydrochloric acid have 
been added. The carbonaceous residue is filtered off, a drop of 
sulphuric acid added, and the solution is allowed to stand twelve 
hours to see if any precipitate of barium sulphate will form. In 
the iatter case, the precipitate is filtered off and tested for barium 
according to Vol. I, p. 62; in the filtrate from the barium sulphate 
the magnesium is determined as described on page 65. 


> 
Determination of Barium. 


If the qualitative tests have shown the presence of barium, 
a larger sample of the substance is weighed out (about 2 gms.) 
moistened in a platinum dish with 10 ¢.c. of sulphurie acid (1:4) 
and 5 c.c. of hydrofluoric acid are added. The liquid is evapo- 
rated on the water-bath, with frequent stirring, until the mineral 
is completely decomposed, which is recognized by there no longer 
being any sandy particles perceptible on stirring with a platinum 
spatula. Frequently a further addition of hydrofluoric acid is 
necessary. When the decomposition is complete, the greater 
part of the sulphuric acid is removed by heating the contents of 
the dish in an air-bath. After cooling, the residue is taken up in 
water, and the barium sulphate is filtered off, and ignited wet 
in a platinum crucible. ‘The precipitate thus obtained always 
contains small amounts of calcium sulphate which must be elim- 
inated. To accomplish this, the residue in the crucible is dis- 
solved in a little hot concentrated sulphuric acid, and after cool- 
ing the solution is diluted with cold water. The barium sulphate 
is now completely free from calcium; it is filtered off, ignited, 
and weighed. 


Determination of the Alkalies. 
(a) Method of J. Lawrence Smith.* 


Principle-—The substance is heated with a mixture of 1 part 
ammonium chioride and 8 parts calcium carbonate. By this 
means the alkalies are obtained in the form of chlorides, while the 
remaining metals are for the most part left behind as oxides, 





- * Am. Jour. Science [2], 50, 269, and Ann. d. Chem. u. Pharm., 159, 82 
(1871). 


DETERMINATION OF THE ALKALIES. 497 


and the silica is changed to ealchum silicate, as represented by the 
following equation: ; 


2K AISi,0,+ 6CaCO,+2NH,Cl= 
=6CaSiO,+ 600,+ Al,O,+2KCl+2NH,+ H,0. 


The alkali chlorides together with calcium chloride can be 
removed from the sintered mass by leaching with water, while 
the other constituents remain undissolved. 

Preparation.—The ammonium chloride necessary for the de- 
termination is prepared by subliming the commercial salt; the 
calcium carbonate by dissolving the purest calcite obtainable in 
hydrochloric acid and precipitating with ammonia and ammonium 
carbonate. This last operation is performed in a large porcelain 
dish. After the precipitate has settled, the clear solution is poured 
off and the precipitate is washed by decantation until free from 
chlorides. The produet thus obtained contains traces of alkalies, 
but the amount present is determined once for all by a blank 
test and a corresponding deduction made from the results of the 
analysis; it is usually sodium chloride and amounts to 0.0012- 
0.0016 gm. for 8 gms. calcium carbonate. The decomposition 
was performed by Smith in a finger-shaped crucible about 8 cm. 
long and with a diameter of about 2 cm. at the top and 14 cm. at 
the bottom. Such a crucible is suitable for the decomposition 
of about 0.5 gm. of the mineral. A larger quantity can be analyzed 
in a somewhat wider crucible. 

Filling the Crucible-—About 0.5 gm. of the mineral is mixed 
with an equal quantity of sublimed ammonium chloride by trit- 
uration in an agate mortar, then 3 gms. of calcium carbonate 
are added and intimately mixed with the former. The mixture 
is transferred to a platinum crucible with the help of a piece of 
glazed paper, and the mortar is rinsed with one gram of calcium 
carbonate, which is added to the contents of the crucible. 

The Ignition—The covered crucible is placed in a slightly 
inciined position and gradually heated over a small flame. until 
no more ammonia is evolved* (this should take about fifteen 





* During this part of the operation the heat should be kept so low that 
ammonium chloride does not escape. The latter is dissociated into ammonia 
and hydrochloric acid by the heat, and the acid unites with the calcium 
carbonate to form calcium chloride.—[Translator.] 


498 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


minutes), then the temperature is raised until finally the lower 
three-fourths (and no more) of the crucible are brought to a 
dull red heat, and this temperature is maintained for 50-60 
minutes. The crucible is then allowed to cool and the sintered 
cake usually can be removed by gently tapping the inverted cru- 
cible. Should this not be the case, it is digested a few minutes 
with water, which serves to soften the cake so that it can be readily 
washed into a large porcelain, of, better, platinum dish. The 
covered dish is heated with 50-75 c.c. of water for half an hour, 
replacing the water lost by evaporation, and the large particles 
are reduced to a fine powder by rubbing with a pestle in the dish. 
The clear solution is decanted through a filter and the residue is 
washed four times by decantation, then transferred to the filter 
and washed with hot water until a few cubic centimeters of the 
washings give only a slight turbidity with silver nitrate. To 
make sure that the decomposition of the mineral has been .com- 
plete, the residue is treated with hydrochloric acid; it should 
dissolve completely, leaving no trace of undecomposed mineral. 

Precipitation of the Calcitum.—The aqueous solution is treated 
with ammonia and ammonium carbonate, heated and filtered. 
As this precipitate contains small amounts of alkali, it is redis- 
solved in hydrochloric acid and the precipitation with ammonia 
and ‘:ammonium carbonate is repeated. The combined filtrates 
are evaporated to dryness in a porcelain or platinum dish, and the 
ammonium salts are removed by careful ignition over a moving 
flame.* After cooling, the residue is dissolved in a little water and 
the last traces of calcium are removed by the addition of ammonia 
and ammonium oxalate. After standing twelve hours, the cal- 
cium oxalate is filtered off and the filtrate is received in a weighed 
platinum dish, evaporated to dryness, and gently ignited. After 
cooling the mass is moistened with hydrochloric acid in order to 
transform any carbonate into chloride, the evaporation and igni- 
tion is repeated, and the weight of the contents of the dish is deter- 
mined; this represents the amount of alkali chloride present. To 
determine potassium, the residue is dissolved in water, and the 





—_—_—— 


* Before igniting, it is well to heat the contents of the dish in a drying-oven at 
110°. By this means there is no danger of loss by decrepitation.—_{Translator.] 


cl 


DETERMINATION OF THE ALKALIES. 499 


potassium is precipitated as potassium chloroplatinate, as de- 
scribed on p. 44. .The sodium is determined by difference. 


(b) The Hydroftuoric Acid Method of Berzelius. 


About 0.5 gm. of the mineral is weighed into a platinum dish, 
2 c.c. of water and 0.5 ¢c.c. of concentrated sulphuric acid are 
added, and mixed with the substance by means of a platinum 
spatula; after cooling about 5 c.c. of pure, concentrated hydro- 
fluorie acid, which has been distilled from a platinum retort with 
the addition of a little potassium permanganate, are added.* 
The liquid is evaporated on the water-bath, frequently stirring 
with the platinum spatula, until no more hydrofluoric acid is 
expelled and no more hard particles can be felt at the bottom 
of the dish. 7 

The dish is heated in an air-bath until the greater part of the 
sulphuric acid is removed; this is necessary to make sure that 
the hydrofluoric acid is completely expelled. It is not advisable; 
however, to remove all of the sulphuric acid, on account of the 
danger of forming insoluble basic salts. The mass is allowed to 
cool, covered with 200 c.c. of water, and digested until all of the 
residue has gone into solution.f The sulphates are now transformed 
to chlorides by precipitation with as slight an excess of barium 
chloride as possible; and then, without stopping to filter off the 
barium sulphate, the aluminium, calcium, and excess of barium 
are precipitated by the addition of ammonia and ammonium 
carbonate. The precipitate is allowed to settle, washed four 
times by decantation, then transferred to the filter and washed 
free from chloride. The filtrate is evaporated to dryness, and the 
ammonium salts removed by gentle ignition. A few drops of 
hydrochloric acid are added, and the magnesium is removed by 
adding barium hydroxide solution until slightly alkaline, boiling 
and filtering. The filtrate is treated with ammonia and am- 
monium carbonate, boiled, and the precipitated barium carbonate 
filtered off. This filtrate is again evaporated to dryness, the 
ammonium salts are expelled, the residue is dissolved in a 





* The permanganate serves to destroy organic matter that is likely 
to be present in commercial hydrofluoric acid. 
+ If barium was present, it is left benind as the sulphate. 


500 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


little water, and a little more barium carbonate is pre 
cipitated by the addition of ammonia and ammonium car- 
bonate. This treatment is repeated until finally the addition 
of ammonia and ammonium carbonate produces no further pre- 
cipitation. The last filtrate is evaporated to dryness, gently 
ignited, moistened with hydrochloric acid, again evaporated, 
ignited and weighed; this represents the weight of the alkali 
chlorides together with a small amount of magnesium chloride. 
The chlorides .are dissolved in a little water, and the potassium 
precipitated as potassium chlorplatinate (p. 44). If the cor- 
responding amount of potassium chloride is deducted from the 
first weight, the amount of sodium chloride plus the small amount 
of magnesium chloride will be obtained. In order to determine 
the latter, the alcoholic filtrate from the potassium chlorplatinate 
precipitate is evaporated to dryness‘on the water-bath (the water 
in the bath must not boil), and the residue is dissolved in a little 
water and washed into a small flask. The latter is now fitted 
with a rubber stopper containing two holes, and through these, two 
right-angled pieces of glass tubing are introduced, one reaching 
to the bottom of the stopper and the other until it almost 
touches the liquid in the flask. The solution is now heated to 
boiling so that steam escapes from both of the tubes. After boil- 
ing two minutes we can assume that the air is completely ex- 
pelled from the flask; the short tube is connected with a hydrogen 
generator and a rapid current of hydrogen is conducted through 
the apparatus, while at the same time the flame is removed from 
beneath the flask and the long tube is closed by means of a piece 
of rubber tubing containing a glass rod. The liquid is allowed 
to cool completely, and the air-space above will be entirely filled 
with hydrogen. As the hydrogen is absorbed by the liquid, the 
sodium and magnesium chlorplatinates are reduced to chloride 
with the deposition of metallic platinum, which floats on the liquid 
in the form of dendrites: 


Na,PtCl,-+ 2H,—4HCl+2NaCl+ Pt 
MegPtCl, + 2H,—4HCl+ MgCl, +Pt 


The flask is placed in a lukewarm water-bath, frequently 


ANALYSIS OF LEPIDOLITE. 501 


shaken, and the hydrogen is allowed to act upon the solution until 
the reduction is shown to be complete by the liquid becoming per- 
fectly colorless. The connection with the hydrogen generator 
is now broken and a rapid current of carbon dioxide is conducted 
through the solution for two minutes through the longer tube in 
order to remove the hydrogen. This is necessary, as otherwise 
on opening the flask there is likely to be an explosion between the 
hydrogen and oxygen, owing to the catalytic action of the plati- 
num. The platinum is filtered off, the filtrate concentrated, and 
the magnesium precipitated by the addition of ammonia and 
sodium phosphate. After standing twelve hours, the magnesium 
ammonium phosphate is filtered off and the magnesium deter- 
mined as magnesium pyrophosphate. The corresponding weight 
of MgCl, is deducted from the weight of NaCl+ MgCl, and n this 
way the amount of NaCl is determined. 

Remark.—This method is in very general use, and the results 
obtained agree closely with those by the J. Lawrence Smith method. 
Many silicates, such as the feldspars, are readily decomposed by 
the action ‘of sulphuric and hydrofluoric acids; * others, such as 
certain specimens of tourmaline, only with difficulty. According 
to Jannasch the members of the andalusite group are not com- 
pletely decomposed by hydrofluoric acid, but this can be effected 
by strongly igniting with ammonium fluoride. For this purpose 
the ignited mineral is placed in a platinum dish, covered with 
10 c.c. of ammonia, evaporated to dryness, diluted with waiter, 
strongly acidified with concentrated hydrofluoric acid, and again 
evaporated to dryness. The dish is placed in a nickel beaker and 
ignited quite strongly, until finally the excess of ammonium fluoride 
is driven off. The residue is now treated with sulphuric acid (1:2) 
in order to decompose salts of hydrofluosilicic acid, evaporated 
on the water-bath as far as possible, and then the greater part of 
the sulphuric acid is removed. From this point the procedure 
is the same as in the regular Berzelius method. 

The Smith method is always applicable and has the advan- 
tage that the magnesium is practically completely removed at 
. the start. eae 





* Many silicates can be decomposed by evaporation with hydrofluoric 
and hydrochloric acids. F. Hinder, Z. anal. Chem., 1906, 332. 


502 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


Analysis of Lepidolite. 


Lepidolite is a member of the mica group and contains lithium 
and fluorine with the following composition: | 


$i,0 AL (Li,K,Na),(F,OH), 
SiO, = 40 to 45 per ct.; Al,O,=19 to 38 per ct.; MnO=O to 5 perct.; 
MgO =0 to 0.5 per ct.; K,O=4 to*11 per ct.; Li,O=1 to 6 per ct.; 
Na,O=0 to 2 per ct.; F=1 to 10 per ct.; H,O=1 to 8 per ct. 


Besides the above, calcium, iron, phosphoric acid, and chlorine 
are frequently found, and in rare cases small amounts of cesium 
and rubidium are present. 

The determination of the silicic acid, aluminium, iron, man-— 
ganese, and magnesium is effected as in the case of the orthoclase 
analysis, except that in this case the manganese must be separated — 
from the iron and aluminium as described on p. 149 or 152. 

Determination of the Alkalies.—The weight of NaCl+ KCl+ LiCl 
is determined by one of the methods given under the analysis of 
orthoclase, and the potassium weighed as potassium chloroplatinate. 
The platinum is then removed by the treatment with hydrogen, 
or the solution is heated to boiling and the platinum is precipitated 
as the sulphide by the introduction of hydrogen sulphide. The 
filtrate free from platinum is evaporated to dryness and the lithium 
separated from the sodium as described on p. 53 or p. 55. 

Determination of Fluorine.—This determination is the same as 
in the case of analysis of fluorine in calcium fluoride (p. 471), 
except that it is unnecessary to add any silica, for the mineral 
itself already contains a sufficient quantity. 

Determination of Water.—This is effected by the method of 
Rose-Jannasch (p. 484). 


Determination of Ferrous Iron in Silicates and Rocks. 


The very finely powdered, but not bolted, mineral contained 
in a platinum dish is covered with 5 to 10 c.c. of dilute sulphuric 
acid (1:4) and placed upon the little triangle (a) Fig. 81, made of 
glass or, better, platinum. This is placed in the lead vessel C and 
the latter rests in a paraffin bath (B). After the cover is placed upon 


DETERMINATION OF FERROUS IRON. 503 


C, a rapid current of carbon dioxide is passed through A, whereby: 
the air within the apparatus will be replaced in about three 
minutes. The cover is quickly removed, and 5 to 10 c.c. of con- 
centrated hydrofluoric acid are added: The cover is immediately: 
replaced, and the current of carbon dioxide continued, while the 
contents of the dish are repeatedly stirred during the whole oper- 
ation by means of a platinum spatula or piece of coarse wire 
introduced through the other hole in the cover.* At the same 
time the paraffin bath is heated to 100° C. and kept at this tem- 
perature for about an hour. As soon as no more gritty particles 











are to be felt, the temperature of the bath is raised to about 
120° C. in order to remove the large excess of hydrofluoric acid. 
This requires about another hour. The dish is then allowed to 
cool in the carbon dioxide atmosphere and its contents are finally 
washed into 400 c.c. of cold distilled water, 10 c.c. of concentrated 
sulphuric acid are added, and the solution is titrated with a potas- 
sium permanganate solution of known strength until a pink color 
is obtained which is permanent for several seconds. This end- 
point is fugitive in proportion to the amount of hydrofluoric acid 
remaining in the solution. 

Remark.—The above method has been used in the author’s 
laboratory with success for several years. It is a modification 
of Cooke’st method in which the decomposition with hydrofluoric 





* in Fig. 81, this second opening is incorrectly shown. It should really 
be in the middle of the cover directly over the evaporating-dish. 
+ J. P. Cooke, Am. J. Science [2], 44, p. 347 (1867). 


504. GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


acid took place under a glass funnel upon the water-bath. In 
this case a large amount of hydrofluoric acid remains in solution 
and it is difficult to obtain a sharp end-point. 

Another method for the determination of the amount of ferrous 
iron present in insoluble silicates is that of Mitscherlich. The 
silicate is decomposed in a closed tube with sulphuric acid 
(8 H,SO,: 1 H,O) under pressure, and the resulting solution titrated 
with potassium permanganate. ‘This method usually gives good 
results in the case of a silicate analysis, but it is worthless for the 
analysis of rocks containing pyrite or other sulphides, which on 
treatment with sulphuric acid are decomposed with evolution of 
SO,.* The latter serves to reduce iron that was originally present 
in the ferric form, so that a too high result will be obtained. 


Determination of Small Amounts of Titanium in Rocks. 


The colorimetric method of A. Weller is best suited for this 
purpose, and is to be preferred over all gravimetric methods. 

Procedure.—The silicic acid is removed exactly as in the analysis 
of orthoclase (p. 491) and in the filtrate the iron, aluminium, 
titanium, zirconium (chromium and vanadium) are separated 
from the manganese, magnesium, and calcium, by the acetate 
method. The precipitate thus obtained still contains traces of 
manganese, so that it is dissolved in dilute hydrochloric acid and 
reprecipitated by ammonia. The precipitate is ignited in the 
same crucible in which the residue from the impure silica is ¢on- 
tained (small amounts of titanium are likely to be in this residue) 
fused with potassium pyrosulphate, and the melt dissolved in water 
containing sulphuric acid. Any insoluble silicic acid is filtered off 
and the titanium determined in the filtrate as deseribed on p. 100 
by treatment with hydrogen peroxide. | 

Remark.—In rock analysis it is convenient to determine the 
titanium after the determination of the total iron. For this 
purpose the solution of the potassium pyrosulphate melt is satu- 
rated with hydrogen sulphide in order to precipitate the platinum 





=~ 


*L,. L. de Koninck, Zeit. fir anorg. Chem., 26 (1901), 125, and Hille- 
brand and Stokes, J. Am. Chem. Soc., XXII (1900), p. 625. See also Stokes, 
Am. J. Sci., Dec., 1901. 


ZIRCONIUM AND SULPHUR IN ROCKS. 595 


and reduce the iron, and the filtrate from the platinum sulphide 
is titrated with potassium permanganate after expelling the excess 
of hydrogen sulphide, as described on p. 109. The solution is 
afterwards concentrated to about 80 c.c., and the titanium de- 
termined as ‘above. 

Of the gravimetric methods, that of Gooch is best suited (p. 116), 
but even this fails in the presence of zirconium (Hillebrand), so 
that it is in all cases better to employ the colorimetric method. 

Tf it is desired to analyze a rock for titanium alone, about one - 
gram should be treated with hydrofluoric and sulphuric acids 
(see p. 499), the greater part of the sulphuric acid removed by 
volatilization, in order to make sure that the hydrofluoric acid 
is expelled, and the residue taken up in water. From this solu- 
tion the titanium is determined as above. 


Determination of Zirconium and Sulphur in Rocks. 
W. F. Hillebrand.* 


About 2 gms. of the substance are fused with 5 or 6 times as 
much sodium carbonate (free from sulphur) and 0.5 gm. potassium 
nitrate in a large platinum crucible. The crucible should be 
placed through a hole in a piece of asbestos and held in an in- 
clined position so that none of the sulphur from the flame can 
come in contact with the contents of the crucible. The melt is 
taken up in water, a few drops of alcohol are added in order to 
reduce any manganate to manganous salt, the solution is filtered, 
and the precipitate washed with dilute soda solution. The filtrate 
contains all the sulphur in the presence of sodium silicate,t while 
the residue contains all the barium and zirconium together with 
the remaining oxides which were present in the rock. 


(a) Treatment of the Filtrate. 


This should amount to 100-250 c.c. in volume; it is acidified 
with hydrochloric acid, heated to boiling, and precipitated with 
hot barium chloride solution. After standing twelve hours the 
barium sulphate is filtered off and weighed. 








* Bulletin of the U. S. Geolog. Survey (1900), p. 73. 
t Besides sulphuric and silicic acids the filtrate may contain chromic 
(yellow color), vanadic, molybdic, phosphoric, arsenic, and tunystic acids. 


506 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


According to Hillebrand it is not necessary to evaporate the 
solution to remove the silicic acid before precipitating the sul- 
phuric acid, for from a dilute solution silicic acid is never precipi- 
tated with the barium sulphate. 


(b) Treatment of the Residue. 


The residue is washed by means of a stream of dilute sulphuric 
acid (1:20) into an evaporating-dish, and, after digesting for some 
time, it is filtered through the original filter. The filtrate con- 
tains aluminium, iron, and the greater part of the zirconium. 
The residue contains the rest of the zirconium together with barium 
sulphate and some silicic acid; after being washed, it is ignited 
in a platinum crucible and freed from silica by evaporation with | 
sulphuric and hydrofluoric acids. The residue in the crucible 
is then taken up in hot dilute sulphuric acid and filtered. The 
insoluble portion can be used for the determination of barium 
(see below). 

The two sulphuric acid filtrates, containing at the most only 
1 per cent. of this acid, are treated with hydrogen peroxide 
and a few drops of disodium phosphate. Aluminium and iron 
are not precipitated on account of the acid present, and only 
traces of titanium are thrown down, while all of the zirconium 
is precipitated as phosphate, after standing 24 to 48 hours. 

If the yellow color of the solution should fade away, a little 
more hydrogen peroxide is added; the precipitate is filtered off, 
and, even when it is small in amount, it is purified from the titanium 
as follows: The filter, together with the precipitate, is ignited, 
fused with a little sodium carbonate, the melt extracted with 
water and filtered. This residue is likewise ignited, but it is now 
fused with potassium pyrosulphate, and the fusion dissolved in hot 
water containing a few drops of dilute sulphuric acid. The solution 
is poured into a small Erlenmeyer flask of about 20 ¢.c. capacity, a 
few drops of 4 per cent.. hydrogen peroxide and a few drops of sodi- 
um phosphate solution are added, and after standing 1 or 2 days the 
precipitate is filtered off. The latter is now free from titanium in 
nearly every case, and after ignition it is weighed as zirconium phos- 
phate. Although zirconium phosphate theoretically contains 51.8 
per cent. ZrO,, there will be no appreciable error introduced if it is 


SEPARATION OF SOLUBLE FROM INSOLUBLE SILICIC ACID. 5°7 


assumed that one-half the weight of the precipitate represents the 
amount of this oxide present. 


Determination of Barium. 


The above-mentioned precipitate containing all the barium as 
sulphate, in the presence of calcium and perhaps strontium, always 
contains a little silicic acid. In order to remove the latter, it is 
heated with hydrofluoric and sulphuric acids and the residue is 
fused with sodium carbonate. The melt is treated with water and 
the carbonates of barium and calcium are filtered off, washed, and 
then dissolved in hot dilute hydrochloric acid. From this solution 
the barium is precipitated by the addition of a slight excess of 
sulphuric acid and ignited wet in a platinum crucible. The pre- 
cipitate thus obtained contains a small amount of calcium sulphate, 
which must be eliminated. For this purpose the residue is dis- 
solved in the crucible by hot concentrated sulphuric acid, and after 
cooling the solution is poured into water. In this way a precipitate 
of barium sulphate free from calcium is obtained. It is ignited 
and weighed. | 


Separation of Soluble from Insoluble Silicic Acid: Lunge and 
Millberg.* 


Frequently a mixture of silicates is to be analyzed which is 
partly decomposed on treatment with acids, with the separation 
of gelatinous silicic acid, and partly unaffected. The silicic acid 
deposited from solution by the addition of acids is soluble in 5 
per cent. sodium carbonate solution, while quartz and feldspar 
are not appreciably attacked by the latter (cf. Vol. I. pp. 356, 357). 

If it is desired to separate the deposited silicic acid from the 
unattached silicate (usually feldspar and quartz), the substance is 
treated with acid (hydrochloric or nitric) and evaporated on the 
water-bath until a dry powder is obtained. This is moistened with 
acid, diluted, boiled, and filtered. After washing, the residue is 
digested with 5 per cent. sodium carbonate solution on the 
water-bath, in a porcelain dish for fifteen minutes. It is then 
filtered, washed first with soda solution and finally with water. 





* Zeitschr. f. angew. Chemie, 1897, pp. 393 and 425 


508: GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


If a turbid filtrate should be obtained, a little aleohol is added to 
the wash water, after which the filtrate will at once run through 
clear. Shae 

The alkaline filtrate contains the soluble silicic acid; this can 
be determined by acidifying and evaporating to dryness. The 
residue from the sodium carbonate treatment, consisting of quartz 
and feldspar, is weighed. In order to determine the quartz, the 
mixture is acted upon by sulphuri® and hydrofluoric acids, the 
excess of the latter is removed by heating with sulphuric acid, and 
the cold residue is dissolved in water, precipitated with ammonia, 
and the alumina weighed. If this weight is multiplied by 5.41, 
the corresponding amount of feldspar is obtained, and if this is 
deducted from the weight of the quartz+ feldspar, the weight of 
the quartz will be found. 


Determination of Soluble Silicic Acid in Clay. 


Clay contains besides alumina, sand (quartz+breccia) and 
small amounts of calcium and magnesium carbonates. 

About 2 gms. of the substance, after having been dried at 120°, 
and being in the form of a not-too-fine powder, are moistened with 
water, and a mixture of 100 c.c. water and 50 ¢.c. concentrated sul- 
phuric acid * is added. The porcelain dish is covered with a watch- 
glass and heated over a free flame until dense fumes of sulphuric 
acid vapors are evolved. The contents of the dish are allowed 
to cool, 150 ¢.c. of water and 3 c.c. of concentrated hydrochloric 
acid are added, the solution boiled for fifteen minutes, filtered, 
washed completely, and the mixture of soluble silicic acid, quartz, 
and insoluble silicate is treated as above. 

Remark.—It was formerly the custom to separate the soluble 
silica from the insoluble silica by boiling with potassium hydroxide 
solution. According to the experiments of Lunge and Millberg, 
however, this is not permissible because quartz is perceptibly 
soluble in caustic potash solution. If, on the other hand, the 
~ substance is obtained in a very finely-divided condition, even 
sodium carbonate solution cannot be used for the same reason. 





* Alexander Subech, Die chem. Industrie 1902. p. 17. 


ANALYSIS OF CHROMITE. 5°09 


Analysis of Chromite. 


Although chromite (chrome iron ore) is not a silicate, it is 
insoluble in all acids, and can be brought into solution by fusion 
with alkali carbonates, or borates, so that its analysis will be dis- 
cussed at this place. 

Chromite contains 18 to 39 per cent. FeO, 0 to 18 per cent. MgO, 
42 to 64 percent. Cr,O,, 0 to 13 percent. Al,O,, and 0 to 11 per cent. 
SiO,. Calcium, manganese, and nickel are also occasionally present. 

Of the finely-powdered and bolted mineral, 0.5 gm. is fused 
in an inclined, open platinum crucible with 4 gms. of pure sodium 
carbonate * for two hours over a good Teclu burner. After cool- 
ing, the melt is leached with water, acidified with hydrochloric 
acid,t evaporated in a porcelain dish until a dry powder is obtained, 
moistened with hydrochloric acid, taken up in water, and the 
silica filtered off. The latter is ignited, weighed, and its purity 
tested with hydrofluoric acid (p. 487). The filtrate from the 
silicic acid is precipitated hot with hydrogen sulphide and the 
precipitate of platinum sulphide and sulphur is filtered off. It 
is then placed in an Erlenmeyer flask, 10 c¢.c. of ammonium 
chloride, enough ammonia (free from carbonate) to make the 
solution alkaline, and a little freshly-prepared ammonium sul- 
phide are added, after which the flask is corked up and allowed 
to stand over night. In the morning the precipitate is filtered 
off, washed twice with water containing a little ammonium sul- 
phide, then dissolved in hydrochloric acid, and the precipitation 
by means of ammonium sulphide is repeated. The ammonium 
salts are removed from the filtrate and the calcium and magnesium 
determined as described on p. 76-8. : 

The ammonium sulphide precipitate is dissolved in dilute 
hydrochloric acid, any residue of nickel or cobalt sulphide is fil- 





* Bunsen fused the chromite with one-third as much SiO: and 6 to 8 
parts NazCO; and then subtracted the amount of silica added from the 
total amount found. This makes the decomposition take place more read- 
ily, but the author prefers not to add the silica on account of the possibility 
of thereby introducing an error. 

+ If a dark residue of undecomposed mineral should remain, it is filtered 
off and again fused with sodium carbonate. 


510 GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


tered off and dried. This residue is then ignited first in auc, 
then in a current of hydrogen, and finally weighed as metal. It 
is not worth while to attempt the separation of the nickel from 
the cobalt on account of the small amount present. The filtrate 
from the sulphides of nickel and cobalt is freed from hydrogen 
sulphide by boiling, the iron present is oxidized by evaporating 
with potassium chlorate and hydrochloric acid, and the iron, 
chromium, and aluminium are separated from the manganese by 
means of the barium carbonate method (p. 149) and from one 
another as described on p. 1lU7 et seq. In the filtrate from the 
barium carbonate precipitate, the manganese is separated from’ 
the barium as described on p. 122, b, and determines! as sulphide 
or as sulphate. 

Remark.—If it is desired to determine the chromium alone, 
this is best accomplished by means of a volumetric process (see 
Part II). 


Determination of Thorium in Monazite, according to E. Benz.* 


Monazite is a phosphate of the rare earths [PO,(Ce,La,Di,Th)]. 
It occurs in so-called ‘“‘monazite sand’”’ mixed with quartz, rutile, 
zircon, tantalates, etc., and is at present the raw material used for 
the preparation of thorium (used in the Welsbach mantle). The 
value of a sample of monazite sand depends upon the amount of 
thorium present, and its determination is best effected as follows: 

Of the bolted monazite sand, 0.5 gm. is intimately mixed with 
10 gms. of potassium pyrosulphate in a spacious platinum cruci- 
ble; the latter is covered and slowly heated until its contents are 
at a gentle fusion. This is best accomplished by placing the 
platinum crucible within a larger porcelain one which is provided 
with an asbestos ring. After no more gas is given off, the cruci- 
ble is gently ignited over the free flame, and, after cooling, its 
contents are treated with water and a little hydrochloric acid 
until it is completely disintegrated. After allowing the residue 
to settle, it is filtered, treated with a little concentrated hydro- 
chloric acid, diluted with water, and again filtered.j In the com- 





* Zeit. f. angew Chem. 15 (1902), p. 297. 
+ This residue is free from thorium, and consists chiefly of silicic and 
tantalic acids. 


DETERMINATION OF THORIUM IN MONACZITE. 511 


bined filtrates the hydrochloric acid is nearly neutralized with 
ammonia (the formation of a permanent precipitate is to be 
avoided, for it will be difficult to redissolve it), the solution is 
heated to boiling, and 3 to 5 gms. of solid ammonium oxalate are 
added while the liquid is vigorously stirred. The oxalates of 
the rare earths are immediately deposited in the form of a coarse 
powder. To make sure that the precipitation is complete, a little 
ammonium oxalate solution is added. After standing twelve 
hours the precipitated oxalates are filtered off, washed once by 
means of water acidified with nitric acid, then transferred to a 
porcelain dish, and the last portions of the precipitate are eventually 
washed from the filter by repeated additions of hot, concentrated 
nitric acid and water; the liquid is evaporated almost to dryness. 
Ten cubic centimeters of concentrated nitric acid (sp. gr. 1.4) and 20 
c.c. of fuming nitrie acid are then added, the dish covered with a 
watch-glass anc. heated on the water-bath. After a short time 
the nitric acid pegins to decompose the oxalic acid, shown by 
the lively evolution of gas. After no more gas is given off, the 
watch-glass and sides of the dish are washed down and the 
solution evaporated to dryness. In order to remove the free 
nitric acid, a little water is added and the solution evaporated 
once more; after this the filter fibres present are removed by 
filtration. It is now necessary to separate the thorium from 
the remaining earths. This is effected by precipitating the former 
with hydrogen peroxide as thorium peroxide. On ignition the 
latter is changed into ThO,, in which form it is weighed. 

The precipitation with hydrogen peroxide takes place as 
follows: The neutral solution of the nitrates is diluted with 10 
per cent. ammonium nitrate solution to a volume of 100 c.c.. 
heated to 60-80° C., and precipitated by the addition of 20 c.c. 
of pure 3 per cent. hydrogen peroxide solution. The precipitate, 
which is colored yellow by traces of cerium peroxide (at the most 
#5 mg. of the latter is present), is immediately filtered, washed 
with hot water containing ammonium nitrate, ignited wet in a 
platinum crucible, and weighed as ThO,,. 

If it is desired to obtain an absolutely pure thorium oxide, 
“he moist precipitate is dissolved in nitric acid and the above 
brecipitation with hydrogen peroxide is repeated. By this method 


512} GRAVIMETRIC DETERMINATION OF THE METALLOIDS. 


E. Benz obtained in the analysis of a South American monazite: 
sand the following results: 4.72, 4.58, 4.50 per cent. ThO,. 

Remark.—The above process for the determination of thorium 
in monazite is quicker and more accurate than either that of 
Glaser * or that of Hintz and Weber,t} so that it is to be recom- 
mended for both technical and scientifie purposes. 

The determination of thorium oxide in thorite is carried out 
in the same way with the difference that instead of fusing the 
mineral with sodium fluoride and potassium pyrosulphate. it is 
decomposed by treatment with hydrochloric acid, and the silica 
removed as usual. The filtrate from the silica is analyzed 
as above.t 

For the 


Analysis of Incandescent Mantles 


consult the work of T. B. Stillman, Chem. Zeit., 1906, 60. 


Determination of Water in Silicates. 


If the mineral on ignition loses nothing but water, the amount 
of the latter can be determined by the loss in weight. In the 
great majority of cases, however, other constituents (e.g. CO,. SO,, 
Cl. I’, etc.) are lost and the substance may undergo an oxidation 
(FeO is changed to Fe,0,. PbS to PbSQO,. ete.). In such cases 
the procedure recommended by Jannasch can be used to advan- 
tage. The substance is heated with lead oxide, the water vapor 
conducted over a heated mixture of lead oxide and lead peroxide 
and absorbed in a weighed calcium chloride tube (see p. 484). 

If the substance on ignition loses simply water and carbon 
dioxide the former may be accurately determined by the method 
of Brush and Penfield. The substance is introduced by means 





* Chem. Ztg.. 1896. p. 612. 

+ Zeitschr. fiir anal Chemie (1897), XXXVI. p. 27, _ 

t As members of the hydrogen sulphide group are usually present, it is 
advisable to first remove them and to effect the precipitation of the rare 
earths with ammonium oxalate from the slightly acid filtrate from the hydro- 
gen sulphide precipitate. 

§ Amer. Journ. Sci. [8], XLVIII (1894), p. 31, and Zeit. fiir anorg. 
Chem , VII (1894). p, 22 


DETERMINATION OF WATER IN SILICATES. 513 


of a long funnel into a bulb blown on the end of a narrow tube made 
of difficultly-fusible glass, and the tube is provided with a second © 
bulb about 2 or 3 cm. from the end one. The open end of the 
tube is connected by means of a short piece of rubber tubing with 
a short tube drawn out into a capillary, and the substance is heated 
in the flame of a good Teclu burner. The water is expelled and 
condenses in the colder portion of the tube, and as a precaution 
the latter is enveloped in moist blotting-paper. As soon as no more 
water can be expelled, the end of the tube is heated until it softens 
and the tube is drawn out between the two bulbs. The front end 
of the tube now contains the water in the presence of a little carbon 
dioxide, and the latter must be removed. For this purpose the 
tube is inclined at an angle of 40°, so that the heavier carbon diox- 
ide will run out of it. The weight of the tube slowly diminishes, 
but at the end of about three hours it becomes constant, losing 
about 0.0003 gm. per hour, due to the evaporation of water. If, 
therefore, the tube is allowed to stand three hours before weighing, 
0.0009 gm. must be added to the weight of the water. If the 
substance contained a large amount of carbonate, the escaping 
earbon dioxide will carry aqueous vapor with it, so that a further 
correction must be made. One gram of CO2 at an average baro- 
metric pressure (760 mm.) and temperature (20° C.) will cause 
a loss of 0.0096 gm. water vapor. Ifthe amount of carbon dioxide 
present is known, it is, therefore, only necessary to multiply its 
weight by 0.0096 to obtain the amount of water that would other- . 
wise escape the determination. 


Determination of Silicon. 


See Steel Analysis, p. 441. 


Determination of Silicon in the Presence of Silicic Acid. 


Gf. M. Phillips, Z. angew. Chem., 14, 1969 (1905). 


TAT LS 11, 
VOLUMETRIC ANALYSIS. 





A GRAVIMETRIC analysis is accomplished by adding to the 
solution of the substance to be analyzed a reagent of only approxi- 
mately-known strength, separating one of the products of the reac- 
tion from the solution and weighing it. On the other hand, a 
volumetric analysis is made by causing the reaction to take 
place by means of a measured amount of a solution of accurately- 
known strength and computing the amount of substance present 
by the volume of the solution which reacts with it (cf. p. 2). 
For the latter sort of analysis accurately-calibrated measuring 
instruments are necessary, as will be briefly described. 


Measuring Instruments. 


1. Burettes are tubes of uniform bore throught the whole 
length; they are divided into cubic centimeters and are closed at 
the bottom, as shown in Fig. 82, by means of a glass stop-cock, or 
with a piece of rubber tubing containing a glass bead h. The latter 
form was devised by Bunsen and is used as follows: The tubing is 
seized between the thumb and forefinger at the place where the 
glass bead is, and by means of a gentle pressure a canal is formed 
at one side of the bead through which the liquid will run out. 
Instead of the glass bead an ordinary pinch-cock is frequently used. 

Besides the above forms of burettes, a great many others are 
in use, but it is unnecessary to describe them here. om 

2. Pipettes—A distinction must be made between a “ full” 


pipette and a “measuring” one. A full pipette has only one 
514 


MEASURING INSTRUMENTS. 515 


-Imark upon it, and serves for measuring off a definite amount of 
liquid. They are constructed in different forms; usually they 
consist of a glass tube with a cylindrical widening at the middle. 
The lower end is drawn out, leaving an opening about 4-1 mm, 


be | 


J 
——_- 


afc ve eeepc yaa 


QUE OD EY EE ED 








SJovefasefanp eeepc fare = 

















Fig. 82. 


wide. Pipettes of this nature are constructed which will hold 
respectively 1, 2, 5, 10, 20, 25, 50, 100, and 200 c.c. 

Measuring pipettes are burette-shaped tubes graduated into 
cubic centimeters and drawn out at the lower end as before. They 
serve to measure out any desired amount of liquid and are obtained 
with a total capacity of 1, 2, 5, 10, 20, 25, and 50 c.c. 

3. Measuring-flasks are flat-bottomed flasks with narrow 
~ necks provided with a mark, so that when they are filled to this 


516 VOLUMETRIC ANALYSIS. 


point they will contain respectively 50, 100, 200, 250, 300, 500, 
1000, and 2000 c.c. They serve for the preparation of standard 
solutions and for the dilution of liquids to a definite volume. 

4, Measuring-cylinders are graduated into cubic centimeters 
and are used only for rough measurements. 

It is clear that accurate results can be obtained by a volu- 
metric analysis only when the instruments used are accurately 
calibrated. It should never be taken for granted that a pur- 
chased instrument is correct, but it should always be carefully 
tested. In the case of measuring-flasks and ‘full’? pipettes, it 
is best for each one to etch for himself the position on the flask 
or tube up to which they should be filled with liquid, | 


Normal Volume and Normal Temperature. 


A liter, which is the volume of a kilogram of water at its 
maximum density, is taken as the normal volume. If it is desired 
to mark on the neck of a liter-flask the point to which-this volume 
reaches, the position of the mark depends upon the temperature 
of the vessel. It is necessary, therefore, to choose for the vessel 
itself a definite temperature, the so-called normal temperature. 
At present the temperature of +15° GC. is almost universally 
taken as the normal temperature. According to this, then, the 
flask should be marked at 15° with the volume occupied by a 
kilogram of water at +4°, and as the kilogram is the unit of mass, - 
the weighing should also take place in a vacuum. 

This experimental impossibility can be avoided inasmuch as 
the weight of a liter of water is known accurately at temperatures . 
other than +4°, also the expansion of the glass with rise of 
temperature, and the buoyancy which the weights and the water 
experience as a result of weighing in the atmosphere. The weights 
which must be placed upon the balance pan in order to determine 
the space occupied by a true liter of water, therefore, depend upon 
the temperature of the water and of the vessel, as well as the 
density of the air at the time of the experiment. The density of 
the air varies somewhat from day to day and depends upon the 
barometric pressure, the temperature, and the amount of moisture. 


MEASURING INSTRUMENTS. 517 


It suffices in most cases, however, to assume the average values of 
these factors corresponding to the locality. 

As regards the density of water at diferent temperatures, this 
is siven in the following table: 


DENSITY OF WATUR AT DIFFERENT TEMPERATURES * 








t Density. t Density t Density. 
0° 0.999867 14° 0.999271 28° 0.996258 
1 9926 15 9126 29 0.995969 
2 9968 16 8969 30 5672 
3 9992 17 8801 31 5366 
4 1.000000 18 $621 32 5052 
5 0.999992 19 8430 33 0.994728 
6 9968 . 20 8229 34 4397 
7 9929 21 8017 35 A058 
8 9876 22 0.997795 36 0.993711 
9 9808 23 7563 37 3356 
10 9727 24 7321 38 0.992993 
1l 9632 25 7069 39 2622 
12 9524 26 0.996808 40 0.992244 
13 3404 27 6538 


























* Thiesen, Scheel and Diesselhorst, 1904. 


With the aid of this table it is possible to tell what the weight 
of a liter of water will be at any temperature. It was stated on 
page 13 that if po is the weight of a body in a vacuum and p 
that of the same body in the air, then 


Bias he 
po=P(1 +2-4), 


81 


in which expression 4 denotes the density of the air under the 
prevailing conditions, s that of the body and s, that of the brass 
weights at 7°. 

At t°, however, the volume occupied by the mass compared 
with that at 10° is 


Vi=Vi5({1 +a(t—15)] 


where a is the coefficient of cubical expansion. The weight of 


518 VOLUMETRIC ANALYSIS, 


water contained in the mass at / in a vacuum (disregarding 
quantitics of the second order) is: 


Visllt+a(é—15 
p= 151 a ; i 
1+4(5-3) 

Baye! 5 | 


If, therefore, it is desired to deverauns the volume of a liter 
by weighing water at 17.35° with brass weights, the computation 
is carried out as follows: 

The density of water at 17.35° is given by interpolation in the 
above table as 0.9987=s, the density of the brass weights can 
be taken as 8.0=s,, the density of the air as 0.001214 and the 
coefficient of cubical expansion of glass as 0.000027, so that by 
inserting these values in the above equation: 





_ 0.9987[1 + 0.000027 (17.35—15)] _ 
PTS OOTY O00EIE 17 
0.9987 8.0 





0.9977 kg., 





or in other words the volume occupied by 997.7 gms. of water 
under the above conditions represents one liter in glass 
at 15°. | 

Invariably the temperature of the laboratory is such tha 
somewhat less than 1000 gms. is used for the calibration in true 
cubic centimeters. It is convenient, therefore, to place the 
1000-gm. weight on one side of the balance together with the 
empty flask, and then place a tare on the opposite side of the 
balance. Then the 1000-gm. weight is removed and in its place 
1000—p gms. are placed, after which equilibrium is restored by 
filling the flask with water. 

To avoid the somewhat tedious calculation of the value of 
p, W. Schlésser * has calculated the following table in which the 
values are given for 1000— p at different temperatures. 





* Z. angew. Chem., 1903, 960; Chem. Ztg., 1904, 4. 


MEASURING INSTRUMENTS. 


Correction Table. 


519 


This table shows in milligrams how much less than 1000 gm. is the weight 
of water which occupies a volume of one liter, on the assumption that the 
coefficient of cubical expansion for the glass is 0.000,027 per degree Centi- 
grade, the normal temperature is 15°, the barometric pressure 760 mm. the 
temperature of the air 15°, and the tension of aqueous vapor is normal. 
The table reads from a temperature of 5.0° to one of 30.9°. 





t 0.0 


0.1 


0.2 


0.3 - 


0.4 


0.5 


0.7 


0.8 


0.9 





-§°| 1341 


1340 


1339 


1338 


1338 


1338 


1338 


1338 


1338 





6 | 1338 
7 | 1350 
8 | 1376 
9 | 1417 


-1339 


1352 
1380 
1421 


1340 
1354 
1384 
1426 


1341 
1356 
1388 
1431 


1342 
1358 
1392 
1436 


1343 
1360 
1396 
1442 


1345 
1366 
1404 
1452 


1346 
1369 
1408 
1458 


1348 
1372 
1412 
1464 





10 | 1471 


1477 


1483 


1489 


1496 


1503 


1517 


1524 


1531 





11 | 1539 
12 | 1619 
13 | 1713 
14 | 1819 


1547 
1628 
1723 
1830 


1555 
1637 
1733 
1841 


1563 
1646 
1743 
1853 


1571 
1655 
1753 
1865 


1579 
1664 
1764 
1877 


1595 
1683 
1786 
1901 


1603 
1693 
1797 
1913 


1611 
1703 
1808 
1925 





15 | 1937 


1949 


1962 


1975 


1988 


2001 


2027 


2040 


2053 





16 | 2066 
17 | 2208 
18 | 2360 
19 | 2525 


2080 
2223 
2376 
2542 


2094 
2238 
2392 
2559 


2108 
2253 
2408 
2576 


2122 
2268 
2424 
2593 


2136 
2283 
2440 
2610 


2164 
2313 
2474 
2645 


2178 
2328 
2491 
2663 


2193 
2344 
2508 
2681 





20 | 2699 


2717 


2735 


2753 


2771 


2789 


2826 


2845 


2864 





21 | 2883 
22 | 3078 
23 | 3283 
24 | 3498 


2902 
3098 
3304 
3920 


2921 
3118 
3325 
3542 


2940 
3138 
3346 
3564 


2959 
3158 
3367 
3586 


2978 
3178 
3388 
3609 


3018 
3220 
3432 
3655 


3038 
3241 
3454 
3678 


3058 
3262 
3476 
3701 





25 | 3724 


3747 


3770 


3793 


3816 


3839 


3886 


3910 


3934 





26 | 3958 
27 | 4202 
28 | 4455 
29 | 4716 


3982 
4227 
4481 
4743 


4006 
4252 
4507 
4770 


4030 
4277 
4533 
4797 


4054 
4302 
4559 
4824 


4078 
4327 
4585 
4851 


4127 
4377 
4637 
4905 


4152 
4403 
4663 
4932 


4177 
4429 
4689 
4959 





30 | 4987 








5014 





5041 





5069 





5097 





5125 








5181 





5210 





5239 





If it be desired to take into consideration the deviation of the 
temperature and barometric pressure from that assumed in the 
above table, it is sufficient to add (or subtract) to the figure given 


520 VOLUMETRIC ANALYSIS. 


in the table 1.4 mgm. for each millimeter that the barometer reads 
above (or below) 760 mm., and to subtract (or add) 4 mgm. for each 
degree that the temperature of the air is above (or below) 15° C. 

If, for example, the temperature of the water is 17.35°, the 
barometer reading 720 mm., and the temperature of the air 23.7°, 
then the correction is computed as follows: 

‘cording to the table the value of 1000—p is 2260 mgm., 
this number, therefore should be diminished by 


(760 —.720)1.4=56 mgm. 
(03 71b\0.4—36;. - a, 
91 mgm. 


The correction becomes 2260—91=2169 mgm.=2.169 gms. 
In order to simplify the matter still further, Schlésser recommends 
preparing a special table for localities where the average barometric 
pressure is considerably less than 760 mm. Thus the following 
table applies to Ziirich and can be used for other places where the 
average barometric pressure is correspondingly low. 


CORRECTION IN GRAMS FOR 1000 c.c. 


under the assumption that the coefficient of cubical expansion for glass is 
0.000027 per degree Centigrade, the normal temperature of glass is 15°, the 
temperature of the water between 5° and 30.5°, the barometer reading 720 
mm., the temperature of the air 15° and the vapor tension normal. 














t Correction t Correction t Correction t Correction 
in Grams. in Grams. in Grams. in Grams. 

5.0° 1.284 115° 1: 522 18.0° 2.303 24.5° 3.552 
5.5 1.281 12.0 1.562 18.5 2.383 25.0 3.667 
6.0 1.281 12:5 1.607 19.0 2.468. 25.5 3.782 
6.5 1.286 13.0 1.656 19.5 2. 553 26.0 3.901 
7.0 1.293 13:5 1.707 20.0 2.642 26.5 4.021 
7.5 1.303 14.0 1.762 20.5 p Ae? 27.0 4.145 
8.0 1.319 14.5 1.820 21.0 2.826 24.0 4.270 
8.5 1.339 15.0 1.880 21.5 2.921 28.0 4.398 
9.0 1.360 15.5 1.944 22.0 3.021 28.5 4.528 
9.5 1.385 16.0 2.009 ae .0 3.121 29.0 4.659 
10.0 1.414 16.5 2.079 23.0 3.226 29.5 4.794 
10.5 1.446 17.0 2.151 23.0 3.304. 30.0 4.930 
11.0 1.482 17.5 2.226 | 24.0 3.441 30.5 5.068 





























MEASURING I..STRUMENTS. 521 


To calibrate a 500 c.c. flask for the normal temperature of the 
glass at 15° by means of water at 19.5° at Ziirich, the cormecbion 
is taken from the above table and divided by 2, 200 = 1.276 ems. 

In most cases it is not necessary to take into consideration 
slight changes in the barometer reading or in the temperature 


of the air. 


The Mohr Liter. 


Before the above tables had been worked out by Schlosser it 
was customary to avoid the computation otherwise necessary by 
adopting a standard other than that of the true liter, and the | 
practice is still adhered to by many chemists. Thus for vol- 
umetric work the liter was taken as the volume of a kilogram of 
water at either 15° or 17.5° C. as weighed in the air. For all titra- 
tions this standard is perfectly satisfactory, but it is not suitable 
for the measurement of the volume of gases in which it is necessary 
to estimate the weight of a gas from the volume, because the 
density of gases is always referred to true liters. 

A Mohr liter measured with water at 15° is 1.0019 and one 
measured with water at 17.5° is 1.0023 times the volume of a 
true liter. In other words the former is 1.9 cm. and the latter 
2.3 cm. too large. 

When in the course of this baok a liter is mentioned, the true 
liter is to be understood in all cases. 

Since, however, many instruments are still graduated at normal 
temperatures of 15°, 17.5° and 20°, a table will. be given (see 
p. 522) which can be used for testing such apparatus. 

Thus, to determine the volume of: a Mohr liter for the normal 
temperature of 15°, the liter flask and 1 kg. in brass weights 
should be counterpoised against a tare. The kilogram weight 
is then removed, the flask is filled with water at 15° and the 
position of the meniscus in the neck of the flask is marked. If, 
however, the temperature of the water is not 15°, but say 25.5°, 
then evidently the Mohr liter will weigh, according to the above 
table, 998.095 gms. 


522 VOLUMETRIC ANALYSIS. 


TABLE FOR PREPARING A MOHR LITER AT THE NORMAL TEMPERATURES OF 
15, 17.5 AND 20° c. ACCORDING TO W. SCHLOSSER. 











Normal Temperatures. 
Temperature 
of Water. 
ti ar mee 175° 20° 
Grams. Grams. Grams. 
15° 1000 . 000 1000. 345 1000. 763 
16 999.871 Ay | . 634 
17 .728 ™ .075 .491 
18 .576 999 .023 339 
19 .413 .760 fe a 
20 . 237 . 584 1000. 000 
21 .053 .400 999.816 
22 998 .858 . 204 .620 
23 .652 998 .999 .414 
24 437 .783 .199 
25 Ne 8 .558 998 .973 
26 997 .977 dao .739 
27 .733 .078 .494 
28 .479 997 .825 . 240 
29 . .218 . 563 997 .978 
30 996 . 946 . 292 707 














Calibration of Measuring-flasks. 


A flask is chosen with a long neck, as cylindrical as possible, 
the diameter of which should not exceed a certain value. 


GREATEST PERMISSIBLE DIAMETER OF THE NECK 


Contents ......... 2000 1000 500 250 200 100 50 2c. 
Diameter......... 25 18 15 15 12 12. 40.) 6 2am 


The flask is very carefully cleansed, and dried, after which it 
is placed upon an accurate balance and counterpoised by a tare. 
Beside the tare weights are placed corresponding to the volume 
of the flask, and on the opposite side of the balance weights 
corresponding to the correction obtained from the table on page 
519 corresponding to the temperature of the water to be used, 
after which equilibrium is again established by pouring distilled 
water into the flask. Care is taken that no drops of water are left 
suspended from the sides of the neck above the water-level; if any 
are present, they are removed by touching with a piece of filter- 
paper wrapped around the end of a glass rod. An exact equi- 
librium is finally established by adding or removing a little water 


MEASURING INSTRUMENTS. 523 


by means of a capillary tube. The flask is then placed upon a level 
surface and a piece of gummed paper with a straight edge is fast- 
ened around the neck of the flask so that its upper edge is just 
tangent to the deepest point of the water meniscus. The flask is 
now emptied, dried, its neck covered with a uniform layer of bees- 
wax, and allowed to cool; this usually requires about fifteen 
minutes. ‘The flask is then held, as is shown in Fig. 83, against the 
piece of wood s, the blade of a pocket-knife is placed firmly against 
the upper edge of the thick paper ring, and the flask is revolved 
through 360° around its horizontal axis; in this way a circle is 
cut in the wax layer. By means of a feather (Fig. 6, p. 22) a 
drop of hydrofluoric acid is placed along this circle while the 
flask is held in the horizontal position. By turning the flask 
around its axis, the drop of hydrofluoric acid is allowed to act 
upon the glass where the wax coating has been cut. At the end 
of two minutes the excess of hydrofluoric acid is washed off, the 
neck of the flask dried by means of filter-paper and heated until 
the wax melts, when the latter can be readily wiped off. The last 
traces of wax are removed by rubbing with a cloth wet with 
alcohol. As it is possible that the etched circle will not exact] 





Fig. 83. | 


| coincide with the upper edge of the paper, the flask should always 
be tested.* 


__ 


* The attempt should not be made to test the correctness of the calibra- 
tion by filling the flask with water which has been brought to a definite 
temperature; it is important, on the other hand, that the flask and water 
should be allowed to remain for some time in the same place in order that 
the temperature of the two may be nearly the same. 





524 VOLUMETRIC ANALYSIS. 


Testing Calibrated Flasks. 


The flask is counterbalanced with a tare and then weights 
are added to the tare corresponding to the volume of the flask. 
The flask is filled with distilled water up to the mark and equi- 
librium is restored by adding small weights. . 

Thus in testing the calibration of a liter flask, it was filled three 
times with water at 21.5° after it ad been counterbalanced with 
a tare when perfectly dry. It was found necessary to place 
small weights on the side with the filled flask amounting to 
2.987, 2.893 and 3.122 gms.; average 3.001 gms. If the flask had 
been perfectly accurate it is found from the table on page 520 that 
the small weights should have been equal to 2.921 gms. for water 
at 21.5°. The flask is, therefore, 3.001—2.921=0.080 cm. too 
small. | 
This is, however, an unusually good agreement. According 
to the Royal Commission of Berlin, the allowable error in cal- 
ibrated flasks is shown by the following table: 


PERMISSIBLE ERROR FOR FLASKS CALIBRATED FOR CONTENTS 
(The allowable error for flasks calibrated for delivery is twice as large.) 


Contents... 2000 1000 500 400 300 250 200 ~~ 100 50 -c.c. 
Error ..... 0.6% 0.25 0:14 0.11 0.11 0.08; 0.08: 0.08 = 0.05 ce 


Liter flasks which are calibrated for contents are marked in 
Germany 11. —> ae =- (E) in case they are calibrated in terms of true 


cubic centimeters, and 11. oa (E),* in case they are calibtatel in 


Mohr liters. Flasks £ se for delivery are marked with an 
A instead of the E. 


Calibration of Pipettes. 


It is best to have pipettes prepared by the glass-blower and 
to etch them for one’s self. First of all, the pipette must be 
scrupulously clean; no trace of fat should be left on the inner sides 
of the tube, for it will cause drops of moisture to adhcre and escape 





ig — pein 
* Ors POs —— H, according to the normal temperature chosen, 





CALIBRATION OF PIPETTES. — 525 


measurement. The pipette is, therefore, cleaned by placing it in 
a tall beaker containing a little soap solution and the latter is 
drawn to the top of the pipette by sucking through a rubber tube 
fastened to its upper end and which is provided with a pinch-cock. 
The solution is allowed to remain in the pipette for about fifteen 
minutes. 

The alkali is then allowed to run out, the pipette washed with 
water and filled with a warm solution of chromic acid in con- 
centrated sulphuric acid.* This is allowed to remain from 
five to ten minutes in the pipette and is then removed, the tube 
washed first with water from the tap and finally with distilled 
water. | 

The pipette is now clean and ready to be calibrated. <A long 
strip of paper is fastened upon the upper part of the tube, the 
lower end is closed with the finger, and the pipette is filled with 
water which has stood for some time in the balance room, from 
another of the same size or from a burette. The position of the 
bottom of the mensicus is noted with a lead-pencil upon the paper 
which was fastened to the side of the pipette. Assume, for 
example, that it is desired to calibrate a 10-c.c. pipette, and that 
the water to be used is at a temperature of 18°. According to the 
table on p. 480 one liter of water at 18° weighs in the air exactly 
1000— 2.303 =997.70 gms., consequently 10 c.c. should weigh 
9.9770 gms. : 

The point of the pipette is dipped in water, and this is sucked 
up into the pipette by placing the mouth at the upper end until 
the water is above the pencil marking. The top of the pipette is 
then closed with the finger, the water adhering to the outside 
carefully wiped off, and that inside is allowed to run into a beaker, 
with the point of the pipette against the walls, until the upper 
meniscus in the stem is exactly on the mark. The contents of 
the pipette are then allowed to run into a tared beaker which is 
covered with a watch glass, or into a glass-stoppered weighing- 
beaker, allowing the water to flow along the walls of the beaker. 
Now on weighing the beaker again it is perhaps found that the gain 





* A solution of potassium dichromate in concentrated sulphuric acid can 
be used. 


526 VOLUMETRIC ANALYSIS. 


in weight is 9.9257 gms. or 9.9770— 9.9257 =0.0513 gms. too little. 
A second mark is therefore made a little higher up on the paper 
attached to the stem of the pipette, and the above process is 
repeated. If necessary a third mark is made until finally the 
weight of the water does not vary more than 5 mgms. from that 
computed. 

The strip of paper is then cut off at exactly the correct mark, a 
strip of gummed paper is placed rotf{nd the pipette at this point, 
and, after the gum has dried, it is covered with a layer of beeswax 
and etched with hydrofluoric acid as described on p. 523. After 
the mark has been etched upon the pipette, it is filled with water 
up to the mark and emptied into the tared flask. This 
operation is repeated three times and the mean value is taken as 
correct. 

Pipettes may be emptied in several ways: 

1. By allowing the contents to run out freely with the pipette 
held vertically. At the end the end of the pipette is touched to 
the sides of the beaker. A drop of the liquid will then always 
remain in the pipette. 

2. The solution is allowed to run out while the hist of the 
pipette is held against the side of the vessel into which the liquid 
is being delivered. 

All other methods of emptying pipettes, especially that of 
blowing at the last, are to be abandoned. At all events, it is 
always necessary to use the pipette in the same way as in the calibra- 
tion. 

The ‘ kaiserl. Normaleichungskommision ” allows the fol- 
lowing error in pipettes. 


Contents of pipette . 100 50 25 20 10 2 


1 lem 
Errot an-Goc2e 0.07 0.05 0.025 0.025 0.02 0.006 0.006 
Error in per cent.... 0.07 0.1 0.10 0.125 0.2 0.3 0.6 


It is possible, however, to prepare pipettes which are mor? 
accurate than this; thus the author by using pipettes as recom- 
mended above obtained the following values: 


CALIBRATION OF BURETTES. 527 


50 c.c. Pipette: 49.9904, 49.9910, 49.9926. Mean 49.9913. 
f*=0.002%, F=0.001%. - 
20 c.c. Pipette: 20.0059, 20.0068, 20.0055. Mean 20.0061. 


f=0.003%, F=0.002%, 
and in the same way: 


10 c.c. Pipette: f=0.008%, F=0.004. 
5 c.c. Pipette: f=0.011, F=0.006%. 


Calibration of Burettes. 


In volumetric titrations it is advisable to begin each titration 
with the solution at the zero point of the burette. It is proper, 
therefore, to calibrate burettes in the same way. The burette is 
filled to the zero point and a definite volume, e.g., 5 c.c., is allowed 
to run into a tared beaker, as described for pipettes on p. 525, 
allowing the tip of the burette to touch the side of the beaker. 
After determining the weight of the water, the burette is filled 
again to the zero point and then 10 c.c. are withdrawn in exactly 
the same way. This process is repeated for each 5 c.c. until 
finally the 50 c.c. mark is reached, each time determining the 
weight of the water withdrawn. In withdrawing liquid from 
a burette until a given point is reached, without waiting for the 
burette to drain, evidently the amount actually withdrawn 
depends upon the rate at which it flows from the burette. It is 
advisable, therefore, to have the tip so narrow that it will take 
80 seconds for 50 ¢.c. to run out. It is true that the burette is not 





* By f is understood the average error of the single determination. It 
X(d?+d7+d,7+ . ) 
n=l 
Leitfaden der prakt. Physik.), in which n represents the number of deter- | 
minations made, and d, d,, d,...represent the deviation of each from the 
arithmetical mean and 2 (d?+d,?+d,?+.. .) the sum of the squares of 


2(?+d/+d,?+... 
the errors. rat ptd,?+.. +) 


n(n—1) 
of the mean. 








is computed by the formula f aN (ef. Kohlrausch: 








and represents the probable error 


528 VOLUMETRIC ANALYSIS. 


drained completely in this time,* but according to Wagner ft it 
is sufficiently so for practical purposes. : 

Burettes with rubber tubing at the bottom gradually change 
with regard to the amount delivered on account of the rubber 
losing its elasticity. For this reason the tubing should be made 
quite short and when it begins to get old it should be renewed. 

The corrections obtained as above are best tabulated by 
means of a plot in which the bvtfrette readings are taken as ~ 
abscisse and the corrections as ordinates. By connecting the 
points, a curve is obtained by means of which the correct reading 
of all parts of the burette can be obtained at a glance. 


Method of Reading Burettes. 


Although in the case of graduated flasks and pipettes the 
marks are carried around the whole circumference of the tube, in 
the case of burettes this is not usually done,t so that it 
is a matter of some difficulty to determine with certainty 
the exact position of the lowest part of the meniscus. 
To avoid a parallax error a number of means have been 
devised. Thus floats are often used such as are shown 
in Fig. 84,a and b; the former represents that of Beuttel 

Fic. 84. and the latter that of Rey. Around the bulb of aa 
circle is etched, and if the eye is in the correct position, it appears 

to the observer as a straight line. The liquid in the burette is at 
the zero-mark, when the projection line from the circle on the float 
exactly coincides with the line at the zero-point on the burette. 
In the case of dark-colored liquids it is difficult to see the circle in 
the case of the float a, but this difficulty is overcome in 6 by the 
circle being etched upon the upper bulb (in the figure the latter 
is drawn too small). Such floats are weighted so that the upper 
bulb rises above the level of the liquid in the burette; it is, 
therefore, easier to make a reading with the float devised by 
Rey than with that of Beuttel. In refilling the burette, the 
former float assumes an inclined position; it must, therefore, 








* W. Schlésser, Chem. Ztg., 1904, 4. 
+ Habilitationsschrift, Leipzig, 1898. 
t Such burettes can be purchased, however, and very accurate readings 


can be made with them. 


CALIBRATION OF BURETTES. 529 


be removed, dried off, and again carefully introduced into the 
liquid.* 

Schellbach hasinvented another method of avoiding the parallax 
error, ea ae: the back of the burette with a dark vertical 





Fia. 85 

line upon a background of milky glass as is shown in Fig. 85. 
When the eye is in the correct position, this dark line is ere ney 
drawn out into two points E 
as shown in 0, whereas if the 
eye is too low the appear- 
ance a is obtained, or c if 
the eye is too high. 

Kreitling ¢ has proved, 
however, that the use of 
floats is likely to lead to 
error, and experiments have 
also shown that the Schell- 
bach burettes are not alto- 
gether reliable. Better than 
these is the Bergmann’s screen as improved by Géckel.t If burettes 








* This difficulty is overcome by Diethelm by placing below the large 
bulb a second “flattened-out” bulb, and in this case the float will not 
attach itself to the sides of the burette, so that it is not necessary to remove 
it in refilling the burette. 

+Z. angew. Chem., 1900, 829, 990; 1902, 4. 

t Chem. Ztg., 1901, 1084. Z. angew. Chem., 1898, 1856. 


53° VOLUMETRIC ANALYSIS. 


are used on which the divisions extend at least half around the tube, 
and the eye is placed so that the line on the back of the burette 
coincides with that on the front, then with the use of this screen 
very exact results are obtained. 

Bergmann’s screen, which can be used to advantage with all 
kinds of vessel, consists of a wooden test-tube holder painted a 
dull black (Fig. 86). The reading is made easier if a piece of 
ground glass or a strip of oiled paper is held behind the burette, 
or fastened to the screen itself. 

The “ kaiserl. Normalaichungskommission ” gives as the 


_ ALLOWABLE ERROR FOR BURETTES. 


Contents. ....... 100 75 50 30 10 2 c.c. 
0.08 0.06 0.04 0.03 0.02 0.008 c.c. 


Normal Solutions. 


By a normal solution is understood one which contains one 
“eram-equivalent” of the active reagent dissolved in one liter of 
solution. By “‘gram-equivalent”’ is meant the amount of substance 
corresponding to: one gram-atom (1.008 gms.) of hydrogen. For 
convenience in computation the concentration of solutions used 
for volumetric purposes are expressed in terms of their normal- 
ity; i.e., a solution is 2 normal, 4 normal, jy normal, etc. The 
letter N is used as an abbreviation for normal. 

A normal solution of the following substances will contain 
dissolved in 1000 c.c.: 


Hydrochloric acid, 1 gm.-mol. HCl= 36.468 gms.=1 gm.-atom H. 
Nitric acid (as an acid), 1 gm.-mol. HNO3=63.018 gms.=1 gm.- 








atom H. 
: 98.086 
Sulphuric acid, + gm.-mol. H2SO4,= 9 = 49.043 gms.=1 gm.- 
atom H. : 
Potassium hydroxide, 1 gm.-mol. KOH=56.108 gms.=1 gm.- 
atom H. 
Le 106.0 3 
Sodium carbonate, 4 gm.-mol. NasCO3= 9 = 53.00 gms.=1 


gm.-atom H. 
Silver nitrate, 1 gm.-mol. AgNO3= 169.89 gms.=1 gm.-atom H. 


NORMAL SOLUTIONS. 531 


How much potassium permanganate or potassium dichromate 
must be dissolved in 1 liter of solution to obtain a normal solu- 
tion of each salt for oxidation purposes? 

First the oxidation equations must be written: 

1. In acid solutions potassium permanganate is reduced to 
salts of potassium and manganous oxides with loss of oxygen; 
the latter is taken up by the substance oxidized: 


2KMnO,=K,0+2Mn0+50. 


Two gram-molecules of KMnO,, therefore, correspond to 5 gm.- 
atoms of oxygen. or 10 gm.-atoms of hydrogen. For the normal 
solution + molecular weight in grams of KMnQ,, we =31.61 
ems., should be dissolved in 1 liter, 7 

2. Potassium dichromate is reduced to salts of potassium 


and chromic oxides: 
K,Cr,0, = K,0+ Cr,0,+ 30. 


One gram-molecule of K,Cr,O, loses 3 gm.-atoms of oxygen, 
corresponding to 6 gm.-atoms of hydrogen. One liter of normal 
solution will contain + molecular weight in grams of K,Cr,0,, 
aes =49.07¢ms.* : : 
If, however, the potassium dichromate is not used as an oxi- 
dizing agent, but as a precipitant, e.g. for the precipitation of 
barium as barium chromate, the case is different: 


K,Cr,0,-+2BaCl,+ 2NaC,H,0, +H,0 = 
—2KCl+ 2NaCl+ 2HC,H,O,+ 2BaCr0,. - 





* The same results will be obtained by considering the change of valence 
which the manganese and chromium undergo on reduction. The former 
is reduced (in acid solutions) from a valence of seven to a valence of two, 
corresponding to five atoms of hydrogen. In the dichromate, each of the 
two atoms of chromium is changed from a valence of six to a valence of 
three, equivalent to six atoms of hydrogen. Consequently from this point 
of view it is evident that 1 of the molecular weight of permanganate and 
1 of a molecular weight of bichromate must be used to prepare one liter 
of normal solution.—_{Translator.] 


532 VOLUMETRIC ANALYSIS. - 


One gram-molecule of K2Cr2Qz, therefore, precipitates 2 gm.e 
atoms of barium, and this corresponds to 4 gm.-atoms of hydrogen, 
so that + gm.-molecule of KsCr2Q7 will now be sufficient to make 1 


An =73.6 gms, 





liter of normal solution, 


It is evident, then, that the amount of substance necessary to 
make a normal solution depends upon the purpose for which it 
as to be used, ss 


Preparation of Normal Solutions. 


The required amount of substance should be dissolved in 
water at 15° and diluted to a volume of 1 liter while at this tem- 
perature. In most cases, however, the water is not at the normal 
temperature of 15°, so that it is customary to dissolve the substance © 
in water at the laboratory temperature and then dilute the solution 
up to the mark in a liter flask. After thoroughly mixing the 
solution, its temperature is taken by a sensitive thermometer. 
If the temperature is above 15°, as is usually the case, the volume 
of the solution would be less than 1 liter if it were cooled to 
exactly 15°, so that the solution as made up is a little too strong. 
The error can be computed as follows: 

Not only the solution, but the glass of the flask should have 
been at the normal temperature of 15°. The coefficient of cubical 
expansion for glass may be taken as a, and that of the solution 
as 8. The volume of the flask, and that of the solution is equal to 
~ 1000[1 +a(f—15)] ¢.c., but this volume of scatroee at ¢ would 
assume at 15° a volume of 3 


1+a(t— 15) 
1+ B(¢— 15) 





1000 


Schlésser * has worked out the following table to show how 
much greater or less a given volume is at different temperatures 
than it would be at exactly 15°. 





* Chem, Ztg., 1904, 4; 1905, 510. 


PREPARATION OF NORMAL SOLUTIONS. 533 


TABLE FOR THE REDUCTION OF THE VOLUME OF WATER, NORMAL, AND TENTH 
NORMAL SOLUTIONS TO THE NORMAL TEMPERATURE OF 15° c 

















Tem- tae */, 0. */, 0. */,n. */, n. /,n. */, 0. 
perature. Wakiticea. HCl. HeC204. H2S80,4 HNOs3 NazCOz3 NaOuil 
5? +0.60 {|+1.26 |4+1.33 |+1.94 |+2.00 |+2.03 |+2.18 
6 0.60 1.18 £25 1.79 1.84 1.87 1.99 
7 0.59 1.10) 1.16 1.63 1.68 1.69 1.80 
8 0.56 1.00 1.05 1.46 1.50 1.50 1.60 
9 0.52 0.88 0.94 1.28 v.31 1-31 1.39 
10 0.46 0.76 0.81 1.09 P14 ane of 1.18 
11 0.40 0.63 0.67 0.89 0.91 0.90 0.96 
12 0.33 0.48 0.52 0.68 0.69 0.69 0.73 
13 0.22 0.33 0.35 0.46 0.46 0.47 0.50 
14 +0.12 |+0.17 |4+0.18 |4+0.23 {40.23 |4+0.24 |+0.25 
15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 
16 —0.13 |—0.18 |—0.20 |-—0.24 |—0.25 |—0.24 |—0.25 
17 0.27 0.36 0.40 0.49 0.50 0.49 0.51 
18 0.42 0.56 0.61 0.75 0.76 0.75 0.78 
19 0.59 0.76 0.82 1.02 1.03 1.02 1.05 
20 —0.76 |—0.97 |-—1.05 |-—1.30 |-—1.30 |-—1.29 |-—1.33 
21 0.95 1.19 1.29 1.58 1.58 ae yg 1.62 
22 1.94 1.41 1.54 1.86 1.87 1.85 1.92 
23 1336 1.64 1.80 2.15 2.17 2.14 2.23 
24 1.56 1.88 2.07 2.45 2.47 2.44 2.04. 
25 1.79 2.14 2:08 |} 2.06 2.78 2.75 2.85 
26 2.02 2.40 2.62 3.08 3.10 3.06 3.17 
27 Re 3 2.67 2.90 3.41 3.43 3.38 3.50 
28 2:82 2.95 3.19 3.75 3.76 3.70 3.83 
29 2315 3:20 3.49 4.09 4.10 4.04 4.17 
30 —3.06 |-—3.52 |-—3.82 |-—4.438 |-—4.44 |-—4.38 |-—4.52 


























The use of the tables on PP- 533-535 can be best explained by 
a few examples: 

1. A liter flask is ialibrated to contain exactly 1000 true c.e. 
at 15°. A normal solution of sodium hydroxide is prepared at 
25°. The table shows that the solution at 25° would occupy at 
15° 2.85 c.c. less, so that in order to make the solution exactly 
normal, 2.85 c.c. of water should be added. 

2. In a titration 47.35 c.c. of normal sodium hydroxide solution 
were used which was at a temperature of 19°; this amount of 
solution would at the normal temperature occupy a volume of 

47.35 X 0.76 - 


47.35 — 1000 = 47.31 ¢ 





VOLUMETRIC ANALYSIS. 


534 


TABLE FOR THE REDUCTION OF THE VOLUME OF A N/10 


(Correction is given in 


cooocooooo 


soocococso | 


coccococooso locococccccs 


COoceoOCOrns ans ‘| 












































° ~ 
© 

= | | | | 
fo cCooo rococo |;ocoooooooooo ;oooooooeoeoeoe ;}ooooocooooeo ;}ooqoqoooccooo|o 
a + + Br ne 5 ae “F 
tH cococoooo |coooooooooo j}oooooooooo ;oooooooooo j}ooooococooo!|o 
em | + 5 33 + + a 
oa Ccoococoooo }cooooooooOoS | COO SCCOOOC ON [RANA NAAN NA | SANNA NN | oS 
= ae + SF “te - + 
w oooocooooo | oooooooOOn | XFAnAnNnAaAnAaAmes as eles here ek oe Oe Be oe Be Be 
=| + + 23 + + 
| COCO COCCOCS | COCOONS | TNR PSN LNNNANNNANANANN IAN 
a | + + + + + + 
2 |coooe coo | COCR RRA | FRR | FR NANAAN | AANAAAAAAN | A 
sie Face . 5 + + 4m & 
° COCO COCOCOS | COR RRR NN | RRR LN NNANNNNNANNA  NANNNNNANNAAA | OO 
aed aa *F + Le + $ 
° coocooooo ak dak eceh ged le, eek cook Sean eB N NANNNANANNANANN NANNANANNANN 09 09 09 ine] 
Oo 1+ + * aa one f 
° DOCDCCOCOCCS | CONN NNN NNN | ARRAN AN NANNAANAAAN AEN NANI OD 09 0 09 OD OD | OD 
caeea el. + 7 + + 4 
e eases scoed © Onn Ane A eee Seas BB NANN NNANNANNNANANANANAN NNAAN OD OD 09 09 OD OD oD 
aed aes + + i + 
° COCCCOCSS | CONN NNN | RAN NNNN | NANNANN NAAN LANA N09 09 09 OF OD OD | OD 
yg dies ek + e ‘Z ee + 
) 

= 

3 

Os MAMAINOR OD | OHNMHOSHMOD | OANMHOOrOM | © 
BS | ANOMMOMDA | SING WSON LA | SHARAAAAAA | Dasa |THAHEAMEG |B 
28 

~~ 

: 

a 























* Computed according te 


535 


PREPARATION OF NORMAL SOLUTIONS. 


SOLUTION TO THE NORMAL TEMPERATURE OF 15° c. 


30° 


CRA NNANNANAS 
| 


(oe eee 


DOOMDNOHHDOH 
| 





29° 


SOR RRANANNN 


OD OD OD OD SH eH SH LO LD LD 


IDO O OP Pee OO 





28° 


SCOnm RRA NANN 


Sh oe er eee 


IDIDIN OOOOK-NES 
| 





27° 


SOOn nA AA NN 


QUN 0D OD 0 OD SH SH SH SH 


HAD 19 19 19 OO OOO 


I~ Ie De re DHODOHODOH 





26° 


COCA HAAAN 
| 


CII OD 69 0 0D CD OD SHH 
| =: 


st HH HD 19.1919 0 


OOOKRKNE DOD 
| 


DDHDABWHAAoOO 
mere 





25° 


COOn ssa HN 
| 


NNANANAN OD OD OD OD OD 


SHH SH HH 1919 191910 
| 


IDIDOOOOOnRNE 
| 


Sah clin ie core aa 





24° 


COONAN 


FHANNANNNANNM OO 
| 


CD OD OD SH SH SH SH SH SH tH 


LD 1D 1D 19 19 19 1 OOO 


OOOK-NENEN® 





23° 


oocOnsAnnrr 
| ; 


MBANNANANANAN 


OD OD 6 CD CD 09 OD SH SHH 


st SH SH SHH 19.20 19 1916 


ee ae 





22° 


ooococOrnnAasArH 
L. 


MRR HNNANNANN 


| 


NN AID 69 69.09 09 09 CD 


A eee ee 


TH LD UD 1D 19 19 19 19 10 





21° 


oooocoeonnarH 


SSeS ASR NNY 


NANANNAANAN OD 


OD OD OY) OY) CY) CYD CY) CY) SH SH 


SSH SH SH Ht eH cH tH 10 
[ 





20° 


ocoooco°o°cons 
| 


Se A oe Oe Oe Oe Oe Oo Oe oe | 


PFNNANNANNNANN 


NAN AN OD 09 OD OD OD 


CD OD CY OD OD OD OD SH MH <H 





19° 


coooocoooso 


Ce Oe Oe ee ee Be Be 


Sess ARR TNN 


NANNANANANANN 


NAAN ATAN 09 09 09 0 OD | 





18° 


ooooooooo 


ooococOrnnNANAsS 
| 


bs ee eB eB | 


SSeS SSS NN 


ee 








reo cubic centimeters.*) 


yaad 





fo ee 





ocoooooeoseo 
l 





SCOnt RANA NAS Ae 





ot et ed ed et et ed 





eet et St St st 





—1 |-2 |-3 4 -5 6 6 6 (9-90 irr ois |—15 





the table on page 493. 


536 VOLUMETRIC ANALYSIS. 


3. If a normal solution of common salt is prepared at 25°, it 
is evident that the volume of one liter when reduced to the normal 
temperature would be 1000—1.79=998.21 c.c. The solution is, 
therefore, too strong, for it contains as much sodium chloride as 
should be present in 1000 c.c. at the normal temperature. 1 c.c. 
of this solution is equivalent to 1.0018 ¢.c. of a normal solution 
prepared at 15°. 

This number is called the factor of the solution, for if the number 
of cubic centimeters actually used is multiplied by it, the result 
represents the corresponding number of cubic centimeters of 
exactly tenth-normal solution. 

Similarly in all future experiments, the actual volume should 
be reduced to the normal temperature when the greatest accuracy 
is desired. This reduction can be accomplished by means of the 
tables on pages 534 and 535 for tenth-normal solutions, and by 
the table on page 533 for more concentrated solutions. 

If 20 ¢.c. of a tenth-normal solution is used at 25° this would 
correspond at the normal temperature of 15° to 20 —0.04=19.96 


c.c. (ef. p. 535). : 
Now calibrated vessels serve not only for the measurement of 


water but for other dilute and concentrated liquids. The question 
arises as to whether the volumes as determined by weighing water 
are accurate for those liquids which differ greatly from water as 
regards viscosity, adhesion and capillarity. In the case of vessels 
calibrated for contents, the only difference is that arising from the 
different nature of the meniscus; but even in the most unfavorable 
instances no appreciable error is occasioned. Schlésser and 
Grimm * have carefully studied the amount of error in vessels 
calibrated for delivery. According to them there is no correction 
needed for tenth-normal solutions except in the ease of iodine. 
With normal solutions a correction is needed at the most only 
with hydrochloric and oxalic acids and with liquids in the nature 
of alkalies and ferric chloride (1 c.c.=0.012 gm. Fe). In the 
case of concentrated liquids it is those containing alcohol in which 
the deviations are most marked. ‘Thus Boutron and Boudet found 
that. with an alcohol soap solution, 0.255 c.c. less was delivered 





* Chem. Ztg., 1906, 1071. 


PREPARATION OF NORMAL SOLUTIONS. 537 


from a 100 c.c. pipette than of water, and from a 25-c.c. pipette, 
0.103 ¢.c. less. With concentrated alkalies and acids the devia- 
tions were also quite marked; thus with 95% sulphuric acid, 
0.442 c.c. less were delivered from a 100-c.c. pipette and 0.085 c.c. 
less from a 10-c.c. pipette than when water was used. The 
amount of time’allowed for the draining of the pipette exerts an 
important effect in this connection. If the pipette is allowed to 
drain for a long time the negative correction becomes smaller 
and may even become positive. It would be well, then, for the 
chemist to determine the amount of time required to drain a 
pipette with water and with any other liquid, and if the difference 
exceeds two seconds to then determine the contents of the pipette 
for the other liquid. If it be desired to avoid this difficulty, the 
pipettes may be graduated both for contents and for delivery. 
The pipette is then filled with the special liquid to the mark 
corresponding to the former graduation, it is allowed to drain, 
and then the remaining solution is carefully washed out. 


SUBDIVISIONS OF VOLUMETRIC ANALYSIS. 


I. Acidimetry and Alkalimetry. 
II. Oxidation and Reduction Processes. 
III. Precipitation Processes. : 


I. ACIDIMETRY AND ALKALIMETRY. 


This covers the analysis of acids and bases. In order to deter- 
mine the amount of acid present, an alkaline solution of known 
strength is required; and conversely, in the analysis of a base, 
an acid solution is required. In both cases the ‘‘end-point”’ of 
the reaction is determined with the help of a suitable indicator. 
The accuracy of the result depends largely upon the choice of 
the indicator, so that at this place a few words will be said with 
regard to the indicators most frequently used for detecting the 
presence of acids or alkalies. 


538 VOLUMETRIC ANALYSIS. 


INDICATORS. 


The indicators used in acidimetry and alkalimetry are dyestuffs 
which are of one color in acid solutions and another color in dilute 
alkali. They are, as a rule, weak acids; though some of them are 
weak bases. It has been found that in organic compounds the 
color can usually be traced to a particular arrangement of atoms 
called a chromophor. The change in color, therefore, is caused 
by a slight rearrangement of the atoms in the molecule. Thus, 
if the salt of an indicator acid is yellow and on treatment with © 
acid it turns red, this is due to the fact that when the free indi- 
cator acid is set free by the action of the stronger acid, it under- 
goes a change whereby a slight change takes place in the way 
the atoms are linked together in the molecule, and in fact thereby 
loses temporarily the ability to dissociate electrolytically as an 
acid. It is not sufficient, however, to assume that this change 
of color is caused solely by the fact that the ions have a color other 
than that of the undissociated molecule; on the contrary it has 
been shown in certain cases that the ions have the same color 
that the undissociated molecule has before the rearrangement of 
the atoms in the molecule has taken place. On the other hand, 
as regards the proper use of indicators it is necessary simply to 
bear in mind how salts of weak acids behave in the presence of 
stronger acids and how the acids themselves behave in the pre- 
sence of alkali. 

The number of indicators which have been discovered and 
used more or. less is very large, but it will be sufficient here to 
consider only methyl orange, methyl red, lacmoid, litmus, and 
phenolphthalein. 


INDICATORS, 539 


1. Methyl Orange.* 


Under methyl orange, Lunge,+ who first proposed the use of 
this indicator, understood either the free sulphonic acid of dimethyl- 
amido-azo-benzene or its sodium or ammonium salt. 

In the free state the free sulphonic acid is obtained in the form 
of reddish-violet scales, soluble in considerable water. If some of 
the solid is dissolved in as little water as possible, a distinct reddish- 
orange colored solution is obtained; but on the further addition of 
water this color gradually changes to yellow. Ifa trace of an acid 
is added to the yellow solution, it becomes red again and on further 
dilution with water the color changes to orange and finally to 
yellow once more, if too much acid was not added. This color 
change can be easily explained. 

In the sensitive neutral solution there is a dendltion of equi- 
librium between two isomeric forms of methyl orange as expressed 
by the equation: 


HSO3-CgH4-N:N-CeH4N(CH3) 2 
sa ¥ CeH4 -NH-N: CoH, :N (CH3)e 
| 





The formula on the left represents the yellow substance and the 
color is due to the azo group N:N, whereas the formula on the 
right represents the red substance which has for its chromophor 
the quinoid group :CgH4:. The formula on the left has a sul- 
phonic group which imparts acid properties to the molecule and 
at the other end is an N(CH3)2 group which has weakly basic 
properties. The formula on the right, therefore, represents an 





* This dyestuff is known commercially as helianthin, orange III, tro- 
piolin D, Poirrier’s orange III, dimethylaniline orange, mandarine orange, 
and gold orange. 

{ Berichte, II (1878), p. 1944; Zeitschr. f. ch. Industrie, 1881, p. 348; 
Handbuch fiir Sodaindustrie, I (1879), p. 52; II (1893), p. 151. 


540 . VOLUMETRIC ANALYSIS. 


inner salt inasmuch as the acid and base forming groups are here 
united. 
The sodium salt of methyl orange is ales and has the formula 


NaSO3-CeH4N : NCgeH4N(CHs3)2 


and when decomposed by acids the free sulphonate at once reverts 
to the red form: 


Be -CeH,: NH. N:CgH4: N(CH3)2 
| 





Methyl orange is an excellent indicator for weak bases, but 
cannot be used for the titration of weak acids.* 

If it is desired to titrate a solution containing sodium hydrox- 
ide with a tenth-normal acid, a little methyl orange is added to 
the alkaline solution and the acid is added until the solution is 
colored a distinct red. The latter color will not appear, however, 
until an excess of the acid has been added. This causes a slight 
error in the analysis which is greater in proportion to the amount 
of indicator employed, and the more dilute the solution. | 

It is apparent that the weaker the acid character of the indi- 
cator the more sensitive it will be, and the opposite is true of in- 
dicators which are bases. 

From what has been said the following rule holds: 

In any titration the smallest amount possible of indicator 
should be used, and inasmuch as the change of color is propor- 
tional to the concentration and not to the absolute amount of 
acid present, the titrated solution should have as nearly as pos- 
sible the same concentration as was the case in the standardiza- 
tion of the normal solution. 

When a normal acid is used for the titration, the change of 
color is very sharp when the volume of the solution titrated — 





* Cf. Stieglitz, J. Am. Chem. Soc., 25, 1117. 


INDICATORS. — 541 


amounts to about 100 c.c. Even with a fifth normal solution the 
change of color is very distinct, but less so with tenth-norma} 
solutions, but these can be titrated provided the standardization 
was made at the same dilution as that used in the analysis. 

How is it with the end-point in the titration of an acid with 
an alkaline hydroxide solution? 

If a few drops of methyl orange are added to 100 ¢.c. of water, 
the latter will be colored distinctly yellow. If we imagine that 
the solution contains the same amount of gaseous hydrochloric 
acid as is contained in 10 ¢.c. of a tenth-normal solution of this 
acid, the solution will be colored a deep red. In order that the 
solution shall assume its original yellow color, it is only necessary 


to add exactly 10 c.c. of a alkali hydroxide solution, but no ex- 


cess of alkali, because the water is itself sufficient to decompose 
the dyestuff sufficiently to produce the yellow color. 

It is evident, then, that it is not a matter of indifference in 
the analysis whether the titration is completed by the addition 
of acid or by the addition of alkali. In the former case, for the 
titration of T c.c. of “ alkali solution, T+? c.c. of x acid would 
be necessary. 

Methyl orange is more sensitive toward alkali than it is toward 
acid, but many prefer to finish the titration by the addition of 
acid, for most eyes can detect the change from yellow to red with 
greater accuracy. In principle it is more accurate to accomplish 
the titration the other way, as was recommended by F. Glaser. 

Preparation of Methyl-orange Solution.—The solution of 0.02 
gm. of solid methyl orange * dissolved in 100 ¢.c. of hot water is 
allowed to cool, and any deposited meta-sulphonic acid is 
filtered off. 

Use.—Methyl orange is suitable for the titration of strong 
acids (HCl, HNO,, H,SO,) as well as phosphoric and sulphurous 
acids. Hydrochloric and nitric acids can be titrated with this 





* Tf the free acid is not at hand, 0.022 gm. of the sodium salt is dissolved 
in 100 c.c. of water, 0.67 c.c. af HCl is added, and after standing some time 
any deposited erystals are filtered off. 


542 VOLUMETRIC ANALYSIS. 


indicator with a sharper end-point than is the case with sulphurie 
acid If free phosphoric acid is titrated with sodium hydroxide 
using this indicator, the solution changes from red to yellow when 
one-third of the phosphoric acid has been neutralized: 


H,PO,+ NaOH =NaH,PO,+ H,0. 


The primary phosphates are neutral toward methyl orange, 
while the secondary and tertiary phosphates react alkaline toward 
it. With half-normal solutions, the end-point of the reaction is 
fairly sharp, with tenth-normal solutions it is less so; in the latter 
case an excess of about 0.3 ¢.c. of the tenth-normal alkali is nec- 
essary to cause the change from red to yellow. 

Sulphurous Acid.—In titrating sulphurous acid with sodium 
hydroxide, the yellow color is obtained when half the acid has 
been neutralized, ; 

H,SO,+ NaOH = NaHSO,+ H,O, 


so that NaHSO, is neutral toward this indicator. 

The weak acids HCN, CO,, H,S, As,O,, B,O,, CrO, when pres- 
ent in considerable amount do not act upon the indicator. CO, 
and HS produce an orange-red coloration only when present in 
large amounts. For this reason the alkali salts of these acids can 
be titrated with accuracy by means of this indicator. 

Organic acids cannot be titrated with methyl] orange. 

The strong and weak bases NaOH, KOH, NH,OH, Ca(OH),, 
§Sr(OH),Ba(OH),, and Mg(OH), can be titrated with great accu- 
racy by means of this indicator, and the same is true of the amine 
bases (methyl and ethyl amines, etc.); on the other hand, such 
weak bases as pyridine, aniline, and toluidine cannot be titrated. 

Nitrous acid ordinarily cannot be titrated with this indicator 
because the acid destroys it. If, however, an excess of alkali is 
first added to the solution of nitrous acid, then the methyl orange, 
the titration can be accomplished with accuracy. 


B. THE SODIUM SALT, 
4 1 4 1 
N(CH,).—C,H,—N==N—C,H,SO,Na. 


This sodium salt can be used as an indicator in the same way 
as the free acid; it should be mentioned, however, that the com- 


INDICATORS. 543 


mercial salt often contains small amounts of sodium carbonate 
as impurity, which causes it to be slightly less sensitive than the 
free acid. : 

The change of the yellow color of the solution in this case 
takes place when the salt has been decomposed by the addition 
of an equivalent amount of a stronger acid, and the dissociation 
of the free acid diminished by increasing the concentration of the 
hydrogen ions. As a matter of fact, however, the amount of 
acid necessary to effect this change in a solution containing a 
drop of the indicator solution is inappreciable. 


2. Methyl Red. * 


; a 1 
( CH3) 2N—CgH4,—N = N—C,H,—COOH. 
Para-dimethyl-amido-azo-benzene-o-carboxylic acid. 


This valuable indicator is suitable for titrating weak organic 
bases and ammonia. The aqueous solution of methyl red is 
orange, but if a few drops are added to 50-100 c.c. of water, the 
latter is colored a pale yellow. The addition of a drop of 0.1 N. 
HCl at once turns the liquid a violet red without passing through 
any intermediate shade and by the addition of a drop of ammonia 
the solution becomes nearly colorless again. Methyl red is not 
very sensitive toward carbonic acid, but more so than is methyl 
orange, so that it is less suitable for the titration of carbonates. 
-The chief advantage of this indicator lies in the sharp color 
change from a very pale yellow to a violet red, even in titrating 
ammonia. 

Preparation of the Indicator. About 0.02 g. of the free acid 
is dissolved in 100 c.c. of hot water, the solution allowed to cool, 
and then filtered. Two or three drops of this solution are added 
for every 100 c.c. of the solution to be titrated. 

H. W. Langbeck + recommends the use of ortho-nitrophenol 
as indicator, but it has no advantages over methyl orange and 
methyl red. It is not at all sensitive towards carbonic acid. 
It is turned yellow by alkalies and colorless by acids. 


* E. Rupp and R. Loose, Berichte, 41, 3905 (1908). 
t Chem. News, 43, 162. 





544 VOLUMETRIC ANALYSIS, 


3. Lacmoid, or Resorcin Blue, 
C,,.H,O,N. 


Lacmoid is prepared by heating resorcin with sodium nitrite 
at not too high a temperature. The constitution of the dye has 
not been completely established. Pure lacmoid is soluble in 
water (the impure product is difficultly soluble), but more soluble 
in aleohol, glacial acetic acid, actorfe, and phenol, and less so in 
ether. To determine whether a sample of commercial lacmoid 
is suitable for use as an indicator, a little of it is boiled with water; 
if the water is colored an intense and deautiful blue, it can be 
used. In this case the alcoholic solution will be of a pure blue 
color, and not with a tinge of violet, as is the case with the impure 
substance. 

Preparation of Pure Lacmoid.—The solution of the good com- 
mercial product in hot 96 per cent. alcohol is filtered and allowed 
to evaporate in vacuo over concentrated sulphuric acid. 

Preparation of the Indicator.—A solution is used containing 
0.2 gm. of the purified lacmoid in 100 c.c. of alcohol. 

Behavior of Lacmoid toward Acids and Bases.—lf the solution 
after it has been colored reddish by acid is treated with a solu- 
tion of an alkali hydroxide, the red color is gradually changed 
to a violet-red, and on further addition of alkali, it suddenly 
changes to a pure blue. If the violet solution is diluted with 
considerable water, it becomes blue. 

Uses.—Lacmoid is suitable for the titration of strong avid 
and bases as well as for ammonia, but is not suited for the titration 
of nitrous acid or weak acids. 


4. Litmus. 


The chief coloring principle of litmus, the azolitmin, is a dark- 
brown powder only slightly soluble in water and insoluble in 
alcohol and ether. With alkalies it forms a readily soluble blue 
' galt. Besides the azolitmin, there are other dyestuffs present in 
litmus which are soluble in alcohol with a red color. 

Commercial litmus is obtained in small cubes mixed with con~ 
siderable calcium carbonate; the dyestuffs are then in the form 
of their calcium salts, soluble in water. If the commercial mate- 


INDICATORS. 545 


rial is dissolved in water, a solution of blue and _ reddish-violet 
coloring matter is obtained, which becomes red on the addition of 
acid. On making alkaline again, a pure blue color is not obtained 
at first, but a reddish-violet, which becomes blue on the addition 
of considerable alkali. Such a solution, therefore, is far from 
being a sensitive indicator and cannot be used for accurate work. 
A number of different methods have been proposed for obtaining 
a sensitive litmus solution, and that of F. Mohr* will be de- 
scribed. 

Purification of Iitmus.—The cubes of litmus are placed in a 
porcelain dish (without powdering), covered with 85 per cent. alcohol, 
and digested on the water-bath for some time with frequent stirring. 
The solution is decanted off and the operation is repeated three 
times. By this means the undesired coloring matter is removed. 
The residue is now extracted with hot water, and as it is very 
difficult to filter the solution, it is poured into a tall cylinder, and 
after standing several days the clear liquid is siphoned off. The 
solution is concentrated to about one-third of its volume and acidified 
with acetic acid in order to decompose the potassium carbonate 
present. It is then evaporated to a syrupy consistency upon the 
water-bath and the mass covered with a large amount of 90 per cent 
alcohol. By this means the blue coloring matter is precipitated, 
while the remainder of the violet substance remains in solution 
with the potassium acetate. The residue is filtered off and dis- 
solved in sufficient hot water so that three drops of the solution 
will be necessary to impart a distinct color to 50 c.c. of water. 

Use.—Litmus can be used for the titration of inorganic and 
strong organic acids, alkali and alkaline-earth hydroxides, and 
ammonia, as well as for the titration of carbonates in hot sola- 
tion. 

5. Phenolphthalein. 


Phenolphthalein is a very weak acid forming red salts which 
contain the strongly chromophoric quinoid group :CgH4:. The 
free acid, however, is unstable and when set free from one of its 
colored salts reverts instantly into a colorless lactoid form, con- 
taining no chromophor group: 


HOOC.: CgH4:C(CeH40H) : CoH: ei : ies -C(CgH40H)2 


* Lehrbuch der Chemisch-Analytischen Titrirmethode. 








546 VOLUMETRIC ANAL YSIS. 


In the case of the free acid, therefore, the condition of equi- 
librium favors the lactoid form and only minimal traces of the 
quinoid acid are present. This trace of quinoid acid is ionized. 
_ and is in equilibrium with its ions: 


4 


HOOCG-CeH4-C(CgH4OH) :CgH4:0@ 
@H' +00€-CeH,-C(CgH4,0H) :CeH4: 0" 


~The addition of an alkali causes the hydrogen ions to disappear, 
so that more of the quinoid molecules must be ionized to preserve 
_ equilibrium, and the quinoid molecules in turn be reproduced from 
the lactoid as fast as the former are converted into the salt. 
Phenolphthalein is a very sensitive indicator towards acids, but 
on account of being such a weak acid it does not form stable 
salts with weak bases. 

Preparation of the Indicator.—One gram of pure phenolphthalein 
is dissolved in 100 c.c. of 86% alcohol. | 

Uses.—Phenolphthalein is particularly suited for the titration 
of -organic and inorganic acids and strong bases, but not for the 
titration of ammonia. | 

If the red-colored solution containing phenolphthalein and a 
little alkali is treated with an excess of concentrated alkali hydrox- 
ide solution, the red color disappears, but returns on diluting the 
solution with water. Phenolphthalein, therefore, cannot be used: 
as an indicator for the titration of concentrated alkali without 
previous dilution with water. 

‘Phenolphthalein is the most sensitive indicator we possess 
toward acids, far more sensitive than methyl orange, for in this 
case not only can the presence of weak acids be detected, but very 
small amounts can be titrated with accuracy. 

- Ordinary distilled water usually contains carbon dioxide, as 


can be shown by slowly adding = barium hydroxide solution, drop 


by drop, to 100 c.c. of water containing a drop of the indicator 
solution. Where the alkali first meets the water, a red color is 
produced which disappears on stirring, so that often as much as 
0.5 to 1.8 ¢.c. of the alkali must be added before a permanent red 
color is obtained. The disappearance of the red shows the pres- 


INDICATORS. 547 


ence of acid (in this case carbonic’ acid), and its amount. corre- 
sponds to the alkali neutralized. 

Phosphoric Acid.—lf a solution of phosphoric acid containing 
phenolphthalein is titrated with normal sodium hydroxide solu- 
tion, a permanent coloration is produced when two-thirds of the 
phosphoric acid is neutralized: 


H,PO,+2NaOH = Na,HPO,+ 2H,0. 


Apparently Na,HPO, reacts neutral toward phenophthalein, 
but this is not quite correct, for a pure solution of disodium phos- 
phate is colored by phenolphthalein a pale pink, and on diluting 
with water the intensity of the color increases owing to progres- 
sive hydrolysis: 

Na, HPO,+ HOH = NaOH’ + Na HPO, 

During the titration of phosphoric acid with sodium hydrox- 
ide, a pale-pink color is obtained somewhat too soon, and this color 
gradually increases in intensity until finally a maximum is reached; 
- the latter point is taken as the end-point. It is possible that this 
hydrolysis could be prevented by the addition of a large excess of 
sodium chloride and cooling to about zero Centigrade. 

Carbonic Acid.—If the solution of a neutral alkali carbonate 
is treated with phenolphthalein a red color is obtained, showing 
the presence of hydroxy] ions in the solution, due to hydrolysis: 

Na Na CO,+H OH@Na OH+ Na HOO. 

If hydrochloric acid is added to such a solution which is not 
too dilute and is at a temperature of 0° C., decolorization 
is effected when half of the soda has been neutralized. At ordi- 
nary temperatures’ a sharp end-point cannot be obtained; the 
color gradually fades. Pure sodium bicarbonate dissolved in 
ice-cold water is not colored by the addition of phenolphthalein; 
if it is warmed to the temperature of the room it turns red, but 
on cooling the color disappears (Kiister). ve 

Silicie acid seems to be without influence upon phenolphtha- 
lein, for alkali silicates (the water-glasses) can be titrated with 
accuracy. 

Chromic Acid and Acid Chromates are changed by the addi- 
- tion of alkali to neutral chromates and the latter have no action 
upon phenolphthalein. 


548 VOLUMETRIC ANALYSIS, — 


Alkali aluminates can be titrated accurately with this indicator, 
cor aluminium hydroxide does not affect it. 

Almost all the problems involved in acidimetry and alkalim- 
etry can be solved by the use of one or the other of these two 
indicators: methyl orange and phenolphthalein. For further 
information with regard to the countless other indicators which 
have been proposed, the student is referred to Glaser’s “ Indica- 
toren der Acidimetrie und Alkalimetfie,’ Wiesbaden, 1901.* 


NORMAL SOLUTIONS. 


For the standardization of the solutions used in acidimetry 
and alkalimetry, a great many different methods have been pro- 
posed, all of which more or less satisfactorily answer the purpose. 
It was Gay-Lussac who first proposed the use of chemically-pure, 
calcined sodium carbonate, ana in simplicity and accuracy this. 
method has never been ecastied yt so that we will content ourselves | 
with its description. 

The chemically-pure sodium carbonate must form a clear solu- | 
tion with water and should contain neither sulphuric nor hydro- 
chloric acids. It is possible to obtain the pure substance com- 
mercially, but as a rule it must be purified. For this purpose 
about 300 gms. of crystallized sodium carbonate are dissolved in 
250 c.c. of water at 25-30° C., and quickly filtered into a two-liter 
flask of Jena glass. After replacing the air by carbon dioxide,t 
the flask is closed by means of a perforated rubber stopper through 
which a short, right-angled glass tube is passed, and the latter is 
connected by means of a long piece of rubber tubing with a 
Kipp-carbon dioxide generator. The contents of the flask are 
shaken until no more carbon dioxide will be absorbed; this usu- 
ally takes from half to three-quarters of an hour. In proportion 
as carbon dioxide is absorbed, sodium bicarbonate is deposited. 
The solution is cooled to 0° C., while the carbon dioxide is continu- 
ally passed through it ; the thick mass of crystals is transferred to a 





* See also J. Wagner, Zeitschr. fiir anorg. Chem., XX VII (1901), p. 138. 

+ According to Sérensen, the standardization takes place with equal 
accuracy by means of anhyrous sodium oxalate, which after weighing is 
heated ‘until the carbonate is formed; ef. page 597. 

t The carbon dioxide is passed through a solution containing sodium 
bicarbonate before it reaches the flask. 


NORMAL HYDROCHLORIC ACID. 549 


filter-plate which is covered with a piece of hardened filter-paper and 
sucked as dry as possible. The sodium bicarbonate thus obtained 
often contains considerable chloride and sulphate. It is washed : 
back into the flask by means of 50 c.c. of distilled water (that has 
been cooled to 0° C. and saturated with carbon dioxide), vigor- 
ously shaken, and the mother-liquor once more removed by suc- 
tion. This operation is repeated until finally 3 gms. of the salt 
will no longer give the test for chlorides or sulphates. 

The pure sodium bicarbonate thus. obtained is dried on the 
water-bath and preserved for further use in a tightly-stoppered 
bottle. 


Normal Hydrochloric Acid. 
1000 ¢.c. contain 1 HC]=36.468 gms. 

Pure, concentrated hydrochloric acid is diluted until its spe- 
cific gravity is about 1.020, and in this way a solution is obtained 
that is slightly more than normal in strength. To obtain an ex- 
actly normal solution, it is titrated against a weighed amount of 
chemically-pure sodium carbonate, and from the result obtained 
the amount of water to be added can be computed. About 8 
gms. of the pure, dry sodium bicarbonate are placed in a large plati- 
num crucible, and the latter is inserted in an inclined position 
within a hole in a piece of asbestos board and over a small flame 
(cf. p. 358). The contents of the crucible are stirred frequently 
with a short piece of heavy platinum wire, and only the bottom of 
the crucible is heated to redness. The mass must not be al- 
lowed to sinter together or fuse, for in that way an appreciable 
amount of the normal carbonate would be decomposed. After 
heating for about half an hour the crucible is cooled in a desicca- 
tor, weighed, and to make sure that a constant weight has been 
obtained, the heating is repeated once or twice more.* 





* If it is feared that-some of the carbon dioxide may be expelled from the 
normal carbonate, the bicarbonate may be heated for half an hour at 270— 
300°. This can be easily accomplished by embedding the platinum crucible, 
which contains the bicarbonate, in sand, so that the latter extends up on the 
outside of the crucible as high as the bicarbonate on the inside, and then 
heat slowly to 230°. The heating is then continued for about half an hour, but 
taking care that the thermometer in the sand beside the crucible does not 
register above 300°. 


550 VCLUMETRIC ANALYSIS. 


The amount necessary to neutralize 35-40 c.c.* of normal acid 
(about 2 gms.) is weighed out from a glass-stoppered weighing- 
tube into a beaker, dissolved in about 100 c.c. of distilled water, 
and enough methyl orange is added (from 5-6 drops) to impart a 
pae-yellow color to the solution. The hydrochloric acid at 17- 
18’ C. is added from a burette, with constant stirring, until the 
color of the solution is changed from yellow to orange. The 
burette is then read and a drop more of the acid is added to see 
whether this will produce a pure pink color. If this is not the case, 
more hydrochloric acid is added until this point is reached, and 
in this way the number of cubic centimeters of the acid that are 
required to neutralize the weighed amount of the sodium carbo- 
nate is determined. Assuming that for the neutralization of 
2.1132 gms. of NagCOs, 39.20 c.c. of hydrochloric acid at 19° were 
necessary, how strong is the acid? 

If the acid were exactly normal, according to definition (p. 
530) 1000 c.c. would neutralize io 3 _ 100.00 = 53.00 ems. of 
sodium carbonate, so that the amount weighed out would require 
for neutralization at 15° 





53 -00:1000 = 2.113: 2 


2113 
= Fa 53007 oo 87 ¢ 


This would be equivalent to 39.90 ¢.c. at 19°.+ 

As, however, only 39.20 c.c. were necessary it is evident that 
our solution is too strong, and for each 39.20 c.c. of the acid, 
39.90 —39.20=0.70 o.c. of water must be added to make it normal, 
and to 1 liter: 


39.2:0.70= 1000: 2 


700 
De 399 399 — 1% 86 c.c. water. 





* It is best not to weigh out more substance than can be titrated with 
one buretteful, and not too small an amount should be taken, for in the 
latter case the error in reading is too great. 

t According to the table on page 533, 1000 c.c. N. HCl at 19°=1000—0.76 
c.c. at 15°; therefore (1000 —0.76): 1000=39.83 : 2, x=39.90 c.ce. 


NORMAL HYDROCHLORIC ACID 551 


A perfectly dry liter flask is, therefore, filled exactly to the 
mark with acid, and 17.9 ¢c.c. of water are added from a burette 
(or measuring-pipette), the solution is thoroughly mixed, and the 
strength of the solution is verified by a second titration with a 
weighed amount of sodium carbonate. Further, it is to be rec- 
ommended that the beginner should convince himself of the accu- 
racy of the result by determining the amount of chlorine present 
gravimetrically as silver chloride. 10 c.c. of normal acid yield 
1.4338 gms. AgCl. 

For practical purposes it is quite unnecessary to spend the 
time necessary for the preparation of an exactly normal solution, 
but its normality * is determined, and if the number of cubic centi- 
meters used is multiplied by this factor, the corresponding amount 
of normal solution will be obtained. Thus in the above case 39.20 

_ ¢.c. of acid were used to do the work that would require 39.90 c.c. 

39.90 
, 39907 1.018 N. Or, 
if instead of using 40.10 c.c. it was found that 40.15 c¢.c. of acid 
were required, the solution would be 470.9987 N. Whatever 
the normality may be, it is written upon a label and pasted upon 
the bottle containing the acid. 

For most purposes, a normal solution is too strong, so that 
4,4 and +, N solutions are used. Obviously a tenth-normal solu- 
tion can be prepared by diluting 100 c.c. of a normal solution to 
1 liter, etc. 


of normal acid. The solution is, therefore 


In order to titrate a - acid solution with sodium carbonate. 


about 0.2 gm. of the salt is placed in a white porcelain dish and dis- 

solved in 50 ¢.c. of water, methyl orange is added until a pale-yellow 

color is obtained, and acid is added until the color becomes orange. 

The carbon dioxide is then expelled by heating to boiling, after which 

the solution is cooled and once more titrated until an orange color 

is obtained; the second titration requires but about 0.1-0.2 c.c. 
, more, but in this’ way the correct end-point is obtained. At this 
‘dilution the carbon dioxide exerts an imperceptible action upon 
‘the indicator. 





* By normality is understood the relation toa normal solution. 


552 VOLUMETRIC ANALYSIS. 


Normal Nitric and Sulphuric Acid Solutions. 


These are prepared in the same way as was described in the 
preparation of normal hydrochloric acid. | 


X Oxalic Acid. 
10 


1000 e.c. contain lice ee 9 ve = 6.303 gms. 





An oxalic acid solution of this strength can be prepared by 
dissolving exactly 6.303 gms. of pure, crystallized oxalic acid in 
water at 17°.5 and diluting to a volume of 1 liter. The com- 
mercial acid, however, must always be purified. 

The chief impurities found in the commercial product are cal- 
cium and potassium oxalates. In order to remove these salts, 500 
gms. are dissolved in 500 c¢.c. of pure, boiling hydrochloric acid 
of specific gravity 1.075 in a porcelain dish. If an insoluble resi- 
due should be obtained, the solution is filtered through a hot-water 
funnel and the filtrate received in a porcelain evaporating-dish, 
the latter placed upon ice and cooled as quickly as possible. The 
fine crystals thus obtained are placed in a funnel provided with a 
platinum cone and the mother-liquor completely removed by suc- 
tion. The above process is repeated, and the crystals obtained 
the second time are washed with a little ice-cold water, re- 
crystallized three times from hot water, and their purity tested. 
A solution of 2 gms. of the purified acid should give no sign 
of a turbidity with silver nitrate, and another portion of 5 gms. 
should leave no weighable residue after ignition in a weighed 
platinum dish. After having been dried as completely as pos- 
sible by suction, the crystals are spread out upon several layers 
of blotting-paper and allowed to stand in the air for several days; 
they then have the formula H2C204+2H20. The strength of the 


solution is tested by titration with a sodium hydroxide solution 


ee ; N 
using phenolphthalein, as indicator (see p. 553), or with To 


potassium permanganate solution (see p. 598). 


NORMAL SODIUM HYDROXIDE SOLUTION. 553 


Normal Sodium Hydroxide Solution. 
1000 c.c. contain 1 NaOH =40.01 gms. 


About 45 gms. of the commercial caustic soda are roughly 
weighed out, the carbonate on the surface is washed off as much as 
possible by a stream of water from the wash-bottle, and the alkali . 
is dissolved in a little more than a liter of water. The solution is 
then allowed to stand for about one hour beside the hydrochloric 
acid against which it is to be titrated, in order that both solutions 
may be at the same temperature. About 40 c.c. of the solution are 
measured off from a burette, and titrated with normal hydrochloric 
acid after the addition of a few drops of methyl orange solution. 
The titration is repeated several times with fresh amounts of the 
sodium hydroxide and from the mean of the results the amount of 
water to be added is calculated. If, for example, 


40 c.c. NaOH = 41.23 c.c. N. HCl, 


it is evident that 1.23 c.c. of water must be added to each 40 c.e. 
of the alkali to make the solution exactly normal, and for one liter 


40:1.23 =1000:z 
pa 1280 
| 40 
After the solution has been diluted with water until it is exactly » 
normal, it must be tested once more with the hydrochloric acid, 
and from it tenth-normal and fifth-normal hydroxide solutions 
can be prepared. 

The solutions thus obtained always contain carbonate, so that 
they are not suitable for titration with phenolphthalein, but with 
methyl orange the results obtained are the same as if all of the 
sodium was present as the hydroxide. With phenolphthalein 
accurate results can be obtained from a boiling-hot solution, or by 
preparing a solution of alkali free from carbonate. 


= 30.75 c.c. water. 


554 VOLUMETRIC ANALYSIS. 


Titration of Alkali containing Carbonate with Phenolphthalein in 
Hot Solutions, 


_ The alkali is measured into a porcelain dish, a drop of phenol- 
phthalein added, and hydrochloric acid of approximately the same 
strength .s run into the solution until the red color disappears. 
The solution is then heated to boiling, when the red color 
soon reappears; it is cooled by placing the dish in cold water,* 
hydrochloric acid is again added until decolorized, and the 
process is repeated until finally the red color does not reap- 
pear on boiling. This method of titration is tedious, but the 


results obtained are accurate. On titrating = acids with methyl 


orange as indicator, there is no sharp change from yellow to 
pink, as is the case with normal and half-normal solutions, but 
first a brownish-orange color is obtained which becomes pink on 
the addition of more acid. The correct end-point is the change 
from yellow to yellowish brown. Only when considerable car- 
bonate is present will this change occur before enough acid has 
been added, for in this case the carbon dioxide exerts an action 
upon the conte orange. 

The disturbing action of carbon dioxide is best cee by 
first titrating in the cold, then heating to remove the carbon © 
dioxide, again titrating the cold solution with acid. If only a 
small amount of carbonate is present, it exerts no appreciable 
effect upon methyl orange. 

The titration of oxalic acid with alkali which contains carbonate 
is best effected with phenolphthalein in hot solution. The process 
is carried out as Iollows: About 40 c.c. of the sodium hydroxide 
are accurately measured into a porcelain dish, a few drops of 
phenolphthalein added, and oxalic acid run in from a burette 
until the solution is decolorized. 

The solution is then heated upon the water bath until the 
red color reappears, whereupon it is decolorized by oxalic acid 





* With phenolphthalein the titration can be finished in the hot solu- 
tion, but the end-point is not so sharp. 


NORMAL SODIUM HYDROXIDE SOLUTION. 555 


and the process continued until finally the color does not reappear 
on heating the solution. This point is reached, however, only 
after the solution has been evaporated to dryness and the 
residue taken up with a few cubic centimeters of distilled water. 
A slight red color will appear after this first evaporation, but 
it will be discharged by the fraction of a drop of oxalic acid 
and will not reappear upon-a second evaporation. 

Remark.—Formerly the author was accustomed. to heat the 
oxalic acid solution over a free flame, but since Christie has found 
in this laboratory that it was impossible to reach an end point in 
this way, the use of a free flame has been avoided. 

Sdérensen met with the same difficulty and attributed the 
reappearance of the red color to the following reaction having 
taken place: | 


2Nael204 + H2O = NaeCOz “f 2HCOONa + COz 


Sodium formate 


It seems more probable, however, that this reappearance of 
the red color after the alkali is all neutralized is not due to the 
decomposition of sodium oxalate in the solution but to its being 
overheated on the sides of the dish, whereby it is decomposed into 
sodium carbonate and carbon monoxide: 


NagC204 = NasgCO3 af CO 


Such a decomposition does not occur when the heating takes 
place upon a water bath. 


Preparation of Sodium Hydroxide Solution Free from Carbonate. 


This is best effected as proposed by Kiister.* About 40 c.c. of 
pure alcohol is placed in a small round-bottomed flask, heated to 
boiling on the water-bath, and little by little 2.5 gms. of bright 
metallic sodium are added, the latter being freed from petroleum 





* Zeit. f. anorg. Chem., 18, 134. 


550 VOLUMETRIC ANALYSIS. 


by rubbing between pieces of blotting-paper. The reaction be- — 
tween the boiling alcohol and the 
sodium is at first very violent and 
large amounts of hydrogen and 
alcohol vapors are evolved. Dur- 
ing this time the flask is, there- 
fore, kept covered with a watch- 
glass. Gradually the reaction be- 
gins to diminish and finally stops. 
In the flask there will be a deposit 
of sodium alcoholate and some 
undissolved sodium on account of 
the insufficient amount of alcohol. 
Small amounts of water free from 
carbon dioxide * are now added, 
a test-tube full at a time. The 
alcohol is almost all boiled away, 
and in order to completely remove 
it, a current of air free from car- 
bon dioxide is passed through the 
solution until the odor of alco- 
hol can no longer be detected. 
The solution is then quickly 
cooled by the addition of water 
free from carbon dioxide, imme- 
diately placed in a liter flask, and 
Fic. 87. diluted to the mark with pure 

water at 17-18° C. This solution 

will give the same value when titrated with phenolphthalein in a cold 
solution as when the latter is hot.t| With methyl orange correct 
resuits are also obtained if the orange color is taken as the end-point. 
Such a solution quickly absorbs carbon dioxide from the air. 

Tn order to prevent this, it is placed in a bottle as shown in Fig. 




















* This is accomplished by boiling the water while a current of air free 
from carbon dioxide is passed through it. 

+ Provided the hydrochloric acid solution was prepared with water free 
from carbonate, otherwise too little acid wili be necessary when the titration 
takes place in the cold. 


BARIUM HYDROXIDE SOLUTION. 557 


87 which is connected with a soda-lime tube, N, and with the burette 
by means of the tubes p and r. The burette is filled by squeezing 
the tube at a. In this way a solution can be kept free from 
carbon dioxide for a long time. In order to determine whether 
the solution is free from carbonate, two parallel titrations are made 
with phenolphthalein as an indicator, one in the cold and the 
other in the hot solution. If the results agree the solution is free 
from carbonate. Otherwise it is necessary either to prepare a fresh 
solution or to make a corresponding correction in each analysis 
after determining the amount of carbonate present as described 
on p. 5638. | 

In many cases it is better to use a ;45 normal barium hydrox- 
ide solution; as long as it remains clear it is free from carbonate. 


Preparation of x Barium Hydroxide Solution. 


Ba(OH),+8H,O _ 315.51 


1000 c.c. contain 30 20 





=15.776 gms. 


The crystallized barium hydroxide of commerce always con- 
tains barium carbonate, so that the solution cannot be prepared 
by simply weighing out the necessary amount and diluting to 1 
liter. About 20 gms. of the commercial product are dissolved in 
the necessary amount of distilled water within a large flask. The 
flask is closed and shaken until the crystals have completely dis- 
appeared and a light, insoluble powder of barium carbonate 
remains. The solution is allowed to stand for two days, until the 
barium carbonate has completely settled, when it is siphoned into 
a bottle through which a current of air free from carbon dioxide has 
been passed for two hours previous, after which the bottle is con- 
nected with a soda-lime tube and with the burette as shown in 
Vig. 87, For the titration, 50 c.c. = hydrochloric acid are 
_ placed in an Erlenmeyer flask, a little phenolphthalein is added, 
and the solution titrated by the addition of the barium hydroxide 
‘solution. The normality found should be written upon the label. 


Tt is not advisable. to make the solution exactly nF for it usu- 


ally becomes turbid on dilution. 


558 VOLUMETRIC ANALYSIS. 


A. ALKALIMETRY. 


t. Determination of Alkali Hydroxides. 


Rule.—If the substance to be analyzed is a solid. an accurately 
weighed amount is dissolved in enough water so that the solution 
is at about the same concentration as that of the acid to be used 
in the titration. If, on the other “hand, a solution of an alkali 
hydroxide in water is to be analyzed, the specific gravity of the 
solution is determined by weighing in a pycnometer or by means 
of an areometer, and then diluted to the amount desired. — 


(a) Determination of Sodium Hydroxide in Commercial 
Caustic Soda, 


NaOH= 40.01. 


For the titration a ~ hydrochloric acid solution can be 


used. Consequently in this case an approximately . norma} 
solution of the alkali is prepared. As sodium hydroxide absorbs 
water and carbon dioxide from the air. the sample for analysis is 
weighed out in a tared watch-glass and dissolved in water to a 
definite volume. After thoroughly mixing the solution a pipetted 
portion is treated with methyl orange and titrated in the cold with 
af hydrechloric acid. 

Example.—4.6623 gms. sodium hydroxide were dissolved in 
1000 ¢.c. of solution and 25 c.c. of the latter, corresponding to 


0.11656 gm. sodium hydroxide, required 28.66 c.c. be hydro- 


10 
chloric acid for neutralization. 


Since 1000 c.c. of > acid correspond to 4.001 gms. NaOH, 


10 


BRO ae Noe ce 6088... 
it is evident that 1 c.c. 0 and To0G =0.004006 gm. NaOH, and 
28.66 c.c. Bs acid correspond to 0.004001 < 28.66 = 0.1147 gm. NaOH. 


ALK ALIMETRY. 559 


This amount of NaOH was contained in 25 c.c. of solution, 
equivalent to 0.1166 gm. of the solid substance, so that the per 
cent. of sodium hydroxide present can be calculated: 


0.1166 :0.1147 =100:2z 


Ss = 98.38 per cent. NaOH. 


TAGE 


(b) Determination of Sodium Hydroxide Present in Caustic 
Soda Solution. 


For the titration assume that a N. solution is at hand: 


2 
NaOH 


1000 c.c. = 9 =20.00 gms. NaOH. 





The alkali solution to be analyzed has a specific gravity of 1.285 
at 15° C., and by consulting the table (see the supplement) we 
find that the solution should contain 25.80 per cent. NaOH by 
weight; i.e., 100 gms. of the solution should contain 25.80 gms. 
NaOH. Usually instead of weighing out the solution it is meas- 
ured and the per cent. by volume is computed. 

As 128.5 gms. of the alkali occupy a volume of 100 c.c. we have 


100 :25.8 = 128.5:2 
2=33.153 gms. NaOH in 100 c.c. 
Now as 1 liter of _ sodium hydroxide contains 20.00 gms. NaOH, 
we can compute how much of the alkali must be taken *o be di- 


luted to 1000 c.c. in order to make a 3B solution: 


2 
100:33.153 =2z:20.00 
2003 
= 99 153 00.32 C.c. 


We measure, therefore, 60 c.c. into a liter flask, dilute with 
water just to the mark, shake thoroughly, and by means of a pi- 
petie 25 c.c. of the solution are removed and titrated with half- 
normal acid, using methyl orange as an indicator. 


560 VOLUMETRIC ANALYSIS. 


Assume that 25 ¢.c. of the solution require 24.3 c.c. of 2 acid 


for neutralization. - As 1000 c.c. . acid contain: sa # 


gms. NaOH, it is evident that 1 c.c. of the acid corresponds 
to 0.02000 gm. NaOH, and 24.3 c.c. Pie is equivalent to 


2 
0.02000 « 24.3 =0.4860 gm. NaOH., | 
25 c.c. of the dilute alkali, therefore, contain 0.4860 gm. 
NaOH, and 1000 c.c. of the dilute solution, or 60 c.c. of the original 
alkali, contain 0.4860 x40=19.44 gms. NaOH, and 100 c.c. of 
the original solution contain 


60:19.44=100:2 


2 == ae = 32.40 gms. NaOH. 





= 20.0 


In order to obtain the per cent. by weight, this number must 
be divided by the specific gravity. 
In the assumed case we have: 


se = 25.21 per cent. NaOH. 

Remark.—The titration of alkali hydroxides with methyl 
orange as an indicator will only give correct results when the 
alkali hydroxide is free from carbonate, which with commercial 
material is never the case. The above results are too high, for 
they represent the total amount of alkali, i.e. the amount of 
NaOH-+ Na,CO,, though the latter is expressed in terms of NaOH. 
For an accurate determination of alkali hydroxide in the presence 
of alkali carbonate, see p. 563. 


(c) Determination of Ammonia in Aquesus Ammonia. 


The procedure is the same as under (0). 


(d) Determination of Ammonia in Ammonium Salts. 


A weighed amount of the ammonium salt is placed in the 
flask K (Fig. 24, p. 59),* dissolved in about 200 c.c. of water, and 





- * Or better, the apparatus shown in Fig. 78, p. 454, may be used. 


TITRATION OF PYRIDINE BASES. | 561 


treated with 10 ¢.c. of a boiled solution of 10 per cent. caustic 
soda. The solution is distilled, and the distillate received in a 
known amount of normal acid in the receiver V, as described on 
p. 59. The excess of acid is titrated with normal alkali, using 
methyl orange as an indicator and the ammonia calculated from 
the difference between the amount of acid now found and that 
originally in the receiver. 

Example.—The amount of ammonia in a sample of commer- 
cial ammonium sulphate is to be determined. As the technical 
product is never entirely pure, a large amount of the substance is 
weighed out, and for the sake of convenience this can amount to 
the gram-equivalent of ammonia, i.e. 17.03 gms. This quantity of 
the salt is dissolved in 500‘c.c. of distilled water, and for the analy- 
sis, 50 ¢.c. of this solution are taken (1.703 gms. of salt). This is 
placed in the flask K (Fig. 23, p.59), diluted with 150 c.c. of water, 
and distilled after the addition of 10 c.c. of 10 per cent. caustic 
soda solution. The distillate is received in 60 c.c. of half-normal 
hydrochloric acid, the excess of the latter titrated with half-normal . 
alkali, and from the difference the amount of ammonia calculated. 
For the titration, ¢ ¢c.c. of z alkali are necessary; consequently 
the amount of ammonia in 1.703 gms. of the substance neutral- 
ized 60—t c.c. - acid. This edrersRoens to Asai t) X0.008515 gm. 
NHsz and in per cent.. 

1.703 : (60 —t) X 0.008515 = 100: x 
_ (60—2)0.8515 60-1 
Se SS Sa re 





=per cent. NH3. 


(e) Titration of Pyridine Bases. Method oj K. E. Schulze.* 
1000 c.c. N. acid=C,H.N =79.05 gms, pyridine. 

The pyridine bases are so weak that they cannot be titrated 
with ordinary indicators. If, however, an aqueous pyridine solu- 
tion is treated with an aqueous solution of ferric chloride, the iron © 
is precipitated as ferric hydroxide: 


FeCl, +3C,H;N + 3HOH =3(C, HN, HCl) + Fe(OH)» 
* Berichte, 20 (1887), p. 3391. 





562 ~ VOLUMETRIC ANALYSIS. 


If normal sulphuric acid is very carefully added with constant 
stirring until the precipitate redissolves, each cubic centimetre of 
C.H,N | 

1000 

Procedure.—5 c.c. of pyridine are dissolved in 100 e.c. of water, 
25 ¢c.c. of the resulting solution are treated with 1 ¢.c. of 5 per cent. 
aqueous ferric chloride solution, and the precipitate of reddish- 
brown ferric hydroxide is titrated "vith normal sulphuric acid until 
completely dissolved. 





the acid required will correspond to = 0.07905 gm. pyridine. 


2. Determination of Alkali Carbonates. 


Alkali carbonates can be titrated in the cold by using methyl 
orange as an indicator, the end-point being taken as the change 
from yellow into reddish orange. When fifth-, half-, and normal 
acids are used this is the correct end-point, but with tenth-normal 
acids this change is obtained a little too soon, for large amounts of 
carbonic acid exert a slight action upon the indicator. In this case 
’ the difficulty is best overcome by titrating the solution until the 
orange color is obtained, then heating to boiling to expel the car- 
bon dioxide, cooling, and again titrating until the now yellow 
solution becomes orange again.* With phenolphthalein, accurate 
results may be obtained by titrating the hot solution (cf. p. 554). 
According to Warder,t sodium bicarbonate solution reacts neu- 
tral toward phenolphthalein in the cold, so that when a sample of 
sodium carbonate is titrated in the cold, with phenolphthalein 
as an indicator, an end-point is obtained when the carbonate is 
changed to bicarbonate: 


Na,CO,+HCl=NaCl+ NaHCO,,t 


If the acid is allowed to run upon the carbonate solution, a 
part of the carbon dioxide from the sodium bicarbonate is lost, 
so that too much acid must be added before the end-point is 
reached. On the other hand, correct results may be obtained if 





* Kuster recommends in titrating carbonates with methyl orange, that 
a blank experiment be made to see how much effect an equal amount of 
water saturated with carbon dioxide has upon the same amount of indicator 
solution. (Zeitschr. fiir anorg. Chem., XIII, p. 140.) 

} Zeitschr. f. analyt. Ch., 21, p. 102. 

t Zeitschr. f. anorg. Ch., XIII, p. 140. 


DETERMINATION OF ALKALI CARBONATES. 563 


the titration is carried out at 0° in the presence of NaCl (cf. p. 
547). This is important, for in this way a convenient method is 
obtained for determining the amount of hydroxide in the presence 
of carbonate. 


3. Determination of Alkali Carbonate and Hydroxide in the 
Presence of one Another. 


(a) Method of C. Winkler. 


Of the many methods which have been proposed for this deter- 
mination that of Winkler is the best. 

In one portion the total amount of alkali present is determined 
by titration with acid, using methyl orange as an indicator, and 
the hydroxide in a second portion is determined as follows: The 
solution is treated with barium chloride solution, when the follow- 
ing reaction take place: 


Na,CO,+ BaCl,=2NaCl+ BaCoO, (insoluble). 


2NaOH + BaCly=2NaCl + Ba(OH) (soluble). 

The sodium of the carbonate is transformed into neutral 
sodium chloride, while insoluble barium carbonate is precipitated 
from the solution; the sodium hydroxide, however, yields an. 
equivalent amount of barium hydroxide. If the solution contain- 

ing phenolphthalein is slowly titrated with hydrochloric acid with 
constant stirring, decolorization is effected as soon as the hydroxide 
isneutralized. The amount of acid used corresponds to the amount 
of hydroxide originally present. : 

Example: 


1. 20c.c. (Na,CO,+ NaOH) require 7’ c.c. x acid for Na,CO,+Na0H, 


2. 20c.c.(Na,CO,+NaQH) “ ¢ “ =F “NaOH alone,—! 
so that 
20 c.c. (Na,CO;+ NaOH) require T —i c.c. 2A acid for Na,CO,; 


20 c.c. of the solution, therefore, contain + 
(a) t< 0.004001 gm. NaOH, 
(b) (’—1t) X0.005300 gm. Na,COs. 
Remark.—It has been proposed to add an excess of barium 
chloride solution to the mixture of alkali carbonate and hydroxide 


564 -. VOLUMETRIC ANALYSIS. . 


contained in a measuring-flask, then dilute to the mark, thoroughly 
mix, and filter through a dry filter; for the titration an aliquot 
part of the filtrate is taken. This method, however, will only give 
accurate results when the water used for the dilution is absolutely 
free from carbon dioxide, and this will be the case only when it 
is previously boiled with a current of air free from carbon dioxide 
passing through it. Further, no attention is paid to the volume 
occupied by the precipitated barifim carbonate, and in the case 
of a large amount of the latter, a considerable error is introduced. 
The method of Winkler does not have these disadvantages. Care 
must be taken, however, with regard to the addition of the hydro- 
chloric acid in the titration; unless it is added very slowly some 
of the barium carbonate a He acted upon before the end-point 
is reached. 


(b) Method of R. B. Warder. 


To the cold* solution containing phenolphthalein, hydro- 
chloric acid is added and the liquid is gently stirred with a glass 
rod. Decolorization takes place when all of the hydroxide and 
half of the carbonate are neutralized: 


- NaOH+HC1=NaCl+H,0, 
Na,CO,-+ HCl=NaCl+ NaHCo,. 


To the colorless solution, methyl orange is added, and the 
solution is again titrated with acid until the other half of the car- 
bonate is neutralized, when the solution turns brownish-red. 

If the amount of acid used for the titration with phenolphtha- 
lein is represented by 7’, and that necessary for the titration with 
methyl orange by t, then 

2t c.c. corresponds to the amount of carbonate present, and 
7 —zt represents the amount of hvdroxide. 


4. Determination of Alkali Bicarbonates. 


“The ‘solution is titrated in the cold unitii an orange color is 
obtained with methyl orange, or unti: a eciorless solution is obtained 
by titrating hot with phenolphthalein. (See page 553.) 





* The results are accurate only when the solution is at 0° and NaCl is 
present. Cf. Kiister. Zeitschr. f. anorg. Chem., XIII, p. 134 (1897). 


DETERMINATION OF ALKALI. CARBONATES. 565 


5. Determination of Alkali Carbonates in the Presence of 
Alkali Bicarbonates. 


(a) Method of C. Winkler. 


The total alkali is determined in one portion by titration 
with hydrochloric acid, using methyl orange as an indicator, and 
in a second portion the amount of bicarbonate is determined as 
follows: 

A definite volume of the solution is treated with an excess 
of sodium hydroxide, by which means the bicarbonate is changed 
to neutral carbonate: 


NaHCO,+ NaOH =Na,CO,+H,0. 


The solution now contains sodium carbonate with the excess 
of sodium hydroxide, and the amount of the latter is determined 
as described under 3. In other words, barium chloride is added, 
then phenolphthalein, and the solution is titrated until colorless. 
The amount of acid now used corresponds to the excess of sodium 
hydroxide added, and if this amount is deducted from the total 
sodium hydroxide, the corresponding amount of bicarbonate 
will be obtained. - 


Example: 
25 ©.¢. Na,CO,+NaHCo, required T' ¢.c. x acid for 3 
Na,CO;+ NaHCo,; 
25 ¢.c. Na,0O0,-+NaHCO,+ 7, ¢.c. ns NaOH +BaCl, 


required ¢ ¢.c. af acid for the excess of NaOH; 


25 c.c. Na,CO, + NaHCoO,, therefore, require ry —t cc. as acid 


for the NaHCO, and T7-—(T,—2) cc. = acid for the Na,CQg. 


25 c.c. of the original solution contain 


(a) (T,—t) 0.008401 gm. NaHCO,, 
(b) (’—T, +4) X0.005300gm. Na,CO,. 


566 VOLUMETRIC ANALYSIS. 


Remark.—In order to make sure that enough sodium hydrox- 


ide solution is present, the same amount of the af alkali is added 


as there were cubic centimeters used of 2 acid in the deter- — 
mination of the total alkali; in this case, then, 7=T7',, and t, the 
excess of alkali, corresponds at the same time to the amount of 
Na,CO, present. The caustic alkali solutions, even when origi- 
nally free from carbonate, gradually absorb it from the air, so that 
in every case the amount of carbonate in the alkali should be deter- 
mined before making the above analysis and a corresponding 
correction applied to the calculation. 


(b) Method of Warder.* 


Using phenolphthalein as indicator, the solution is titrated 
with hydrochloric acid until colorless, and in this way half of the 
carbonate is determined. Methyl orange is then added and the 
solution titrated until a brownish-red color is obtained, and in 
this way the total amount of the bicarbonate together with half 
of the carbonate is determined. If ¢ represents the amount of 
acid used in the first titration, and 7’ the total amount used, 
then: 

2t c.c. of acid correspond to the amount of carbonate and 
(T'—2t) c.c. correspond to the bicarbonate. 


6. Determination of Alkaline-earth Hydroxides. 


The solution containing phenolphthalein is titrated until color- 
less. : 


7. Determination of Alkaline-earth Carbonates. 


The carbonate is dissolved in an excess of the standard acid, 
boiled to remove the carbon dioxide, and the excess of acid titrated 
with alkali, using methyl orange as indicator in cold solution. 





* Cf. Am. Ch. Journ., 8, No. 1, and Chem. News, 43, 228. 


DETERMINATION OF ALKALINE-EARTH OXIDE. 567 


8. Determination of Alkaline-Earth Oxide together with 
Alkaline-Earth Carbonate. 


Suppose, for example, it is desired to determine the amount of 
oxide and carbonate in a sample of ‘‘quicklime.” The lime is 
broken up into pieces about the size of a pea, exactly 14 gms. are 
accurately weighed out and slaked with boiled water, the paste is 
washed into a 500-c.c. flask and diluted to the mark with water 
free from carbon dioxide. After thoroughly mixing, 50 c.c. of. 
the turbid liquid is transferred to a second 500-c.c. flask and 
again diluted to the mark. 

Determination of the Total Calcium.—50 c.c. (0.14 gm. of sub- 
stance) of the last solution are treated with 60 c.c. of x hydro- 
chloric acid and heated until there is no further evolution of. car- 
bon dioxide, the solution is cooled, and the excess of the acid 
titrated with as caustic soda solution, using methyl orange as 
an indicator. For this purpose ¢ ¢.c. of the latter are required; 
consequently 60—¢ c.c. a acid were necessary to neutralize the 
calcium hydroxide and calcium carbonate in the 50 c.c. of the 
solution taken for analysis. 3 

Determination of the Calcium Oxide.—A second portion of the 
freshly-shaken solution is titrated with hydrochloric acid 
added drop by drop to the cold solution, using phenolphthalein 
as an indicator. Assume that ¢, c.c. of the acid were necessary to 
neutralize the calcium oxide. 

Consequently, for the neutralization of the CaCO,+CaO= 
60—t c.c. a acid were required, and for the CaO, t, ¢.¢. - acid 
were necessary. For the neutralization of the CaCQs, therefore, 
60— (t+ ¢,) ¢.c. - acid were necessary. 

50 c.c. solution (0.14 gm. lime) contain: 


(a) ty X0.002805 gm. CaO, 
(b) [(60—(¢+t)] 0.5005 gm. CaCOs, 


568 VOLUMETRIC ANALYSIS. 


and in per cent. 
0.14:4 «0.002805 = 100: 
eas = 2t, per cent. CaO 
and 
0.14 :[60— (¢+4,)] 0.005005 = 100: a, 
_ [60— (+4) ] 0.5005 
0.14 





= per cent. CaCOs. 


9. Determination of Alkaline-Earth Bicarbonates. 


This determination finds a practical application in the deter- 
mination of the temporary hardness of water. 

The hardness of a water is caused by the presence of alkaline- 
earth salts, either those with strong acids (CaSO,, MgCl,) or bicar- 
bonates. A hard water is recognized by the fact that it gives 
with a clear soap solution a turbidity or even a precipitate, and 
considerable soap must be added before a lather is obtained on 
shaking. As in a majority of cases calcium salts, and chiefly 
ealcium bicarbonate, predominate in such a solution, its hard- 
ness is usually expressed in parts of calcium carbonate (or caletum 
oxide) in 100,000 parts of water. 

If the solution contains 1 part of calcium carbonate in 100 000 
parts of water it is said to possess one degree of hardness (French) ; 
if such a water contains n parts of CaCO, in the same quantity of 
water it possesses n degrees of hardness. In Germany the hard- 
ness is expressed in parts of CaO per 100,000 parts of water, while 
in England the hardness is expressed in grains of calcium carbon- 
ate per gallon. When magnesium salts are present, these are 
expressed in terms of the equivalent amounts of CaCO, or CaO. 
The error caused by this assumption is not great, for the amount 
of magnesium present is usually small compared with the amount 
of calcium. If a water containing calcium bicarbonate and cal- 
cium sulphate i is heated to boiling, the former is gece with 
the precipitation of calcium carbonate, 


Ca(HCO,),, =H,0+CO,+ CaCO, 


while the calcium sulphate remains in solution. In other words, 
the hardness produced by the presence of alkaline-earth bicar- 


DETERMINATION OF ALKALINE-EARTH BICARBONATES. 569 


bonates disappears on boiling, and is designated, therefore, as 
“temporary hardness” to distinguish it from ‘‘permanent hard- 
ness,’ which is usually caused by alkaline-earth salts of the 
stronger acids, usually calcium sulphate. The sum of the tem- 
porary and permanent hardness of a water represents the total 
hardness. 

According to C. Hehner, the temporary as well as permanent 
hardness may be determined accurately by an alkalimetric process. 


(a) Determination of Temporary Hardness. 


100 ¢.c. of the water to be examined are placed in a white por- 
celain evaporating-dish, a few drops of methyl orange are added 


and the solution is titrated with — - 9 hydrochloric acid until the 


first change from yellow to orange ark place. From the amount 
of hydrochloric acid used the amount of calcium carbonate present 
is calculated. 

Example: 


100 ¢.c. water required 2.5 c.c. 7 hydrochloric acid. 





As 1000 c.c. 2 hydrochloric acid neutralize Se 


CaCOs, 1 c.c. a hydrocholrie acid will neutralize 0.005005 gm. 


CaCO3 and 2.5 c.c. of Bl hydrochloric acid corresponds to 


0.005005 x 2.5 =0.0125 gm. CaCQs. 

Then if 100 c.c. of water contain 0.0125 gm. CaCOs, 100,000 c.c. 
of water will contain 12.5 gms. CaCOs3. 

The hardness of the water corresponds to 12.5 French degrees, 


ar as 
CaCO3:CaO 
100.09 :56.09 = 12.5:2 
__ 56.09 x 12.5 


=7.0 German degrees. 





100.09 


570 ~ VOLUMETRIC ANALYSIS. 


(b) Determination of the Permanent Hardness. 


Another portion of 100 c.c. of the water is treated with an 
excess of . sodium carbonate solution, evaporated on the 
water-bath to dryness, and taken up in a little freshly-boiled, 
distilled water. The residue is filtered and washed four times 
with hot water, while the filtrate is,allowed to cool and afterwards 
x hydrochloric acid. If the amount of hydrochloric 
acid used for the titration is deducted from the total amount of 
sodium carbonate added to the water, the difference represents the 
amount of sodium carbonate required for the precipitation of the 
alkaline-earth salts of the strong acids. 


titrated with 


Ezxample.—100 c.c. of water+10 c.c. ~ Na,CO, were evapo- 


rated to dryness, the residue extracted with water, and the filtrate 
titrated with al hydrochloric acid; this required 8.7 ¢.c. of HCl. 
Consequently, for the precipitation of the calcium sulphate 
10 — 8.7 =1.3 c.c. ANa,00, were necessary, which corre- 
sponds to 


1.3<0.005= 0.0065 gm. CaCO, per 100 c.c. water and 
6.5 gms. CaCO, per 100,000 ¢c.c. water. 


The permanent hardness amounts to 6.5 French degrees or 
6.5 <0.56= 3.64 German degrees. 

Remark.—The above methods of Hehner for the determina- 
tion of hardness will give reliable results only when the water 
contains no alkali carbonates in solution, as is usually the case 
with drinking-waters. For the determination of the amount 
of alkaline earth present in many mineral waters it is obvious 
that these methods cannot be used. 


10. Determination of Alkaline-earth Salts of Strong Acids, 


The determination is practically the same as was indicated 
above. The alkaline-earth salt is precipitated by means of an 


ACIDIMETRY. : 571 


excess of titrated sodium carbonate solution, and after filtration 
the excess of the latter is determined by titrating back with acid. 

Procedure.—A solution eontaining calcium chloride and hydro- 
chloric acid is to be analyzed. It is placed in a measuring-flask, 
treated with a few drops of methyl orange and with sodium 
hydroxide solution until the neutral point is reached, after which 
an accurately measured amount of sodium carbonate solution is 
added. The solution is heated until the precipitated calcium 
carbonate becomes crystalline, allowed to cool, diluted up to the 
mark, mixed, filtered through a dry filter, and the excess of sodium 
carbonate titrated in an aliquot part of the filtrate. From the 
amount of sodium carbonate required for the precipitation of the 
calcium the amount of the metal can be calculated. 

Remark.—Other metals which are precipitated by sodium ecar- 
bonate can be determined in this way. 


B. ACIDIMETRY. 


Acids are determined either by titration with standard alkali 
solution or a known amount of the latter is added and the excess 
titrated with standard acid. The latter method requires more 
burette readings and is, therefore, less satisfactory than the 
former. 

Determination of the Acid Contents of Dilute Mineral Acids 
(HCl, HNO., H,SO,). 


The specific gravity of the acid is determined by means of an 
areometer and from the tables in the back of this book the approx- 
imate amount of acid present is determined. A weighed amount 
of the acid is then diluted so that the solution will have approxi- 
mately the same concentration as that of the alkali to be uscd 
for the titration. It is analyzed by one of the following methods: 

1. An accurately-measured portion of the diluted acid (20 to 
25 c.c.) is placed in a beaker, methyl orange is added, and the 
solution is titrated with sodium hydroxide solution until a yellow 
color is obtained. zt 

2. The dilute solution to be analyzed is placed in a burette, 
and with it a definite amount of normal alkali is titrated. 


572 VOLUMETRIC ANALYSIS. 


eee debuite: volume of the diluted acid is titrated with x 


10 
-Ba(OH), solution or with sodium hydroxide free from carbonate, 
using phenolphthalein as an indicator.* 


Example.—F¥or the analysis a NaOH is at hand. 


The hydrochloric acid to be analyzed had at 15° C. a specific 
gravity of 1.122, corresponding to*about 24 per cent. HCl by 
weight. 

1000 c.c. x sodium hydroxide are equivalent to oe = 
= 18.23 gms. HCl, and 100 c.c. * NaOH neutralize 1.823 gms. HCl. 
Consequently 


100:24=2:1.823 


r= —* =7.595 gms. of the above acid 





would be required to make 100 c.c. of acid, if it contained exactly 


24 per cent. HCl. About this quantity (say 8 gms.) is, therefore, 
weighed out, and as the specific gravity of the solution is 1.122,. 


this will require 





8 : 
Lie "1 c.c. About 7 c.c. of theacid are placed 


in a tared, glass-stoppered weighing-tube, the tube and its con- 
tents weighed, the latter washed into a 100-c.c. measuring-flask 
and diluted with distilled water up to the mark. After thoroughly 
mixing, 25 ¢.c. of the acid are measured off and analyzed by one 
of the above methods. Assume that the original weight of the acid 
amounted to 7.9623 gms. and that 25 c.c. of the diluted acid re- 


quired. 25.80 c.c. of = alkali, then 100 c.c. would require 25.80*4= 


103.2 c.c. of > alkali, corresponding to 103.20.01823= 1.8813 





* When phenolphthalein is used as an indicator in cold solutions the 
acids must be diluted with water free from carbonate. 


COMMERCIAL HYDROUS STANNIC CHLORIDE, 573 


gms. HCi and in per cent. 


7.962:1.881=100:2 


ERGA 


= 796277 23°8 per cent. HCl. 





ar 1 


Remark.—Instead of weighing out the acid for the analysis, 
it can be measured and from the per cent. by volume found the 
per cent. by weight calculated. As, however, the specific grav- 
ity as determined by an areometer is not very accurate, it is better 
to weigh the acid.* 


Analysis of Commercial Hydrous Stannic Chloride. 


Stannic chloride, as used for a mordant in dyeing, is obtained 
as the solid salt SnCl,+5H,O, or in a concentrated aqueous solu- 
tion of about 50° Bé. 

The latter is obtained by dissolving metallic tin in hydro- 
chloric acid and oxidizing the stannous chloride formed either 
with potassium chlorate or potassium nitrate. The preparation 
should contain no free acid, especially nitric acid, no stannous 
chloride, and no iron. The substance is, therefore, tested quali- 
tatively for these substances as follows: 

For stannous chloride, by dissolving in water (or diluting the 
concentrated solution) and adding mercuric chloride; a white 
precipitate of mercurous chloride shows the presence of bivalent 
tin. : 
For nitric acid, by means of ferrous sulphate and concentrated 
sulphuric acid. 

For sulphuric acid (caused by the use of impure hydrochloric 
acid in the preparation of the salt) with barium chloride. 

For iron, with potassium sulphocyanate. 

The solid salt SnCl,+5H,O, made by treating anhydrous 
stannic chloride with the calculated amount of water, is almost 
always found to be very pure. 





* If the specific gravity of the acid is-taken with a pycnometer, using 
all necessary precautions (cf. Kohlrausch, Leitfaden der praktischen Physik), 
it is a matter of indifference whether the acid used for the analysis is weighed 
or measured, 


574 VOLUMETRIC ANALYSIS. 


The gravimetric determination of both the tin and the chlo- 
rine has been described on p. 321, but here will be given a method 
for determining the amount of the latter volumetrically. : 

If stannic chloride is diluted with water, the salt is hydro- 
lytically decomposed, and the solution reacts acid: 


SnCl,+ 4HOH & Sn(OH),+ 4HCl. 
> 


Consequently if methyl orange is added to the diluted solu- 
tion, the amount of acid may be titrated with caustic soda solu- 
tion, and from the amount used the chlorine combined with the 
tin can be calculated, provided no other acid is present. If the 
stannic chloride was prepared by oxidation with potassium 
chlorate or nitrate,* the solution will also contain chlorine com- 
bined with potassium. The total chlorine can be determined by 
adding a few drops of neutral potassium chromate solution to 
the solution which has been titrated with sodium hydroxide, and 
titrating with silver nitrate solution. If in this way more chlo- 
rine is found than corresponds to the amount of hydrochloric 
acid neutralized by the alkali, the difference is expressed in terms 
of potassium chloride. If, on the other hand, less chlorine is found, 
the presence of some other acid in the tin solution is assured. 

To illustrate the accuracy of such an analysis, the following 
results will. be given: A sample of solid stannic chloride 
(SnCl,+5H,O) was analyzed gravimetrically, as described on 
p. 321. It was found to contain 42.02 per cent. of chlorine and 
34.73 per cent. of tin. | 

Two portions were then analyzed volumetrically by titration | 
first with sodium hydroxide and then with silver nitrate: 


A. 0.8533 gm. of tin salt required 20.06 c.c. * sodium hy- 


droxide ‘and 20.34 c.e. “ silver nitrate. As 1 ¢.c. solution corre- 


sponds to 0.01773 gm. of chlorine, it is evident that 20.06 c.c. 3 





* The potassium nitrate is acted upon by the excess of hydrochloric acid 
present forming the chloride, and the excess of the acid is afterwards removed 
by evaporation as much as possible. 


COMMERCIAL HYDROUS STANNIC CHLORIDE. 575 


sodium hydroxide represent 20.06 0.01773 =0.3556 gm. chlorine 
_ or 41.67 per cent. Cl, and 20.34 c.c. 2 silver solution show 20.34 
0.01773 =0.3605 or 42.25 per cent. Cl. 


B. 0.8383 gm. of tin salt required 19.79 c.c. = sodium hydroxide 


and 19.92 c.c. = silver nitrate. 19.79 c.c. _ sodium hydroxide 


represent 19.79 X 0.01773 =0.3508 gm. chlorine or 41.84 per cent. Cl. 
19.92 c.c. x silver solution show 19.92X0.01773=0.3531 gm. 


chlorine or 42.12 per cent. Cl. 

The above analysis shows that the tin salt was practically free 
from potassium chloride by the comparative agreement of the 
results obtained by titration with sodium hydroxide with those of 
the silver nitrate titration. In the absence of free hydrochloric 
acid, the tin can be determined from the amount of chlorine found: 


4Cl Sn 
141.84:119.0— 41.75 *:x 


_ x=35.03 per cent. tin instead of 34.73 per cent. as found gravi- 
metrically. | 

Remark.—It is only permissible to compute the amount of tin 
present from the amount of chlorine found by titration when 
there is no free hydrochloric acid present. It is never possible — 
to know whether this is the case or not, so that the volumetric 
determination is only useful as a check upon the gravimetric 
method. | 


Determination of the Acid Contents of Fuming Acids. 


Highly concentrated acids must be always weighed and not 
measured, in order to avoid loss by evaporation. The weighing 
is best accomplished by means of the Lunge-Rey pipette, shown 
in Fig. 88. | 





* 41.75 is the mean of the values obtained by titration with alkali. 


576 VOLUMETRIC ANALYSIS. 


The lower tube is removed, 4 c.c. of water is placed within 
it, and this is weighed together with the dry upper pipette, but the 
two parts areleft unconnected. The lower stop-cock is closed, the - 
upper one opened, and a slight vacuum is produced 
in the bulb by sucking through the upper tube and 
then closing the stop-cock. The dry point of the 
pipette is now introduced into the fuming acid 
(in the case of solid pyrosulphuric acid it is first 
liquefied by warming slightly) and the lower stop- 
cock is opened. As soon as the widened part of the 
pipette below the lower bulb is 4 to ? full, the stop- 
cock is closed, taking care that none of the liquid 
reaches up to it. : 

The acid on the outside of the pipette is care- 
fully wiped off with filter-paper; the two parts of the 
pipette are now connected for the first time and 
again weighed. The amount of acid taken for the 
analysis should amount to from 0.5 to 1 gm. The 
point of the pipette is then dipped into about 
100 c.c. of distilled water contained in a beaker. 
and, by opening first the upper stop-cock and then 
the lower, the acid is allowed to run into the water. The amount 
remaining in the two parts of the pipette is also washed into the 
beaker. 

If the acid to be analyzed is hydrochloric or sulphuric acid, 
methyl orange is added and the solution is titrated with a half- 
normal sodium hydroxide. If it is nitric acid, an excess of sodium 
hydroxide is first added, then a little methyl orange, and the titra- 














tion is completed with > hydrochloric acid.* When one of the 


above pipettes is not available, the weighing out of the sample 
for analysis can be effected as follows: A thin-walled bulb with 
about 1 ¢.c. capacity is blown between two ends of capillary tubing. 
After weighing, the upper piece of capillary tubing is connected 
with a small, ordinary pipette, at the ends of which are attached 





* In this way the action of the ever-present nitrous acid upon the indi- 
cator is avoided. 


ACID CONTENTS OF FUMING ACIDS. 577 


pieces of rubber tubing, and the latter are closed with pinch-cocks. 
The bulb is filled as follows: 

The lower pinch-cock is closed, the upper one eitiak and a 
vacuum produced by sucking through the upper tube and then 
closing the pinch-cock. The lower point of the weighed tube is 
introduced into the acid and the lower pinch-cock opened. When 
the small bulb is one-third full the pinch-cock is closed, the upper 
end of the capillary tubing is melted together, and, after wiping 
off the acid from the outside, the lower end is likewise sealed, 
and the bulb weighed. About 100 c.c. of water are placed in a 
flask with a closely fitting ground-glass stopper, the weighed bulb 
is thrown in, and it is broken by shaking. In this way the very 
strongest, fuming sulphuric acid can be dissolved in water with- 
out loss. On the other hand, the pipette shown in Fig. 88 is not 
so good for the weighing out of an acid containing 70 per cent. or 
more of SO,. If the acid is not too concentrated, this bulb may 
be emptied as was described for the pipette. — 

For the analysis of the solid anhydride, Stroof places a little 
in a dry weighing tube, and concentrated sulphuric acid of known 
strength is added until a fuming acid of about 70 per cent. SO, 
is obtained. To effect solution, the mixture is warmed to about 
30° to 40° C. in a loosely stoppered bottle. The acid thus. 
obtained is analyzed as above.* WW 


Computation of the SO; Contents of a Fuming Sulphuric Acid. 


The above titration gives not only the sulphuric anhydride 
present, but also the never-failing SO,. In a separate portion, 
therefore, the amount of the latter is determined by titration 
with a Be iodine solution (see lodimetry), an ‘equivaient aniount 
is subtracted from the total amount of sodium hydroxide, used, 
and from the difference the total SO, present is computed. . 

_ With regard to the SO,, the following reactions take place 
during the titrations: 


H,SO,-+ NaOH= NaHSo,+ ape 4 
H,SO,+H,0+ 21=2HI+H,S0,* 
* G. Finch (Chem.’ Ztg.,°1910,-297) and R. Hs: Vernon (ibid., 1910, 702) 
use a different apparatus and larger ‘samples, thus getting more accurate 
results. 


4 wa ~ 
ou 
satis » Tree J 





578 VOLUMETRIC ANALYSIS. 


It is to be noted in the first reaction that, although sulphurous 
acid is a dibasic acid, the end-point is reached, with methyl orange 
as an indicator, when the first hydrogen atom has been neutralized. 
From the two equations, then, it is evident that 2 gm. atoms of 


iodine are equivalent to 1 gm. molecule of NaOH, or 1 c.c. be iodine 


10 
solution is equivalent to 4 ¢.c. a sedium hydroxide. 
Since, in general, 5 ¢.c. 2 solution=1 cc. solution, then 


N : N ; 
1.6. T0 solution=4 c.e. a: solution, 


and in the given case 1 c.c. x iodine= ;\; ¢.c. x sodium hydroxide; 


so that if 7 cc. alkali were used in the first titration of the 


total acid present, and ¢ c.c. of a iodine solution for the oxidation 


of the sulphurous acid, it is plain that T -- represents the amount 


of alkali necessary for the neutralization of the total sulphuric acid. 
The SO, is determined by an indirect analysis. 
We will assume that the fuming acid consisted of 


H.SO,=2 
SO,=y 
SO,= a 


100 





then 100—a=z+y. 

In order to determine x and y a second equation is necessary, 
and this is found from the. titration of the total sulphuric acid. 
Assume that, after the deduction corresponding to the amount of 
SO, has been made, the total amount of H,SO, was found to be 
p per cent., then: 

1. z+y =100—a 
2. z+my= p 


y= py an epee cent. SO, 


ACID CONTENTS OF FUMING ACIDS. 579 


x= 100—(a+y)=per cent. H,SO, 


H.SO4 98.086 


SOs 80.07 ieee 





In equation2, m= 


and 
m—1=0.2250 


Example.*—3.5562 gms: of fuming acid were diluted to 500 c.c., 
and of this amount 100 c.c.=0.7112 gm. were taken for analysis. 


1. 100 e.c. required 5.40 c.c. = iodine = 5.4 0.003203 = 


0.01730 gm. SO,=2.43 per cent. SO,=a. 

2. 100 c.c. required 34.40 c.c. = sodium hydroxide. 

from the latter must be deducted 0.54 c.c. to correspond to 
the amount of alkali necessary for the SO,. 34.40—0.54=33.86 c.c. 


33.86 X0.02452=0.8305 gm. H,SO,=116.7 per cent.=p. 


If these values are introduced in the above equations we 
obtain 

--119.16—100.. 19.16 

~ -» 0.2250 0.2250 





= 85.15 per cent. SO3 


and 
x= 100— (85.15 +2.43) =12.42 per cent. H2SOxz. 


The acid contains, therefore: 


H.SO4= 12.42 + 





SO,= 85.15 
SO,= 2.43 
100.00 





* Lunge, Zeitschr. f. angew. Ch., 1895, p. 221. 

+ Like all indirect analyses, the results obtained are not absolutely accu- 
rate. Almost all fuming acids contain solid constituents which are neg- 
lected in the above calculation. It would be more accurate to determine 
the amount of the latter in a separate portion, by weighing the residue on 
ignition. 


580 VOLUMETRIC ANALYSIS. 


Preparation of Concentrated Sulphuric Acid Mixtures (M. Gerster.) 


It is often necessary to prepare fuming sulphuric acid of 
definite concentration. 

Given: 

(a) Fuming sulphuric acid (A) with a per cent. free SO,. 

_ (0) Sulphuric acid (B) with c per cent. H,SO, and 100—e per 
cent. water. 

A fuming acid containing b per cent. free SO, is desired. 

To obtain the latter, 100 gms. of the acid A are mixed with 
xz gms. of the acid B. It must be remembered, however, that the 
water in the acid B requires SO, in order to form 100 per cent, 
H.SO;: 


H,O+S0,=H,S0O,. 
The acid B requires for the water present in each 100 gms 
-H,O:80,=(100—e):y 


__(100—c) SO, (100—c) 80.06 
oo BA ee 





= 4.44 (100—c) gms. SO,. 


If 100 gms. of the acid Brequire 4.44 (100—c) gms. SO, from A, 
then 
x gms. of the acid B require 0.0444 (100—c) 2 gms. SO, from A. 


Now 
A+B 
(100+ 2) :[a—0.0444 (100—c)xz]=100:b 
oe 100 (a—b) 
444+ b—4.44¢ 





gms. of B. 


Example.—The fuming acid A contains 25.5 per cent. free 
SO,=a. 

Sulphuric acid B contains 98.2 per cent. H,SO,=c. 

The acid desired is to contain 19.0 per cent. SO,=b. : 

If these values are inserted in the above equations, we obtain 


2 100(25.5719.0) 650 _o4 97 gms. H,SO0,, Bo 





= 4444 19—4.44%98.2 27 


TITRATION OF HYDROXYLAMINE SALTS, ETC. 581 


We must add, therefore, 24.07 gms. of the 98.2 per cent. sulphuric 
acid to 100 gms. of the fuming acid in order to obtain an acid 
containing 19.0 per cent. of free SO,. 


Titration of Hydroxylamine Salts. 


Hydroxylamine hydrochloride reacts neutral towards methy] 
orange and acid towards phenolphthalein. If the latter is added 
to an aqueous solution of the salt, and the titration is made with 
af alkali, the end-point will be obtained when the total amount of 
acid present has been neutralized by the alkali. It is impossible 
to determine the amount of free hydrochloric acid present 
when phenolphthalein is used, but it can be done with methyl 
orange. Romijn * recommends for the titration of the acid a 


N 
To borax solution. 


Hydrofluoric Acid. 
1000 ¢.c. normal alkali=HF=20.01 gms. HF. 


Hydrofluoric acid can be titrated with phenolphthalein as an 
indicator, but not with litmus or methyl orange. The acid is 
measured out into a platinum dish by means of a pipette which is 
coated with beeswax, an excess of sodium hydroxide free from 
alkali is added, and the excess of the latter is titrated in hot 
solution with an acid of known strength. 


Hydrofluosilicic Acid. 


The titration of this acid may take place according to either 
of the following reactions: 


I. H,SiF,+2KOH=K,SiF,+2H,0, 
Il. H,SiF,+6KOH=6KF+2H,0-+Si(OH),. 





* Z. anal. Chem., 36 (1897), 19. This method has not been tested in 
the author’s laboratory. 
+ Cf. Winteler, Z. angew. Chem., 1902, p. ‘33. 


582 VOLUMETRIC ANALYSIS. 


According to Equation I. 
1000 ¢.c. normal KOH or Ba(OH),=72.16 gms. H,SiF,. 


Author’s Method. 


If hydrofluosilicic acid is titrated in the cold with caustic 
potash, using phenolphthalein as indicator, a red color appears 
after a time, but disappears later on account of the excess alkali 
reacting according to the equation: 


K,SiF,+4KOH=6KF-+ Si(OH),. 


This last reaction, however, takes place so slowly that it is 
impossible to obtain a distinct end point. If, however, the 
solution is diluted with an equal volume of alcohol, then 2 or 3 
drops of phenolphthalein added, it can be titrated with tenth- 
normal potassium or barium hydroxide. The insoluble potassium 
or barium fluosilicate separates out and is not acted upon by an 
excess of the alkali, so that a sharp end point is obtained. Sodium 
hydroxide forms a soluble salt so that the titration cannot be 
made with this reagent. 


Indirect Method of Penfield.* 


Penfield treats the solution to be titrated with an excess of 
KCl, dilutes with an equal volume of alcohol, and then titrates 
the hydrochloric acid set free in the reaction, 


H,SiF,+2KCl= K,SiF,+2HCI, 


with tenth-normal sodium hydroxide solution, using cochineal 
as indicator. Methyl red is preferable to the cochineal. 


According to Equation II. 
1000 c.c. normal NaOH= 24.05 gm. H,SiF,. 


(a) Method of Sahlbom and Hinrichsen.t 
The solution is titrated at the temperature of the water-bath 
with tenth-normal sodium hydroxide solution, using phenol- 
phthalein as indicator. 


* Chem. News, 39, 179. 
t Ber., 39, 2609 (1906). 





DETERMINATION OF ORGANIC ACIDS. 583 


(b) Method of Schucht and Méller.* 


The solution to be titrated is treated with an excess of neutral 
calcium chloride solution (25 ¢.c. of 4N. CaCl,) and titrated with 
tenth-normal sodium hydroxide, using methyl orange as indi- 
eator. The following reaction takes place in the cold: 


H,SiF,+3CaCl, + 6NaOH = 3CaF, + 6NaCl+Si(OH) ,+2H,0. 


During the titration the solution remains clear, for the CaF, 
and the Si(OH), remain in colloidal solution. Phenolphthalein 
should not be used as indicator, as it is hard to decide upon the 
correct end point. 

In the titration of salts of te iki acid, however, 
the titration must always be carried out with Shenalphihaler as 
indicator: 


Na,SiF,+8CaCl,+4NaOH = 3CaF,+6NaCl+Si(OH),. 


In this case 


1000 c.c. of normal NaOH = 47.08 ems. of Na,SiF,. 


Determination of Organic Acids. 


Methyl orange cannot be used for the titration of organic acids. 
but either phenolphthalein or litmus may be employed. If car- 
bonic acid is present ai the same time, the titration is made in a hot 
solution (cf. p. 554). It is best to dilute the organic acid with 
water free from carbon dioxide, add phenolphthalein, and titrate 
with half-normal barium hydroxide in the cold. 

To wlustrate.—It is desired to analyze a sample of acetic anhy- 
dride. The only impurity that the distilled product is likely to 
contain is acetic acid, so that it is a question of determining the 
amount of acid and anhydride in the presence of one another. 
' Such a problem can be solved only by an indirect analysis. 
The mixture is weighed out in a small glass bulb and then 
thrown into an accurately-measured amount of standard barium 
hydroxide solution. The latter is contained in a flask which is 





* Ber., 39, 3693. This method has not been tested by the auther. 


584 VOLUMETRIC ANALYSIS. 


connected with a return-flow condenser and at the top of the 
condenser a soda-lime tube is fitted. The contents of the flask — 
are warmed gently until the oil has completely dissolved; it is 
thereby changed to acetic acid, : 


CH,CO\, 
CH'co 0 + H.0= 2CH,COOH, 


and the latter is neutralized by the alkali. After the reaction is 
complete, a drop of phenolphthalein is added and the solution 
is decolorized by the addition of a titrated acid. From the amount 
of the latter used, tho excess of the alkali is known, and if this 
is deducted from the total amount of alkali in the flask, the amount 
necessary for the complete neutralization of the acetic acid, whether 
originally present as the free acid or in the form - its anhydride, 
can be calculated: 
¢ 
C,H,O, C,H,O, 

1 « + y = p (original weight) ; 

2.mz + y =4q (weight acetic acid after the action of water) ; 


and from this x can be calculated, 


C= 





1 (q—-P); 


2C02H4O2__ 120.06 


o Sd: d 
COs len 





and in these equations m= 


Ezample.—The absolutely clear preparation of acetic anhy- 
dride from a well-known firm gave the following results, 0.9665 gm. 
being taken for the analysis: 


. 200 e.c. of barium hydroxide solution required 187.79 c.c. + HC; 
- 200 c.c. of barium hydroxide + 0.9665 gm. of substance 


required 6.03 c.c. a HCl; 


so that the 0.9665 gm. of substance was equivalent to 181.76 c.c. 


DETERMINATION OF ORGANIC. ACIDS. 585 


~ HCl, and this amount of a Ba(OH), solution would have been 


required to neutralize it. This corresponds to 
181.76 X 0.006003 = 1.0911 gms. acetic acid=q. 


If, now the values of p and q are introduced in the previous 
equations, we obtain 


2 =5.665(1.0911 — 0.9665) =0.7059 gm. anhydride, 


and in per cent. 
0.9665 :0.7059 = 100: 2 


x=73.04 per cent. acetic anhydride. 
The preparation, therefore, contained 


Acetic anhydride= 73.04 per cent. 


_ 26.96 per cent. 
~ 100.00 per cent. 





Acetic acid 


Remark.—Acetic acid anhydride is also hydrolyzed by water 
at the ordinary temperature. If a weighed amount of the sub- 
stance is shaken with water in a flask until no more drops of 
anhydride are to be recognized, and the acetic acid formed is 
then titrated with barium hydroxide, using phenolphthalein as 
indicator, correct results are obtained if the water used is entirely 
free from carbon dioxide. It is always safer, however, to carry 
out the determination as outlined above. 

In some factories the analysis of acetic acid anhydride is 
carried out by the method of Menschutkin and Wasiljeff. This 
is based upon the fact that when acetic acid anhydride is treated — 
with freshly distilled aniline, acetanilide is formed in accordance 
with the following equation. 


CH3-CO 


CH,.co D0 + CeHsN He = CoH sN (C2H30) H + CH;COOH 


586 | VOLUMETRIC ANALYSIS. 


whereas acetic acid itself does not form acetanilide under the same 
conditions. ‘Two or three grams of commercial acetic anhydride 
are shaken in a dry weighing beaker with from 4 to 6 c.c. of freshly 
distilled aniline. The anhydride immediately begins to combine 
with the aniline, liberating considerable heat. After cooling, the 
solidified contents of the weighing beaker are rinsed by means of 
absolute alcohol into an ordinary beaker, phenolphthalein is 
added and the total amount of acé@tic acid present titrated with 
half-normal alkali. 
We have then 


C4H,03 CoH4Oo 
2 ee Be 


mz + y = 4q (acetic acid); 





In this equation 


C2H40.__ 60.03 


ee ao Oa. gies 





It is true that concordant results are obtained by this 
method, but they are much too high; in fact as much as 14 
to 16 per cent. too high. This is due to the fact that although 
acetic acid itself does not react with aniline in the cold, it does. 
react very readily when heated. When, therefore, a mixture 
of acetic anhydride and acetic acid are allowed to remain in 
contact with aniline, there is so much heat liberated from the 
reaction between the anhydride and the analine that a part of 
the acetic acid itself reacts and takes part in the formation of — 
acetanilide: ave 


CH3CO2H + C6 HsN He = H20+ Ce Hs N (CoH30) H, 


so that evidently too little acetic acid is found in the subsequent 


DETERMINATION OF SULPHUROUS ACID. 587 


titration and consequently too high values are obtained for the 
amount of anhydride present. 


Determination of Sulphurous Acid. 


For the determination of sulphurous acid by itself, the analy- 
sis is always accomplished, as recommended by Volhard, by an 
iodimetric process, i.e., it is oxidized to sulphuric acid. In many 
cases, however, it is necessary to titrate the sulphurous acid with 
alkali (cf. p. 577), and here the choice of an indicator is important, 
for the end-point is very different in the case of methyl orange 
from that obtained when phenolphthalein is used: 


H,SO,+ 2NaOH = Na,SO,+ 2H,O (with phenolphthalein), 
H,SO,+ NaOH = NaHSO,+H,0 (with methyl orange). 


NaHSO, reacts acid toward phenolphthalein, but neutral 
toward methyl orange, so that twice as much alkai would be 
added in the first case. The most accurate results are obtained 
with the use of methyl orange, for the carbon dioxide which is 
almost always present does not exert much of an effect upon this 
indicator, whereas it does upon phenolphthalein. 


Determination of Orthophosphoric Acid. 


NaH,PO, reacts acid toward phenolphthalein, and neutral toward 
methyl orange, while Na,H PO, is neutral toward the former indi- 
cator and basic toward the latter. 

Therefore, on titrating free phosphoric acid with alkali one of 
the following reactions will take place: 


1. H,PO,+2Na0H = Na,HPO,+2H,0 (phenolphthalein). 
2. H,PO,+ NaOH =NaH2PO,+H,0 (methyl orange). 


The first reaction is not sharp, because pure Na,H PQ, is disso= 
ciated to a slight extent, so that it becomes alkaline to phenol- 
phthalein: 


Na, HPO,+H OH@Na H,PO,+Na OH. 


588 VOLUMETRIC ‘ANALYSIS. 


To prevent this hydrolysis, the titration is best effected in a 
cold, concentrated solution containing sodium chloride. 


Alkalimetric Determination of Phosphorus in Iron and Steel * 
1000 c.c. normal NaOH= 1.348 gm. P. 


The phosphorus is precipitated in a 2 gm.sample withammonium 
molybdate according to p. 437, filtered, washed with 1 per cent. 
nitric acid and then with 1 per cent. potassium nitrate solution 
until the washings no longer react acid. The filter and precipitate 
are then transferred back to the Erlenmeyer flask in which the 
precipitation took place, covered with an excess of tenth-normal 
NaOH (T c.c.), stirred until solution is complete, and then the 
excess of alkali titrated with tenth-normal nitric acid, (t c.c.) 
using phenolphthalein as indicator. 2 

~The reactions taking place are as follows: 


2[(NH,),PO,-12Mo0,]+-46NaOH = 2(NH,),HPO,+ (NH,),Mo0, 
+ 23Na,Mo0, +22H,0, 


from which it is clear that 46 gms. mols. of NaOH are equivalent 
to 2 gm. atoms of P and 1 e.c. of the tenth-normal NaOH= 
0.0001348 gm. P. | 

Since the precipitate was produced from 2 gm. steel, and T c.c. 
of NaOH and t c.c. of HNOg were used, the percentage of phos- 
phorus is 





(T ae oP. 

Remark.—To obtain accurate results, it is advisable to deter- 
mine the percentage of phosphorus in a steel gravimetrically, 
then to standardize the alkali against this steel, carrying out the 
titration exactly as described above. 


Determination of Boric Acid. 


Free boric acid has no action upon methyl orange, conse- 
quently alkali borates may be titrated with hydrochloric and 
nitric acids, using this indicator; with sulphuric acid the results 





*See Blair, Analysis of Iron and Steel. The method was proposed by 
J. O. Handy. : 


DETERMINATION OF BORIC ACID. 589 


are not as satisfactory, for there is in this case no sharp color 
change. If phenolphthalein is used as the indicator, the red 
color fades gradually and the end point cannot be determined 
with certainty. If, on the other hand, sodium hydroxide is | 
slowly run into an aqueous solution of boric acid containing phe- 
-nolphthalein, after some time a pale-pink color is noticeable which 
becomes deeper on the addition of more alkali. The first pink ~ 
color is formed before all of the boric acid has been neutralized, 
for sodium borate is perceptibly hydrolyzed. Free boric acid cannot 
be titrated by itself, but if, as proposed by Jorgensen,* a sufficient 
amount of.glycerolf (or mannitol {) is added to the solution, the 
hydrolysis is prevented, so that when 1 mol. of NaOH is present 
for 1 mol. of H3BO3 the solution suddenly changes from colorless 
to red; probably a stronger acid is formed by the addition of | 
the glycerol, the glyceryl-boric acid (C3H;020H) B(OH). 

If the solution does not contain sufficient. glycerol the color 
change takes place too soon, as can be shown by the addition of 
more glycerol. If the red color disappears on adding the lat- 
ter, more alkali is added until it reappears. The right end-point 
is reached when the red color no longer disappears on the een 
of glycerol. Inasmuch as commercial glycerol reacts acid, 
must be just neutralized with alkali before being used for bes 
determination. Furthermore, in order to obtain accurate results 


it is necessary that the solutions should be absolutely free from 
carbonate. 


Application. Determination of Boric Acid in an Alkali Borate 
‘Free from Carbonate. § 

_ About 30 gms. of the borate are dissolved in water free from 

carton dioxide, diluted to 1 liter, and the total alkali is determined 


in an aliquot part by titration with ’ hydrochloric acid, using 


methyl orange as an indicator. A fresh portion of the borate is 


* Zeitschr. f. Nahrungsm. IX, p. 389, and Zeitschr. f. eee Ch, +1897, 
p. 5. 

{ Zeitschr. f. angew. Ch., 1896, p. 549. 

t¢ Jones, Am. J. Sci. [4] 7, 147 (1899). 

§ M. Honig and G. Spitz, Zeitschr. f. angew. Ch., 1896, p. 549. 





59° VOLUMETRIC ANALYSIS. 


taken and exactly neutralized by the amount of hydrochloric 
acid found necessary by the previous titration; by this means the 
solution will contain free boric acid. After adding about 50 c.c. of 
glycerol for each 1.5 gms. of the borate, the solution is titrated 


with 2 sodium hydroxide, using phenolphthalein as indicator. 


- After the end-point is reached, 10 ¢.c. more of glycerol are added, 
and this usually causes the solution ¢0 become colorless. ‘The end- 
point with sodium hydroxide is again obtained and the process 
repeated until finally the addition of glycerol causes no fore 
action upon the end-point. 

If the borate contained carbonate, the portion taken pos analysis 
is neutralized with acid as before, then boiled for a few minutes, 
taking the precaution of connecting the flask containing the solu- 
tion with a return-flow condenser.* After the carbon dioxide is 
expelled, the sides of the condenser are washed down with water 


and the titration with sodium hydroxide made as before.f 
For the 


Determination of Boric Acid in Insoluble Silicates. 


see E. T. Wherry and W. H. Chapin, J. Am. Chem. Soc., 30, 1687 
(1908). 
Determination of Carbonic Acid. 


(a) Determination of Free Carbonic Acid. 


To determine ‘the amount of free carbonic acid present in a 
dilute aqueous solution, an excess of titrated barium hydroxide 
solution is added, and the excess is determined by means of a HCl, 
using phenolphthalein as an indicator: 


H,CO,+ Ba(OH),=BaCO,+2H,0 


1 c.c. a HCl=0.0022 gm. CO,. 


* The condenser serves to keep back‘any boric acid escaping with the 
steam. 

+ Instead of the glycerol, about one gram of mannitol may be wes to 
advantage. 





DETERMINATION OF CARBONIC_ ACID. 59f 


(6) Determination of Carbon Dioxide Present as Bicartonate. 


The solution is titrated with = HCl in the presence of methyl 


orange: 
NaHCO,-+ HCl=NaCl+ H,CO, 


£6c. oe HCl=0.0044 gm. CO,. 


(c) Determination of Carbon Dioxide Present as Carbonate. 
The titration is effected with af HCl and methyl] orange: * 


Na,CO,+ 2HC]=2NaCl+ H,CO, 
N 


1 ¢.0. = HC1=0.0022 gm. CO, 


(d) Determination of Free Carbonic Acid in the Presence of 
Bicarbonate. 


One portion is titrated with 55 s 9 HCL using methyl orange as 


indicator, and the amount of oe is determined as 
under (0). 

A second portion is treated with an excess of barium chloride,f 
then with an excess of barium hydroxide, and the excess of the 
latter titrated back with HCl, using phenoiphthalein as indicator. — 


If the amount of a acid used for the first titration is deducted 


from the amount of 5 i ~ barium hydroxide solution found to be 


necessary by the last oo the difference multiplied by 0.0022 
will give the amount of free carbonic acid.f 





* Alkaline-earth carbonates are dissolved in an excess of standard acid 
and the excess titrated back with standard alkali. 

+ The addition of barium chloride is only necessary when free carbonic 
acid is titrated in the presence of alkali bicarbonates. Without it free alkali 
would then be formed: NaHCO,+ Ba(OH),=BaCO,+H,O+ NaOH. 

t This method cannot be used when magnesium salts are present. 


592 VOLUMETRIC ANALYSIS. 


(e) Determination of Bicarbonate in the Presence of Carbonate. 
Method of C, Winkler. 


In one portion the total alkalinity is determined by titration 


with a HCl, using methyl orange as indicator. This requires 


N 
T c.c. of To HCl. 


In a second portion the bicarbonate is determined by adding 


an excess of = NaOH, then neutral barium chloride solution, 


and afterward titrating the excess of the former with phenol- 


phthalein and es HCl. We will assume that for this purpose 


Ty ¢.¢: ms NaOH and ¢ c.c. a HCl were used, then evidently 


(T,-—2) c.c. a NaOH were necessary to convert the bicarbonate 


into carbonate: 
NaHCO,+ NaOH = Na,CO,+H,0. 


1NaOH corresponds, consequently, to 1CO,, or 


LCX. a NaOH =0.0044 gm. CO,, 


and therefore (7',—t)-0.0044=CO, as bicarbonate. 
For the decomposition of the normal carbonate 


T(T, 1) =(T+i- Tyee: HCl 


were necessary, and from the equation 
Na,CO,+ 2HCl=2NaCl+ H,O+CO, 


it is evident that 


and 


1 c.c. x HCl=0.0022 gm. CO,. 


The carbon dioxide as carbonate=(T'+ {— T',) 0.0022 gm. 


DETERMINATION OF CARBONIC ACID IN THE AIR, 593 


Remark.—It has been proposed to determine volumetrically 
the free and bicarbonate carbonic acid in drinking and mineral 
waters; with the former accurate results can be obtained, but 
_with the latter this is not the case. In the determination of the 
total alkalinity not only the bicarbonate but also the ever-present 
silicate and borate are likewise determined, so that this in many 
cases causes considerable error in the analysis of mineral waters. 
Thus in analyzing a sample of mineral water containing in reality 
4.63 gms. of carbonic acid as bicarbonate per kilogram, the titra- 
tion showed 5.42 gms., a difference of 0.61 gm. CO,! 


Determination of Carbonic Acid in the Air. Method of 
Pettenkofer. 


Principle—A large, measured volume of air is treated with 
an excess of titrated barium hydroxide solution whereby the car- 
bon dioxide is quantitatively absorbed, forming insoluble barium 
carbonate. Phenolphthalein is added, and the excess of barium 
hydroxide is: determined by titration with hydrochloric acid 
until the solution is colorless. From the amount of alkali 
used to absorb the carbon dioxide, the amount of the latter is 
calculated. 

Requirements.—1. A calibrated flask of 5 liters capacity. 

2. Standard solutions of barium hydroxide and hydrochloric 
acid. The acid is prepared so that 1 c.c.=0.25 e.c. CO2 at 0° C. 
and 760 mm. pressure; this is accomplished by diluting 224.7 c.c. * 
hydrochloric acid to 1 liter. The barium hydroxide solution 
should be of about the same strength. 

Procedure.—The flask, with its capacity etched upon it, is 
placed in the space from which the air is to be taken, and by means 
of a bellows, the mouth of which is connected with a piece of 
rubber tubing, the air in the flask is changed; about 100 strokes are 
made with the bellows. The flask is then stoppered with a rub- 
ber cap, and at the same time the temperature and barometer 
readings are noted. 


594 VOLUMETRIC ANALYSIS. 


By means of a pipette, 100 c.c. of barium hydroxide solution 
are run into the flask, the rubber cap replaced on the bottle, and 
tie solution is gently shaken back and forth in the flask for fif- 
teen minutes. The turbid liquid is then poured into a dry flask, 
25 c.c. are pipetted out, phenolphthalein is added, and hydrochlorie 
acid stowly run in with constant stirring until the solution is 
coloriess, This requires n c.c., so that for the 100 c.c. of alkali 
solution, 4Xn c.c. would be necéssary. The strength of the 
barium hydroxide in terms of acid is now accurately determined; 
25 c.c. of barium hydroxide require N c.c. of the standard hydro- 
chloric acid, or 100 c.c. would neutralize 4X N c.c. of acid. ~ 

Calculation.—Assume the contents of the flask to be V c.c. 
at ¢° C. and B mm. pressure. By the introduction of 100 c.c. barium 
hydroxide solution the same volume of air was replaced, so that 
the amount of air taken for analysis amounts to (V—100) c.c. 
at (© C. and B mm, pressure. At 0° C. and 760 mm. pressure the . 
volume is ti 
(V —100) B 


( 
Vo 760 (i-+a-f) 





100 c.c. of barium hydroxide solution require 4 N c.c. HCl, 
while 100 c.c. of the alkali after treatment with V, c.c. of air require 
4.nc.c. of the acid and this corresponds to 4 (N—n)-0.25=(N —n) 
c.c. CO, at 0° C. and 760 mm. pressure. 

The amount of CO, present in 1 liter of air measured at standard 
conditions amounts to 


V.:(N—n)=1000:2 | 


x= sro) gms. CO, 
° 





PERSULPHURIC._ ACID. 595 


Persulphuric Acid. | 


1000 c.c. be Potassium Hydroxide 





10 
_ K2820g__ 270.34 _ 
= 50 = 0 = 13.517 gms. KoS20sg. 


If an aqueous solution of either potassium, sodium, or barium 
persulphate is boiled for some time, the salt is decomposed in 
accordance with the equation: 


2KoS20g + 2H2O =2K.2SO4+ 2HSO4 +O 


into neutral sulphate and free sulphuric acid. The latter can be 
titrated with tenth-normal potassium hydroxide solution. 

Procedure.—About 0.25 gm. of the persulphate is placed in an 
Erlenmeyer flask of Jena glass, dissolved in about 200 c.c. of water, 
and the solution boiled for twenty minutes. It is then cooled, 
- methyl orange added, and the solution titrated with tenth-normal 
potassium hydroxide. Or, an excess of the alkali may be added 
and the amount of excess titrated with tenth-normal acid. 

The results correspond with those obtained by the ferrous 
sulphate method (cf. p. 629) provided the persulphate is not con- 
taminated with potassium bisulphate. 

Remark.—Ammonium persulphate cannot be analyzed by the 
above method because when a solution of this salt is boiled, two 
reactions take place. The principal reaction, to be sure, is - ! 


2(NH,4) 29208 +2H20 =2 (NH4) 9804+ 2H 2S04+ Oo, 


but the oxygen is evolved to some extent in the form of ozone and 
the latter oxidizes a part of the nitrogen, so that besides sulphuric 
acid, the solution will contain more or less nitric acid. 


8(NH4) 2520s + 6H20 = 7(NH4) 2804+ 9H2S04+2HNO3. 


II. OXIDATION AND REDUCTION METHODS. 


All processes considered under this heading are those in which 
the substance analyzed is either oxidized or reduced by means 
of the solution with which the titration is made. As a standard 
for measuring the normality, we tonsider the oxidation of two 
gm.-atoms of hydrogen by 1 gm.-atom of oxygen. The norma] 
solution, therefore, will be one which for each 1000 c.c. gives up 


or requires 58 gms. of oxygen = 1 gm. of hydrogen. 


OXIDATION METHODS. 
A. The Permanganate Methods. 


‘These are based upon the fact that 2 gm.-molecules of potas- 
sium permanganate in acid solution give up 5 gm.-atoms of 
oxygen, equivalent to 10 gm.-atoms of hydrogen: 


2KMnO,=K,0+ 2Mn0O-+ O,(=10H). 


Or, what amounts to the same thing, each atom of manganese is 
reduced from a valence of 7 to a valence of 2, so that 1K MnO, will 
oxidize the equivalent of 5H. 

The solution must always contain enough sulphuric acid in 
order that the metals will be left in the form of sulphates and not 
as oxides; otherwise less oxygen is available from the perman- 
ganate (cf. p. 613). . 

The amount of potassium permanganate required for the prep- 
aration of a liter of normal solution is shown by the above equa- 
KMnO, 158.03 

Bad ier 6 


For the great majority of oxidation analyses x and rarely - 


=31.61 gms. 





tion to be 


solutions are used. 
The Preparation of = Potassium Permanganate Solution 


was described on p. 90. 596 


OXIDATION METHODS. 597 


Standardization of Permanganate Solution. 


1. Against Sodium Oxalate (Sérensen).* 
1000 c.c. of normal permanganate solution =67.00 gms. NagC204. 


. Sodium oxalate can be purchased in a very pure condition. 
The traces of moisture present may be expelled by heating the 
oxalate for two hours at 130° and cooling in a desiccator; but for 
ordinary work this is usually unnecessary. : 

A weighed amount of sodium oxalate is dissolved in 200 c.c. 
of distilled water at 70°, about 20 c.c. of double-normal sulphuric 
acid are added, and the hot solution titrated with permanganate. 
At the start the titration should proceed very slowly, waiting 
after the addition of each drop until the color has disappeared 
before adding more permanganate 


2KMn0,44+5Na2C204+ 8HsS04= 
= K,804,+2MnS0,4+ 5NaeS04+ 10CO2+8H20. 


The purity of the sodium oxalate may be tested by heating 
a weighed sample in a covered platinum crucible for thirty 
minutes over a small flame, so that the bottom of the crucible 
is barely red. It is best to use an alcohol lamp or else insert 
the crucible in a disk of asbestos as in a sulphur determina- 
tion. Otherwise the sulphur in the illuminating gas may cause 
the formation of some sodium sulphate in the crucible. By 
the heating the oxalate is converted quantitatively into car- 
bonate, but there is a separation of some carbon which should 
be removed for the most accurate work. This may be accom- 
plished by heating the contents of the crucible to a much higher 
temperature with free access of air, or more readily by adding a 
few cubic centimeters of water, evaporating the solution to dry- 
ness on the water bath, and then very carefully heating the 
crucible over a free flame. In about ten minutes the carbon 
will all disappear without the carbonate being melted. The 
crucible is then allowed to cool, its contents dissolved in hot 





* Z. anal. Chem., 42, 352, 512 (1903); 45, 272 (1906). 


598 : VOLUMETRIC ANALYSIS. 


water, the crucible and cover thoroughly washed, and the cold 
solution titrated with tenth-normal hydrochloric acid, using 
methyl orange as indicator. 


1000 c.c. tenth-normal hydrochloric acid=6.700 gms. Nag€2O0x4. 


Kzmark.—Sodium oxalate crystallizes without water of crys- 
tallization, is not hygroscopic, and js especially suited for the 
standardization of permanganate solutions. Sorensen, in fact, 
has strongly recommended this substance as a standard for 
acidimetry, although it has no advantage over the standardization 
by means of sodium carbonate. The titration of the carbonate 
may be carried out with methyl orange as an indicator, but 
Sdérensen recommends phenolphthalein as somewhat more reliable. 


2. Against Oxalic Acid. 


Tenth-normal oxalic acid solution is excellent for the standard- 
ization of a permanganate solution. By means of a pipette 25 c¢.c. 
are measured into a beaker, 10 ¢.c. of dilute sulphuric acid (1:4) 
are added, the solution is diluted with water at about 70° C. to 
a volume of 200 c.c., and the permanganate is run into it, with 
constant stirring, from a glass-stoppered burette. At first the solu- 
tion is colored red for several seconds, then it becomes colorless 
but after the reaction is once started the permanganate is rapidly 
decolorized until an excess is present. The permanent pink color 
is imp2rted to the solution by the permanganate as soon as al] 
the oxalic acid is oxidized; this is taken as the end-point. 

The oxidation is expressed by the following equation: 


_ 2KMnO,+ 5H,C,0,+ 3H,SO, = K,SO,+ 2MnSO,+ 8H,0-+ 10C0,. 
Since for the oxidation of 1 gm. molecule of oxalic acid, 


COOH 
[  (4£6~900,4H,0, 
COOH 


1 gm.-atom of oxygen is necessary, and 1 liter aS oxalic acid con- 


tains 35 gm.-molecule of the acid, it is evident that 1000 c.c. of > 


OXIDATION METHODS. 599 


oxalic acid are equivalent to 34 gm.-atom of oxygen=0.8 gm. and 
1 c.c. of the solution =0.0008 gm. O. 


If for the oxidation of 25 c.e. si oxalic acid 24.3 ¢.c. of per- 


| 10 
manganate solution were required, these 24.8 cc. correspond to 
— 25X 0.0008 = 0.0200 gm. oxygen or I c.c. KMn0,=7 =0.0008230 
gm. O. 


Instead of expressing the concentration of the permanganate 
solution in terms of oxygen, it has been the custom to express 
it in terms of iron. The following consideration will show how 
the calculation may be made. 

From the oxidation equation 


2FeO+0=Fe,0, 


it follows that 1 gm.-atom of oxygen is necessary for the oxida- 
tion of 2 gm.-atoms of iron; consequently 4 gm.-atom of oxygen 


(=1H = 10,000 c.c. ss! oxalic acid) corresponds to 1 gm.-atom iron, 


10 
so that 25 c.c. X oxalic acid = 24.3 c.c. permanganate = 25 x 0.00559 
=0.1398 gm. iron; or 1 ¢.c. of the permanganate solution a 


=0.005751 gm. Fe. 

Remark.—Against the use of oxalic acid solution for the stand- 
ardization of a permanganate solution is the fact that the con- 
centration of the aqueous solution is not permanent; for this 
reason, E. Riegler * proposed the addition of 50 ¢.c. of concen- 
trated sulphuric acid to each liter of the oxalic acid, by which 
means the solution can be kept unchanged for a much longer 
length of time. That this is the case is shown by the following 
experiments: A solution of oxalic acid in water was prepared, 
and also one in dilute sulphuric acid. Both solutions were titrated 
on the same day with permanganate solution which had been 
standardized against electrolytic iron. At the end of eight months 





*Z. anal. Chem., 1896, p. 522. 


600 VOLUMETRIC ANALYSIS. 


the same solutions were titrated against a freshly-standardized 
permanganate solution, with the following results: 





7 


Aqueous Oxalie Acid. gat ontain ing 





Freshly-prepared .| 1000 c.c. = 1000.6 c.c. ~ sol. | 1000 c.c. =1002.5 c.e. x sol, . 
After 8 months,. ./ 1000 ¢c.c.= 994.9¢.c. “ # |1000c.c.=1001.8 “ “ 











At the end of eight months, therefore, the aqueous solution 
had depreciated 0.56 per cent. in strength, while the solution con- 
taining the sulphuric acid had only weakened to an extent of 
0.12 per cent. of its original concentration. 

From this it is evident that a solution of oxalic acid contain- 
-ing sulphuric acid can be used for the standardization of a per- 
manganate solution, provided the former has not stood more than 
eight months since it was prepared. The use of old aqueous 
solutions of oxalic acid is to be discouraged. 


3. Against Metallic Iron. 


It has been a favorite practice to standardize permanganate 
solutions against iron wire. The wire, however, is never abso- 
lutely pure, and there is a chance of the impurities reducing 
permanganate so that the actual iron content of the wire does 
not suffice to show exactly how much oxidizing agent it will need. 
Classen,* therefore, has recommended that pure iron be prepared 
by the electrolyis of ferrous ammonium oxalate. This iron is 
dissolved in dilute sulphuric acid out of contact with the air and 
the solution titrated with permanganate (cf. p. 93). 

The standardization of a potassium permanganate solution can 
be correctly accomplished by means of iron wire, provided the 
apparent tron content of the wire has been determined by a com- 
parison of the values obtained in a titration with a standardization 
by either electrolytic iron or sodium oxalate. Every time a new 
supply of iron wire is purchased, the comparison should be made. 





* Mohr Classen, Lehrbuch der chem. anal. Titrirmethode. 


OXIDATION METHODS. 6or 


Determination of the Apparent Iron Value of Iron Wire.—The 
wire is cleaned as described on p. 98, and a weighed portion of 
about 0.2 gm. is introduced into a flask of not more than 250 c.c. 
capacity as shownin Fig. 89. The air is displaced by the intro- 
duction of a stream of carbon dioxide, which has passed through 
a bottle containing water and another containing copper sulphate 
solution (cf. p. 93, foot-note); the wire is then dissolved in 55 ¢.c. 


@ 
































Fig. 89. : Fie, 90. 


of dilute sulphuric acid (1 part concentrated acid to 10 of water). 
During the solution of the wire, the flask is supported somewhat 
as shown in the drawing, and is closed by a rubber stopper which 
carries a bulb tube connected with a Bunsen valve.* The con- 


* A Bunsen valve consists of a short piece of rubber tubing with a cut 
along a few centimeters of one side, and the outer end of the tubing is closed 
by a glass rod. This valve prevents the entrance of air from without. A 
flask larger than 250 c.c. capacity is likely to be so thin as to break during 
the cooling of the iron solution. In Fig. 80a, instead of using a glass rod at 
the end of the valve, a glass tube is used which is sealed at one end, and has 
a hole on one side. This tube serves to prevent the collapse of the rubber 
tubing at the place where the slit is formed. 





602 VOLUMETRIC ANALYSIS. 


tents of the flask are heated by means of a low flame until the wire 
has all dissolved, after which the solution is boiled gently fora 
short time. It is then allowed to cool, the stopper is removed, and 
the permanganate added until a color is obtained which is per- 
manent for thirty seconds. 

Instead of using a Bunsen valve, the Contat-Géckel valve may 
be used as shown in Fig. 90. The funnel contains a cold, saturated 
solution of sodium bicarbonate, thftough which the hydrogen from 
the flask passes. When the flame is removed sodium bicarbonate 
solution is drawn into the flask, and this causes the evolution of 
carbon dioxide, which prevents the entrance of more of the solution. 

S. Christie, by following the above procedure, found the 
apparent iron content of a wire to be 99.985 per cent., and Dr. 
Schudel found 100:21 for another wire. ae 

It must be.mentioned, however, that the apparent iron value 
varies considerably with the way in which the solution of the wire 
is effected. If the volume of the liquid is large (cf. p. 96), there 
is more chance of hydrocarbons remaining in solution, and the 
same is true if the solution is not boiled as in the above direction, 
but merely heated upon the water bath. 

Remarks Concerning the Standardization by Means of Electro- 
lytic Iron.—The objection has been raised that electrolytic iron is 
contaminated with hydrocarbons. According to Avery and Benton 
Dales,* the iron obtained by the electrolysis of ferrous ammo- 
nium oxalate contains from 0.2 to 0.4 per cent. carbon on an 
average; according to Skrabal + considerably more. Verwer and 
Groll,t however, assert that electrolytic iron contains no carbon 
-provided the bath still contains an excess of iron at the end of the 
electrolysis. Christie has carried out extensive experiments in 
the author’s laboratory and found that the electrolytic iron pre- 
pared by the Classen method does often contain carbon, but the 
amount is so small that it may be disregarded. Christie, further- 
more, standardized a solution of permanganate by four different 
methods and obtained the following values: 





* Ber., 32, 64 (1899). 
7 Z. anal. Chem., 42, 395 (1903). | 
t Ber., 32, 806 (1899). See also H. Verwer: Chem. Ztg., 25, 792 (1901). 


STANDARDIZATION OF PERMANGANATE SOLUTION. 603 























Against. Value 1 c.c. in Terms of Oxygen. 
Electrolytic iron. ........ -0.0007972 0.0007960 
OGRE eae aes oh ss 0.0007977 0.0007982 
Oxalic acid. .... Pe eR 0.0007978 0.0007967 
Sodium oxalate.......... 0.0007970 0.0007975 : 








4. Against Sodium Thosulphate. 
See Iodimetry. 
5. Against Hydrogen Peroxide. 
See Gasometric Methods. 
Permanence of Potassium Permanganate Solutions. 
As mentioned on p. 90, a permanganate solution will keep 
indefinitely, provided it is kept free from dust and reducing vapors. 
In order to test the permanence.of such a solution,* it was stand- 
ardized against electrolytic iron and after eight months it was 
again tested.j It had lost only 0.17 per cent. of its original value 
and could be used for all ordinary analyses. For very accurate 
work, however, it is advisable to standardize the solution fre- 


- quently. 


USES OF PERMANGANATE SOLUTION. 


1. Determination of Iron (Margueritte 1846). 


N ‘0.005585 gm. Fe 
1¢.¢. 75 KMn0O, corresponds to } oars gm. FeO 
0.007985 gm. Fe2O3 

In this determination the iron is oxidized from the ferrous 
to the ferric condition: 
2KMnO,-+ 10FeSO,+ 8H,SO,= K,SO,+ 2MnS0O,-++ 5Fe,(SO,),+ 8H,O 
The solution of the ferrous salt is strongly acidified with sul- 
phuric acid (about 5 ¢.c. of concentrated sulphuric acid should 


be present for each 100 c.c. of the solution), diluted with boiled 
water to a volume of 400 to 500 c.c., and titrated in the cold by 





* The solution was already three months old. 

+ In June, 1899, 1 c.c. of the KMnO, solution=0.0054853 gm. Fe; in 
March, 1900, 1 c.c of the KMnO, solution=0.0054761 gm. Fe. See also 
Morse, Hopkins and Walker, Am. Chem. Jour., 18, 401. 


604 VOLUMETRIC ANALYSIS. 


the addition of potassium permanganate from a glass-stoppered 
burette until a permanent pink color is obtained. If the perman- 
ganate solution is tenth-normal, the number of cubic centimeters 
used multiplied by 0.005585, 0.007185, or 0.007985 will give 
respectively the amounts of iron, ferrous or ferric oxide. 

This determination affords very accurate results and is un- 
questionably one of the best methods for determining iron. 

Remark.—The titration of irofi in hydrochloric acid solution 
gives high results unless particular precautions are taken. If 
dilute permanganate solution is allowed to run into a cold dilute 
solution of ferrous chloride containing hydrochloric acid, the 
former is decolorized and the iron is oxidized, but there is a 
noticeable evolution of chlorine.* More permanganate is used up 
than is necessary to oxidize the ferrous salt to the ferric condition. 

If, however, permanganate is run into cold, dilute hydrochloric 
. acid, in the absence of ferrous salt, there is no evolution of chlorine. — 
Furthermore, tne presence of a ferric salt does not cause evolution 
of chlorine. The chlorine, therefore, is not a result of the direct . 
action of the permanganate upon the hydrochloric acid, but is 
due to the intermediate formation of a peroxide. : 

When permanganate is run into a dilute hydrochloric acid 
solution containing ferrous chloride and considerable manganous 
salt, the ferrous iron is quantitatively oxidized to ferric iron 
and there is no evolution of chlorine. This was shown by Kessler ¢ 
in 1863 and by Zimmermann ¢ in 1881. It has since been con- 
firmed by many other chemists. § 

This can be explained as follows: Permanganate reacts with 
manganous salt and forms, as Volhard || found, MnOg. This 
MnOz oxidizes the ferrous iron to ferric iron. 


2FeO + MnOs= Fe203+ MnO 


more quickly than it is able to react with hydrochloric acid. 





* Lowenthal and Lenssen, Z. anal. Chem., 18638, 329. 
t+ Pogg. Ann., 118, 779, and 119, 225. 
t Ber., 14, 779, and Ann. Chem. Pharm., 213, 302. 
§ For example, J. A. Friend, J. Chem. Soc., 95, 1228 (1909). C.C. Jones 
and J. H:Jeffery. The Analyst, 34, 306 (1909). 
|| Ann. Chem. Pharm., 198, 337, 


STANDARDIZATION OF PERMANGANATE SOLUTION. 605 


Zimmermann * suspected that, in the presence of manganous 
salts, ferrous iron is converted into a peroxide which immediately 
breaks down into ferric iron and oxygen, and the latter acts upon 
the hydrochloric acid. This has more recently been confirmed 
by the interesting work of Manchot.{ 

According to Manchot, there is formed in all oxidation 
processes a ‘‘ primary oxide’’ which has the character of a 
peroxide. These primary oxides are as a rule unstable compounds 
which cannot be isolated and which constantly tend to give up 
oxygen and pass to a more stable lower state of oxidation. When 
an acceptor { is present, it will take up the oxygen which is lost by 
the primary oxide as it passes to a lower and more stable state of 
oxidation; in the absence of an acceptor this oxygen is evolved as 
gas. 

According to the method of oxidation, iron tends to form 
different primary oxides. Thus, in the direct oxidation by means 
of oxygen, the primary oxide is FeO2; in the oxidation by means 
of permanganate, chromic acid, or hydrogen peroxide, it is Fe2O5; 
whereas FeO3 is probably formed in the oxidation by means of 
hypochlorous acid. 

The oxidation of ferrous oxide to ferric oxide, therefore, does 
not take place directly, as has been usually assumed, but the 
primary oxide FeOz is first formed, and this reacts with more of 
the ferrous oxide, which therefore plays the part of an acceptor, 
to form ferric oxide: 


2FeO + Oz = 2FeOo, 
2FeO + 2FeO, = 2Fe2O0s. 


Potassium permanganate causes the formation of Fe2O5 as 
‘primary oxide: 
2FeO + Mn207 —Fe.0; + 2MnOz. 





* Ber., 14, 779 and Ann. Chem. Pharm., 2138, 302. 

+ Ann. Chem. Pharm., 325, 105 (1902). 

{ An acceptor is a substance which is not oxidized by oxygen alone, but can 
be thus oxidized by the aid of some other substance present called an auto- 
oxydator. A substance which tends to be peroxidized may play the part of 
an acceptor. Cf. Engler, Ber., 38, 1097 (1900). 


~ 606 VOLUMETRIC. ANALYSIS. 
The MnOz at once oxidizes more ferrous oxide to ferric oxide 


2FeO + MnOo= Fe203 + MnO 


and the Fe.O; converts the MnO to MnOg again. 

If, however, the concentration of the manganous salt in the 
solution is too low, hydrochloric acid begins to play the part of an 
acceptor, so that a part of the Fe2@; is used up in the oxidatisn of 
hydrochloric acid: 


Fe20; + 10HCl= 2FeCls + 5H:O + 2Cle 


The action of the manganese sulphate is twofold. On the one 
hand it regulates the velocity of the reaction between ferrous 
oxide and permanganic aicd, for, according to Volhard, the © 
HMnO, acts upon the manganous salt with the formation of 
manganese peroxide, which then reacts with the ferrous salt; 
on the other hand it takes up the oxygen from the iron peroxide 
and carries it to the unoxidized ferrous salt. In both cases it is 
essential that manganese peroxide does not react with hydro- 
chloric acid very rapidly, and it is necessary, too, that the amount 
of manganous salt shall greatly exceed the amount of iron 
present. 

Zimmermann suggested a similar explanation, but it seemed 
to meet with but little approval, so that the hypothesis of Wagner * 
was quite generally adopted. ‘The latter claimed that the excess 
of permanganate required for the titration of ferrous chloride in 
the absence of manganous sulphate was due to the intermediate 
formation and rapid oxidation of a ferrous-hydrochloric acid, 
FeCl, -2H Cl. | 

Manchot’s explanation, however, seems to be the better one. 

Although it is possible, then, to titrate iron in hydrochloric 
acid solutions in the presence of manganous sulphate, the method 
possesses the disadvantage that the end-point cannot be seen so 
distinctly as when no chloride is present, since ferric chloride , 
forms a much more yellow solution than does ferric sulphate. 





* Zeitschr. f. physikal. Chem., 28, 33. 


STANDARDIZATION OF PERMANGANATE SOLUTION. 607 


This difficulty can be overcome by the addition of phosphoric 
acid, as suggested by C. Reinhardt.* 


TITRATION OF FERROUS SALTS IN HYDROCHLORIC ACID SOLUTION. 
METHOD OF ZIMMERMANN-REINHARDT. 


From 20 to 25 c.c. of the manganese sulphate solution + pre- 
pared as described below are added to the solution, and after 
diluting with boiled water to a volume of 500 c.c. it is titrated 
with potassium permanganate which is added so slowly that the 
drops can be counted. Care is taken toward the last not to add 
a drop of permanganate until the color of the preceding one has 
disappeared. 

The manganous sulphate solution is prepared as follows: 67 gms. 
of crystallized manganous sulphate (MnSO,+4H20) are dissolved 
in 500 to 600 ¢c.c. of water, 138 c.c. of phosphoric acid (of specific 
gravity 1.7) and 130 c.c. of concentrated sulphuric acid (sp. gr. 
1.82) are added, and the mixture is diluted to 1 liter. 

If the iron is present as ferric salt, it must be reduced com- 
pletely to the ferrous condition before it can be titrated with 
potassium permanganate. 


THE REDUCTION OF FERRIC SALTS TO FERROUS SALTS 
can be accomplished in a number of different ways. 
1. By Hydrogen Sulphide. 
This reduction has already been described on page 99. 
2. By Sulphur Dioxide. 


The solution containing the ferric salt is neutralized with 
sodium carbonate,t an excess of sulphurous acid is added, 
the solution boiled, and a current of carbon dioxide is passed 





* Stahl und Eisen, 1884, p. 709, and Chem. Ztg., 13, 323. 

t It is well to add one cubic centimeter of manganese sulphate for each 
cubic centimeter of HCl (sp. gr. 1.12) present. Cf, J. A. Friend or Jones 
and Jeffery, loc. cit. 

t Ferric salts are not completely reduced by sulphurous acid in the pres- 
ence of considerable hydrochloric or sulphuric acid. 


608 VOLUMETRIC ANALYSIS. 


through it until the excess of the reagent is completely removed.* 
The reduced solution is then cooled in an atmosphere of carbon 
dioxide and titrated. | 


3. By Metals. 


The acid solution of the ferric salt, contained in a small flask 
fitted with-a Bunsen valve, is reduced by heating on the water- 
‘bath with the addition of small pieces of chemically-pure zine until 
the solution is completely colorless and a drop of it, removed by 
means of a piece of capillary tubing, will no longer give any color 
with potassium sulphocyanate solution. After cooling, the solu- 
tion is poured through a funnel containing a platinum cone (no 
paper), and the undissolved zine remaining in the funnel is washed 
several times with boiled water. 

Remark.—Since zine often contains iron, a blank experiment 
must be made by dissolving 3 to 5 gms. in the sdme way and titra- 
ting the solution with permanganate. If iron is present, as shown 
by the fact that a measurable amount of potassium permanga- 
nate is decolorized, the reduction of the ferric salt must be effected 
by means of a weighed amount of zinc and a correction made for 
the iron. It is self-evident that in this case the titration must 
not take place until all of the zinc has dissolved. Instead of zinc, 
cadmium and aluminium are frequently used. 

Remark.—Against this method objections can be raised. In 
the first place, the fact that a foreign metal is introduced into 
the solution is in many cases unfortunate. Furthermore, by 
means of zinc, titanic acid is reduced to Ti,O,, only to be oxidized 
again by the permanganate solution, so that more permanganate 
solution will then be required than corresponds to the amount of 
iron present. By means of H,S or SO,, titanic acid is not reduced 
and there is no foreign metal introduced into the solution. Con- 





* It is not advisable to depend upon the sense of smell. The escaping 
gas is tested by passing it through dilute sulphuric acid containing a few 


drops of af KMn0O, solution. If the latter is not decolorized at the end of 


two or three minutes, the excess of sulphurous acid has been removed. 
+ The reduction by means of zinc may be satisfactorily accomplished with 
a “Jones reductor.” Cf, Fig. 91, p. 637. 


REDUCTION OF FERRIC SALTS TO FERROUS SALTS. 609 


sequently, for accurate mineral analyses, it is necessary to use 
one of these methods, and in fact the reduction by means of 
hydrogen sulphide is to be preferred. By means of the latter the 
ferric salt is completely reduced, independent of how little or how 
much free acid is present in the solution; again, any metals of 
the hydrogen sulphide group are precipitated at the same time; 
while finally it is easy to recognize the fact that the excess 
of the gas has been removed by the use of the sensitive lead 
acetate paper test. 


4. By Stannous Chloride. 


This method proposed by Zimmermann and Reinhardt * is 
especially suited for metallurgical purposes, because it can be 
accomplished most rapidly. 

Principle-—The method depends upon the fact that ferric 
chloride in hot solution is easily reduced by stannous chloride: 


SnCl, + 2F eClez = SnCl4 — 2FeCle. 


The complete decolorization of the solution shows the end- 
point of reduction. The excess of stannous chloride is afterwards 
oxidized by means of mercuric chloride: 


SnCl, + 2HgCle = SnCl,+ HgeCle. 


After this treatment, which consumes but a few minutes, some 
manganese sulphate solution is added and the solution imme- 
diately titrated with potassium permanganate, which is added 
slowly. 


Requirements. 


(a) Stannous chloride solution. 50 gms. of stannous chloride 
are dissolved in 100 c.c. of concentrated hydrochloric acid and 
diluted with water to a volume of one liter. 

(b) Hydrochloric acid, sp. gr. 1.12. 





* Loe. cit. 


610 VOLUMETRIC ANALYSIS. 


(c) Mercurie chloride solution. A saturated solution of the 
pure commercial salt in water is used. 

(d) Manganese sulphate solution. See p. 607. 

Procedure.—The ferric salt is dissolved in 20 to 25 c.c. of the 
hydrochloric acid (b) heated to boiling, the flame removed, and the 
stannous chloride solution (a) is added drop by drop until the 
iron solution just becomes colorless. The solution is cooled to at 
least the room temperature and 10 ec. of mercuric chloride (c) are 
quickly added, whereby a slight silky precipitate of HgsClo* is 
formed. After ten minutes the solution is diluted to about 500 
c.c., 20 to 25 e.c. of the manganese sulphate solution (d) are added, 
and the mixture is titrated (very slowly) with potassium per- 
manganate until a pink color permanent for one minute is 
obtained. 

Example: Determination of Iron in Hematite, Fe203.—About 
0.25 to 0.3 gm. of the finely-powdered mineral is weighed out into 
a beaker, 3 c.c. of the stannous chloride solution (a) + are added 
and 25 c.c. of the acid (b). The beaker is covered with a watch- 
glass and its contents heated nearly to boiling until all of the iron 
oxide has dissolved and a white sandy residue is obtained. This 
operation seldom requires more than ten minutes. The slightly 
yellow colored solution thus obtained is carefully treated with 
stannous chloride drop by drop until it becomes colorless and the 
reduced solution is analyzed as above. 





* If the precipitate produced by mercuric chloride is at all grayish in 
color, the portion must be thrown away; too large an excess of stannous 
chloride was used. Moreover, the end point with permanganate is difficult 
to see if the solution contains much precipitate. 

+ The stannous chloride greatly facilitates the solution of the hematite: 
If too much is used, strong permanganate should be added drop by drop 
until the yellow color of ferric chloride appears, and the solution then care- 
fully decolorized again. 


DETERMINATION OF METALLIC IRON. 611 


Determination of Metallic Iron in the Presence of Iron Oxide. 


This method is useful for testing ferrum reductum which is ob- 
tained by the reduction of Fe,O, in a stream of hydrogen. Usually 
the reduction is not complete and the preparation contains, besides 
the metallic iron, some oxide, usually assumed to be Fe,O,. The 
value of the preparation depends upon the free iron content. 


(a) Method of Wilner *-Merck.t 


Principle.—The method is based upon the fact that a neutral 
solution of mercuric chloride dissolves iron according to the equa- 
tion 

Fe+ HgCl,= Hg+ FeCl, 


while the Fe,O, is not attacked. The solution of ferrous chloride 
is titrated with permanganate solution. 

Procedure.—About 0.5 g. of ferrum reductum, in the form of a 
fine powder,t is placed in a 100 c.c. graduated flask, from which 
the air is replaced by CO,, 3 gms. of solid mercuric chloride ‘are 
added and 50 c.c. of water. The contents of the flask are then 
heated to boiling, by means of a small flame, and the liquid boiled 
for a minute. The flask is then filled up to the mark with boiled 
water. After cooling to 15° the solution is again carefully brought 
to the mark, well shaken, and then allowed to stand in the stop- 
pered flask until the precipitate has settled. The liquid is then 
poured through a dry filter and the filtrate caught in a flask filled 
with carbon dioxide. Of this filtrate, 20 c.c. are taken, acidified 
with 20 c.c. of sulphuric acid (1:4), treated with 10 ¢.c. of man- | 
ganese sulphate solution,§ diluted to 200 c.c., and treated with 
tenth-normal permanganate solution. 





* Farm. Tidskrift, 1880, 225. 

t Z. anal. Chem., 41, 710 (1902). 

t A coarse powder is not decomposed quantitatively. 
§ See page 607. 


612 VOLUMETRIC ANALYSIS. 


The Ferric Chloride Method.* 


Principle.—A neutral solution of ferric chloride dissolves 
metallic iron with the formation of ferrous chloride: 


Fe+ 2FeCl, =3FeCl, 


and the ferrous chloride formed is titrated with permanganate 
solution. One-third of the iron thys found corresponds to the 
weight of metallic iron present in the sample. 

Procedure.—About 0.5 g. of ferrum reductum are placed in a 
100 c.c. graduated flask, which has been filled with CO,, and 50 c.c. 
of ferric chloride are added (1 gm. anhydrous ferric chloride in 
20 c.c. water).¢ The flask is stoppered and its contents 
frequently shaken during the next fifteen or twenty minutes. 
The solution is then brought to the mark with cold, boiled water, 
mixed, the flask stoppered, and allowed to stand over night. Of 
the clear supernatant liquid, 20 c.c. are removed by a pipette and 
titrated, as in the previous method, with tenth-normal perman- 
ganate solution. t 


2. Determination of Manganese. Method of Volhard.§ 


M 3 
1000 c.c. N. KMn0,|| = siete a nabde = 16.48 gms. Mn. 
10 10 
If an almost boiling, slightly acid solution of manganese sul- 


phate is slowly treated with a solution of potassium permanga- 





* A. Christensen, Z. anal. Chem., 44, 535 (1905). 

+ The ferric chloride must give a clear solution in cold water. As it 
often contains a little ferrous chloride, a blank test must be made and a 
correction, corresponding to the amount of iron found, applied to the analysis 
proper. 

{ For other methods of analyzing ferrum reductum, see E. Schmidt, 
Chem. Ztg , 21,700 (1897). A. Marquardt, /bid., 45, 743 (1901). L. Wolfrum, 
Inaug. Dissert., Erlanger, 1896. F. Férster and V. Herold, z. Elektrochem. 
1910, 461. 

§ Ann. d. Chem. und Pharm., 198, 318. 

|| Strictly speaking, the normality of the permanganate is different when 
used to oxidize manganese in slightlv acid or neutral solution. In this case 


DETERMINATION OF MANGANESE. 613 


nate, each drop will cause the formation of manganous acid 
(H2MnOs3), which is formed under certain conditions, as described 
below, according to the following scheme: 


K:20 : Mn207 + 3Mn0O =K20 + 5MnOg 
VU -—_~ 
2KMnO,4 
According to this equation, therefore, 2KMnO, will oxidize 
3 gm.-atoms of manganese, and as 1000 c.c. of N. KMnO, con- 
tain + gm.-mol. KMnO,, evidently this amount of permanganate 
corresponds to a = 16.48 gms. Mn. 
A. Guyard, who first determined manganese by this method, 
assumed that the oxidation took place according to the following 
equation: 


2K MnO, + 3MnSO4 + 7H2O =e 2KHSO4 a HeSO4 + 5HeMnOs. 


In reality, however, the reaction does not take place in this 
way, but instead of pure manganous acid being precipitated, dif- 
ferent acid manganites of varying composition are formed; e.g., 


OH 
MnZ0 
4KMn0O,+11MnS0,4+14H,0 =4KHS04+7H»S0,+5 Yo Mn. 
MnO 
OH 


Volhard has shown that if calcium, barium, or, better still, zine 
salts, are present, manganites of these metals are precipitated 





the manganese of the permanganate is reduced to the quadrivalent form instead 
of to bivalent manganese, so that a normal solution would now contain 
KMn0O, KMn0O, 

3 5 
ardize permanganate as outlined on pages 597-603, we shall understand by 
normal KMnO,, a solution containing one-fifth of the molecular weight. 
—([Translator.] 


gms. instead of gms. Inasmuch as it is customary to stand- 


614 VOLUMETRIC ANALYSIS. 


The precipitate, although varying in composition, contains all 
of the manganese in the quadrivalent form; e.g., 
/ou 
Mn=0O 


4KMn0,-+ 5ZnSO,-+ 6MnSO,+14H,0= . 
MnZO 
\OH 
In case iron is present, the reaction does not take place quan- 
titatively in the direction from left to right, so that a different 
procedure is then necessary. 


(A) PROCEDURE WHEN IRON IS ABSENT. 


Requirements.—1. A x potassium permanganate solution. 


Zz. A manganese sulphate solution, obtained by dissolving 4.530 
gms. of anhydrous manganous sulphate in one liter of solution: 
lc.c. of this solution=1 c.c. of KMn0O,.* 

3. A zine sulphate solution obtained by dissolving 200 gms. 

zinc sulphate in one liter of water. 
4, Zine oxide suspended in water, obtained by precipitating 
pure zinc sulphate by means of caustic potash solution in such a 
way that the solution does not react alkaline. The residue is 
washed several times with hot water, then transferred to a tightly- 
stoppered bottle, and kept suspended in water. 


Standardization of the Permanganate Solution. 


20 c.c. of the manganese sulphate solution are placed in an 
Erlenmeyer flask, 40 c.c. of zine sulphate solution and 2 or 3 drops 
of nitric acid ¢ are added, after which the mixture is diluted to 





* Strictly speaking, this solution is 4, normal. By definition, a x solu- 


of manganese sulphate contains = 7.550 gms. MnSO, in oneliter. Such 
a solution, however, would not be equivalent to a KMnO, solution which is 
tenth normal in acid solution. Cf. foot-note to page 527 (Translator). 

+ The addition of the nitric acid causes the precipitate to settle much 


more quickly. 





DETERMINATION OF MANGANESE IN STEEL. 615 


200 c.c., heated to boiling, and treated with potassium permanga- 
nate solution, added with constant shaking, until the supernatant 
liquid remains a permanent pink. 


Titration of Manganese. 


If a neutral solution of manganese sulphate is to be analyzed, 
the same procedure is used as in the above standardization. — If 
the solution contains manganous chloride, it should be freed from 
hydrochloric acid by evaporation with an excess of sulphuric 
acid. The acid solution thus obtained is neutralized with the 
zinc oxide until a little of the latter remains suspended in the 
liquid. From this point the procedure is the same as before. . 


(B) PROCEDURE WHEN IRON IS PRESENT. 


If a hydrochloric acid solution is to be analyzed containing ail 
of the iron in the ferric form, it is evaporated to dryness with the 
addition of sulphuric acid, the dry mass is moistened with nitric 
acid and warmed until complete solution is effected. The greater 
part of the acid is neutralized with sodium hydroxide solution, 
the solution placed in a measuring-flask, and an excess of the zinc 
oxide is added whereby all of the iron is precipitated as hydroxide. 
The liquid is diluted up to the mark with water, filtered through a 
dry filter,and an aliquot part of the filtrate is titrated as before 
with potassium permanganate solution.* 


Determination of Manganese in Steel. 
(a) Volhard Method. 


The solution is prepared for titration by dissolving the steel 
borings + in nitric acid (sp. gr. 1.2), evaporating the solution, after 
the addition of 20 ¢.c. of 50 per cent. sulphuric acid, allowing the 
residue to cool, and then adding 150 c.c. of cold water. The 
water is boiled until the ferric sulphate is all dissolved, the solu- 
tion filtered, the filtrate nearly neutralized with sodium carbonate 
and the zinc oxide added exactly as described above. 





* The first few cubie centimeters of the filtrate should be discarded, for 
the dry filter absorbs some of the dissolved substance. 

j Three gms. of steel are used when the manganese content is about 
1 per cent, less when the content is higher. The process is not well suited 
for low manganese steels. 


616 VOLUMETRIC ANALYSIS. 


(b) The Bismuthate Method. 


This method originated with Schneider,t who used bismuth 
tetroxide as the oxidizing agent, but as the oxide is difficult to 
prepare free from chlorides and traces of chloride interfere with the 
end point of the titration, it was abandoned by Reddrop ‘and 
Ramage,t who proposed the use of sodium bismuthate, NaBiOs. 
The product sold under this namé is of more or less indefinite 
composition. It may be prepared by heating 20 parts of caustic 
soda nearly to redness in an iron or nickel crucible, adding in 
small quantities from time to time ten parts of dry basic bismuth 
nitrate, followed by two parts of sodium peroxide, pouring the 
yellow fused mass on an iron plate to cool. When cold, the 
fusion is extracted with water, collected on an asbestos filter, 
washed five times by decantation with water, and dried in the 
hot closet at 110°. After grinding and sifting the product is ready 
for use. 

The process is based on the fact that a manganous salt in 
the presence of an excess of nitric acid is oxidized to perman- 
ganic acid by sodium bismuthate. The permanganic acid 
formed is very stable in nitric acid of 1.135 sp. gr. when the 
solution is cold, but in hot solutions the excess of bismuthate 
is rapidly decomposed and then the nitric acid reacts with the 
permanganic acid; as soon as a small amount of manganous 
salt is formed the remainder of the permanganic acid is decom- 
posed, manganous nitrate dissolves and manganese dioxide 
precipitates. 

In the cold, however, the excess of the bismuth salt may 
be filtered off and to the clear filtrate an excess of ferrous sul- 
phate added; the excess of the latter is determined by titrating © 
with permanganate. The end-reactions are very sharp and the 
method is extremely accurate. | 


1000 c.c. N. KMnO4,=10.99 gms. Mn. 





* A. A. Blair, J. Am. Chem. Soc. 26, 793. 
+ Ding. poly. J. 269, 224. 
t Trans. Chem. Soc. 1895, 268. 


DETERMINATION OF MANGANESE IN STEEL. 617 


Procedure for Steels—Dissolve 1 gm. of drillings in 50 e.c. 
of nitric acid (sp. gr. 1.135 *) in an Erlenmeyer flask of 200 c.c. 
capacity, cool, and add about 0.5 gm. of bismuthate. The 
bismuthate may be measured in a small spoon and experience 
will soon enable the operator to judge of the amount with sufficient 
accuracy. Heat for a few minutes or until the pink color has | 
disappeared, with or without the precipitation of manganese 
dioxide. If the solution now shows precipitated manganese 
dioxide, add crystals of ferrous sulphate free from manganese, 
sulphurous acid or sodium thiosulphate until it becomes clear. 
Heat for two minutes to remove oxides of nitrogen and cool to 
about 15°. Now add 2 or 3 gms. more of sodium bismuthate 
and agitate the contents of the flask for several minutes. Dilute 
with 50 c.c. of 3 per cent. nitric acid and filter through an asbestos 
filter, using gentle suction. Wash the asbestos with 50 to 100 
c.c. of cold 3 per cent. nitric acid.t Run into this solution 50 
c.c. of standardized ferrous sulphate solution { and titrate back 
to pink color with potassium permanganate. § 

The value of the ferrous sulphate solution in terms of potas- 
sium permanganate must be determined in the following manner: 

Measure into a 250-c.c. Erlenmeyer flask 50 ¢.c. of cold nitric 
acid (sp. gr. 1.13), add about 0.5 gm. of sodium bismuthate, 
agitate, dilute with 50 c.c. of 3 per cent nitric acid and filter 
through asbestos. To the filtrate add 50 c.c. of ferrous sulphate 
solution and titrate with permanganate solution to pink color. 

Having determined the value of the permanganate solution 
in terms of ferrous sulphate, the manganese in the sample is 
represented by the difference between the amounts of perman- 
ganate solution actually used in the determination and in the 





* Concentrated nitric acid mixed with three times as much water. This 
is often called 25 per cent nitric acid (by volume). It contains 22.5 per cent 
nitric acid by weight. 

+ 30 c.c. HNOs, sp. gr. 1.42, in one liter of water. 

t An approximately 0.03 N solution made by dissolving 9 gms. crys- 
tallized ferrous sulphate, FeSO,-7H:O or 12 gms. of ferrous ammonium 
sulphate, FeSQ,-(NH,)280,-6H20 in 950 c.c. water and 50 c.c. of concen- — 
trated sulphuric acid. 

§ 1 gm. KMnQ, to the liter. 


618 VOLUMETRIC ANALYSIS. 


titration of a volume of ferrous sulphate equivalent to that used 
in the determination. 

Pig Iron. — Dissolve 1 gm. in 25 c.c. of nitric acid (sp. gr. 
1.135) in a small beaker and as soon as the action has ceased 
filter on a 7-cm. filter into a 200-c.c. Erlenmeyer flask, wash with 
30 c.c. of the same acid as proceed as in the case of steels. 

In the analysis of white irons it may be necessary to treat the 
solution several times with bismuthate to destroy the combined 
earbon, The so!ution, when cold, should be nearly colorless; if 
not, another treatment with bismuthate is necessary. 

Iron Ores Containing Less than Two Per Cent. of Manganese.— 
Treat 1 gm. in a platinum dish or crucible with 4 c.c. of strong 
sulphuric acid, 10 ¢.c. of water and 10 to 20 c.c. of hydrofluoric 
acid. Evaporate until the sulphuric acid fumes freely. Cool 
and dissolve in 25 c.c. of 1.135 nitric acid. If no appreciable 
residue remains, transfer to a 200-c.c. Erlenmeyer flask, using 
25 c.c. of 1.135 nitric acid to rinse the dish or crucible and proceed 
as usual. If there is an appreciable residue, filter on a small 
filter into a beaker, wash with water, burn the filter and residue 
in a crucible and fuse with a small amount of potassium bisul- 
phate. Dissolve in water with the addition of a little nitric acid, 
add to the main filtrate, evaporate nearly to dryness, take up 
in 1.135 nitric acid and transfer to the flask as before. 

Manganese Ores and Iron Ores High in Manganese.—Treat 
1 gm. as in the ease of iron ores, using a little sulphurous acid, 
if necessary. Transfer the solution to a 500-c.c. flask, dilute to 
the mark, mix thoroughly and measure into a flask from a care- 
fully calibrated pipette such a volume of the solution as will give 
from 1 to 2 per cent. of manganese and enough strong nitric 
acid (sp. gr. 1.4) to yield a mixture of 1.135 acid in a volume 
of 50 to 60 c.e. 

Ferro-manganese.—Treat 1 gm. exactly like steel. Dilute 
to 500 or 1000 c.c. and proceed as in manganese ores. 

Ferro-silicon.—Treat 1 gm. with sulphuric and hydrofluoric 
acids and proceed as with iron ores. 

Special Steels,—Steels containing chromium offer no special 
difficulties, except that it must be noted that while in hot solu- 
tions the chromium is oxidized to chromic acid, which is reduced 
by the addition of sulphurous acid, the oxidation proceeds so 


DETERMINATION OF| MANGANESE IN STEEL | 619 


slowly in cold solutions that if there is no delay in the filtration 
and titration the results are not affected. Steels containing 
tungsten are sometimes troublesome on account of the necessity 
for getting rid of the tungstic acid. Those that decompose 
readily in nitric acid may be filtered and the filtrate treated like 
pig iron, but when it is necessary to use hydrochloric acid it is 
best to treat with aqua regia, evaporate to dryness, redissolve in 
hydrochloric acid, add a few drops of nitric acid, dilute, boil, 
and filter. Get rid of every trace of hydrochloric acid by repeated 
evaporations with nitric acid and proceed as with an ordinary 
steel. : 
(c) Williams Method.* 
1000 ec.e. - KMn0y= =o 2.747 gms. Mn.» » 

Principle.—If a nitric acid solution of a manganous salt is 
heated with potassium chlorate, all of the manganese is pre- 
cipitated as the dioxide: 


Mn(NOs3)2+ 2KC103 + H2O0 = MnOg-H2.0+2KNO3 + 2Cl0e. 


The MnOz is dissolved in a measured volume of acid ferrous sul- 
phate, and the excess is titrated with tenth-normal permanganate. 

Procedure.—From 2 to 3 gms. of an ordinary steel, about 1 
em. of “‘spiegel”’ or 0.3 to 0.5 gm. of ferromanganese are weighed 
into a 600 c.c. Erlenmeyer flask and dissolved in 60 ¢.c. of nitric 
acid, sp. gr. 1.2. To prevent loss by spattering, a small funnel 
is placed in the neck of the flask. After evaporating the solu- 
tion to a volume of about 15 c.c., 50 c.c. of concentrated nitric 
acid, sp. gr. 1.42, and 3 gms. of solid potassium chlorate are added, 
and the solution is boiled for fifteen minutes. It is then removed 
from the source of heat and the treatment with 50 c.c. concentrated 
nitric acid and 3 gms. of potassium chlorate is repeated, after 
which the solution is boiled for fifteen minutes longer. The 
solution is cooled quickly by placing the flask in cold water, and 
the precipitated manganese dioxide is filtered on asbestos, 

* Trans. Inst. Min. Eng., 10, 100. See also W. Hampe, Chem. Ztg., 7, 
73 (1883), 9, 1478 (1885), and Ukena, Stahl und Eisen, 11, 373 (1891). 


+ A satisfactory filter is obtained by placing a little glass wool in a funnel, 
and on this a little asbestos, such as is used for Gooch crucibles. 





620 VOLUMETRIC ANALYSIS. 


washed with concentrated nitric acid till free from iron, and with 
water till free from acid. The asbestos pad and precipitate is 
transferred to the original flask, covered with 50 c.c. of standard- 
ized ferrous sulphate solution,* and diluted with water to a 
volume of 200 c.c. The contents of the flask are shaken with 
glass beads until all the precipitate is dissolved, and the solution 
is then titrated with tenth-normal permanganate. 
The amount of manganese pres®nt is computed as follows: 


50 c.c. of ferrous sulphate solution require T e.c. = KMn0O,4 


50 c.c. of ferrous sulphate solution +a g. of iron require t c.c. 


N ikMnO,. 
10 


| N 
Consequently a g. of the substance =(7'—1?) ¢.c. rT KMn0O4 
and 


0.002747(7 —t) x 100 
a 


(d) G. v. Knorre’s Persulphate Method.} 


Principle.—If a solution of manganous sulphate containing 
a little free sulphuric acid is treated with ammonium persulphate, 
the manganese is precipitated quantitatively as hydrated man- 
ganese dioxide. | 





=% Mn. 


MnSO,4 + (NH,4) 29208 + 3H2O = MnO, > H2O + (NH4) 204 + 2H2504 


and the latter can be estimated as in the above determination. 
Procedure.—In the case of the harder alloys of iron and 
manganese, the sample is pulverized as much as possible in a steel 
mortar. The weights of sample taken correspond to those recom- 
mended for the previous determination. The weighed substance 
is treated in a beaker with sulphuric acid (1:10) at the boiling 
temperature, using 50 c.c. of the dilute acid for the harder alloys 
and 60 c.c. for the softer ones. As soon as the evolution of 


*10 gm. FeSO,-7H20, 50 ¢.c. concentrated HeSO,4, and 950 c.c. water. 
+ Z. angew. Chem., 14, 1149 (1901). 





ADDITIONAL METHODS FOR DETERMINING MANGANESE, 621 


hydrogen ceases, the solution is filtered through a small filter 
which is washed with cold water until the washings give no test 
for iron with potassium ferricyanide. Frequently, especially in 
the case of ferro-manganese rich in silicon, the insoluble residue 
still contains a little manganese, so that for an accurate analysis 
Ledebur ignites the filter and precipitate in a platinum crucible, 
treats the residue with hydrofluoric acid and about 0.5 e.c. of 
concentrated sulphuric acid, and evaporates in an air bath until 
sulphuric anhydride vapors are evolved. After cooling the 
contents of the crucible are added to the main solution. Then 
from 150 to 250 c.c. of ammonium persulphate solution are added 
(60 gms. per liter) the solution diluted to about 300 c.c. and heated 
to boiling. After boiling for fifteen minutes, the precipitate is 
allowed to settle and is filtered, washed, treated with an excess of 
ferrous sulphate solution and titrated exactly as in the previous 
Williams method. 


3. Determination of Uranium. Method of Belhoubek,* 
Zimmermann,}{ Hillebrand.{ 


1000 c.c. N. KMn0O, = = 28" = 119.3 ems. U. 


This method is saccade: suited for testing the purity of a 
precipitate of U,0; obtained in the analysis of uranium minerals. 
It is based upon the fact that when U,O, is heated in a closed tube 
with dilute sulphuric acid at 150° to 175° C. it is readily decom- 
posed according to the equation 


U,0;+4H,SO,=2U0,S80,+ U(SO,),+4H,0, 


forming uranyl and uranous sulphates. The latter compound 
is oxidized to the former by means of potassium escent 


2KMnO,+ 5U(SO,),+ 2H,0= 
—2KHSO,-++ 2MnSO,-+ H,SO,+5U0,80,. 





* Journ. f. prakt. Chem., 99, 231. 
7 Ann. der Chem. u. Pharm., 232, 285. 
t U.S. Geol. Survey, No. 78, 90 (1889). 


622 VOLUMETRIC ANALYSIS. 


From this equation it follows that 2 gm.-mols. of KMnO, are equiva- 
lent to 5 gm.-atoms of uranium, and 1000 c.c. N. KMnO, solution 
(=1KMn0O,) =4 gm.-atom of uranium = —Se si 119.25 gms. U. 

Procedure.—The weighed amount of U,O, is placed in a tube 
closed at one end, 10 to 15 c.c. of dilute sulphuric acid (1:6) are 
added, and the open end of the tube is made narrower by heating 
in a blast-lamp and drawing it*out somewhat. The air in the 
tube is removed by inserting a long capillary so that it reaches to 
the bottom of the tube containing the substance, and conducting 
a current of carbon dioxide through it; the larger tube is finally 
sealed without removing the capillary. The tube is then heated in 
a ‘‘bomb furnace” at 150-175° C. until everything has dissolved 
to a clear green liquid. After cooling, the tube is opened by 
making a scratch with a file and touching it with a hot glass rod. 
The contents are poured into a large porcelain dish, diluted with 
distilled water to 500-700 c.c., and titrated with a KMn0O, solu- 
tion until a permanent pink color is obtained. 


l cc. x KMnO,= 0.011925 gm. U =0.013525 gm. UOg oxidized.* 


Remark.—The above method gives very exact results. 


4. Determination of Oxalic Acid. 


C,0,-2H,O 126.05 
2 aa 





1000 c.c. N. KMn0,= 2] — 63.02 gms. H,C,0,-2H;0. 

The procedure is exactly the same as was described under 
the standardization of permanganate by means of oxalic acid 
(page 598) 





* It must be remembered, however, that only one-third of the total uranium 
in U,O, has been oxidized by the KMnO, (U;O,=2U0,+ UO,). Consequently, 


with regard to the total uranium, 1e.c. X KMnO,—0.03578 gm. U—0.04218 
gm. U,O,.—{Translator]. 


ANALYSIS OF RED LEAD. 623 


5. Determination of Calcium. 


Ca 40.09 
1000 c.c. N. KMnO=- = 





= 20.05 gms. Ca. 


The calcium is precipitated as described on p. 70 in the formn 
of its oxalate, filtered, and washed with hot water. The still moist 
precipitate is transferred to a beaker by means of a stream of 
water from the wash-bottle, and the part remaining on the filter is 
removed by allowing warm dilute sulphuric acid to pass through 
it several times. To the turbid solution in the beaker, 20 c.c. of 
sulphuric acid (1:1) are added, and after dilution with hot water 
to a volume of from 300 to 400 ¢.c. the oxalic acid is titrated with 
N 


i0 KMn0, solution. 


1 €.¢: as KMnO,=0.002005 gm. Ca. 


6. Determination of Pho, in Minium [Red Lead, Pb,O,]. Method 


of Lux.* 
239.1 
1000 c.c..N. KMnO ae =~ =119.55 gms. PbO, 


Principle.—If lead peroxide (PbO,) is treated with oxalic acid 
in acid solution, the latter is oxidized according to the following 
equation: 

PbO,+ HC,O, = PbO+ H,0-+ 2CO,. 

If the decomposition takes place with a measured amount of 
- titrated oxalic acid solution and the excess of the latter is titrated 
by means of potassium permanganate solution, the difference 
shows the amount of oxalic acid necessary to effect the reduction 
of the lead peroxide. 

_ Procedure.—About 0.25 gm. of minium (red lead) is weighed 
into a porcelain dish and heated with 20 to 30 c.c. of double normal 
nitric acid.f The original oxide is thereby changed into soluble 
lead nitrate and brown, insoluble HoPbO3: 


Pb304+ 4HNO3 = 2Pb(NO3)2+ H20+ HoPbOs. 


After solution is effected, 50 cc. > oxalic acid are added, the 





* 7. anal. Chem., 19, p. 153. 
t Nitric acid sp. gr. 1.2 diluted with two volumes of water. 


624 VOLUMTRIC ANALYSIS. 


solution is heated to boiling, and titrated hot with 2 KMnO,. 


it % 0.0. KMn0O, solution were used, then 50—# c.c. af H.C.O, 


were necessary for the reduction of the amount of PbO, contained 
in the minium (a gm.) taken for analysis. 


Since 1000 c.c. N. HeC204= 119.55 gms. PbOs, then 1000 ee. 





H,C204= at?? =23.91 gms. PbOg and 1 c.c.=0.02391 gm. PbOs. 
Consequently | 

(50—f) c.c. ~ oxalic acid correspond to (50—1t) 0.02391 gm. 
PbOsz. 


The per cent. of the latter is 
a: (50—t) X 0.02391 = 100:2 


7p 00— X2.391 
r* 





per cent. PbOs.* 


7. Determination of MnO, in Pyrolusite. 


MnO, 86.93 
Bein. 





1000 c.c. N. KMn0O,= = 43.47 gms. MnQ,. 


(a) Method of Levol and Poggiale, Modified by G. Lunge.t 


After drying at 100° to constant weight, 1.0866 gms. of the 
finely powdered pyrolusite are placed in a 250 c.c. flask which is 
provided with a Contat valve (see page 602). The air is expelled 
by conducting CO, into the flask, and then 75 c.c. of the ferrous 
sulphate solution, prepared as described below, are added, the 
flask closed, and its contents heated over a small flame until there 
is no longer any dark-colored residue. The flask is cooled quickly, 
the contents diluted with 200 c.c. water, and the excess of ferrous 
sulphate titrated with 0.5N KMn0O, solution. Immediately be- 
fore the analysis, the titer of the ferro-sulphate solution is deter- 
mined by taking 25 c.c. of it, diluting to 200 c.c. and titrating with 
permanganate. . 





* To express the results in per cent. Pb3;O4, the number 2.391 should be 
replaced by,6.853.—[Translator.] 
t Chem.-techn. Untersuchungsmethoden. Edition 6, Vol. I., p. 569. 


DETERMINATION OF MnO, IN PYROLUSITE. €25 


By the treatment of the pyrolusite with ferrous sulphate, the 
following reaction takes place: 


MnO, + 2FeSO,+2H,SO,= Fe,(SO,),+ MnSO,+2H,0. 
The computation of the percentage of MnO, is as follows: 


Toe PESO, BOIUTODS. cei is «6 require T c.c. 0.5N KMnO, 
75 c.c FeSO,+ 1.0866 gms. pyrolusite.. “  tc.c.0.5N KMnO, 


.. 1.0866 gms. pyrolusite require T —tc.c.0.5N KMnO, | 





~ corresponding to (T —t) X0.02173 gm. MnO, and in percentage 


(T —t) X0.02173 x 100 
1.0866 





=2(T —t)% Mn0O,,. 


The ferrous sulphate solution is prepared as follows: 200 c.c. 
of concentrated sulphuric acid are slowly poured, with stirring, 
into 500 c.c. of water and while the mixture is still hot 100 gms. 
of powdered FeSO4-7H20O erystals are added; on stirring, solution 
should take place within a few minutes. The solution is finally 
diluted to one liter, and when cold is ready for use. 


(6) The Oxalic Acid Method of Fresenius-Will, Modified by Mohr.* 
About 0.4 gm. of finely powdered pyrolusite, which has been 


dried at 100°, is heated on the water-bath with 50 c.c. x oxalic 


acid and 20 c.c. sulphuric acid (1:4) until no more black particles 
remain undissolved. The solution is diluted with 200 c.c. of hot 


water and titrated with s KMn0O, solution. The reaction which 


takes place between the manganese dioxide and the oxalic acid is 
_ expressed by the following equation: 


MnO,+H,S0,+H,C,0,=MnSO,+2CO,+2H,0. 


I ¢.¢; - KMn0O,= 0.0087 gm. MnO,. 


* Fresenius-Will carried out the analysis in an alkalimeter and determined 
the CO, evolved by loss in weight. 





626 VOLUMETRIC ANALYSIS. 


8. Determination of Formic Acid (Lieben).* 


1000 c.c. N. KMnO ee axe. 02 _ 13.80 gms. HCOOH.+ 


In cold acid solutions permanganate reacts only slowly with 
formic acid, while in a hot solution the latter is lost by volatiliza- 
tion, so that the titration in open vessels is impossible; in alkaline 
solutions, on the other hand, the oxidation takes place readily and 
quantitatively in the cold: r 


KMnO, + 3HCOOK id KeCOz + KHCOs + 2MnOe + H,0. 


Procedure.—The formic acid is neutralized by an excess of 
sodium carbonate, and permanganate is run into the hot tf sodium 
formate solution until the clear liquid above the precipitate is 
colored reddish. 





g. Analysis of Nitrous Acid (Lunge). 


HNO, 47.018 
2 





1000 c.c. N. KMnO, = = 23.51 gms. HNO,, 


On account of the volatility of nitrous acid, the aqueous solu- 
tion of the nitrite, or the solution of nitrous acid in concentrated 
sulphuric acid (nitrose), is measured from a burette into a known 
amount of permanganate solution, which has been made acid with 
sulphuric acid, diluted to a volume of about 400 c.c. and warmed to 
40° C. The nitrous acid is thereby oxidized to nitric acid: 


2KMnO,+ 5HNO,+ 3H,SO,= K,SO,+ 2MnSO,+ 3H,0+ 5HNO,, 
and the decolorization of the solution shows the end-point. 


Toward the end the nitrous acid must be added slowly, for the 
change from red to colorless requires some time. 


10. Analysis of Hydrogen Peroxide. 


1000 c.c. N. KMn0, = O; 4-17. 01 gms. H,0,. 


Ten cubic centimeters of pevaiek 3 per cent. hydrogen 
peroxide are placed in a 100-c.c. measuring-flask, diluted up 





* Monatshefte, XIV, p 746, and XVI, p. 219. 

+ In reality, the normal solution of formic acid would contain 4 (not 8, 
the molecular weight. See foot-note to page 612. 

{ The titration 1s made in hot solution because the manganous acid 
formed does not settle well from a cold solution. % 


ANALYS!S OF HYDROGEN P&ROXIDE. 627 


to the mark with water, and, after thoroughly mixing, 10 c.c. 
(=1 c.c. of the original solution) are placed in a beaker, and diluted 
with water to a volume of 300 to 400 c.c. After adding 20 to 30 


c.c. of sulphuric acid (1:4), the solution is titrated with 75 2 g KMn0, 


_ until a permanent pink color is obtained. The ag reac 
tion takes place: 


2KMnO,+ 5H,0,+ 4H,SO,=2KHSO,+ 2MnS0,-+ 8H,0-+ 50,,. 


Frequently it happens that the first drop of the permanganate 
causes a permanent coloration of the solution. This shows that 
either not enough sulphuric acid is present, or else there is no more 
_ hydrogen peroxide left in the solution. In this case a little more 
sulphuric acid is added, when if the coloration still remains the 
preparation is surely spoiled, as can be shown by the titanic or 
chromic acid tests (cf. Vol. I, p. 42). 

The amount of hydrogen peroxide is expressed either as per 
cent. by weight or as per cent. by volume. 

Example.—10 ¢.c. of the above-mentioned dilute solution of 
hydrogen peroxide (=1 c.c. of the original solution) required 


17.86 c.c. - KMn0O, solution, corresponding to 


10 
17.86 X 0.001701 =0.03038 gm. H,0,. 


As the specific gravity of the original hydrogen peroxide solution 
can be assumed to be 1, it therefore contains 3.04 per cent. H,0,. 

When expressed in “per cent. by volume” the result shows 
how many cubic centimeters of oxygen can be obtained from 
100 ¢.c. of the solution. 

In this case 100 c.c. of the hydrogen peroxide solution con- 
tain 3.04 gms. of H,O, and, on being decomposed, 1 gm.-mol. H,O, 
sets free 1 gm.-at. O: 

H,0,= H,O + O 
34.02 = 18.02+ 16, 
or 11195 ¢.c. of oxygen at 0° C. and 760 mm. pressure; conse- 
quently 3. Us gms. HO, will evolve 
. 34.02 :11195 =3.04:2 
ae 1000 ¢.c. oxygen measured under standard con: 


ditions of temperature and pressure. 


628 VOLUMETRIC ANALYSIS. 


100 c.c. of the commercial hydrogen peroxide, therefore, will 
evolve 1000 c.c. of oxygen, i.e., ten times its own volume. This 
is somewhat anomalously designated as hydrogen peroxide of 10 
per cent. by volume. 


100 c.c. 3 per cent. hydrogen peroxide=10 per cent. by volume. 
100 CC. 6 KE ig a cc = 20 cc 66 ia; aa 


100¢ re 9 ‘c a“ sc <9 = 30 (a3 bc ce 66 


11. Analysis of Barium Peroxide. 


BaO, 169.4 
1000 c.c. N. KMn0,= oe - =84.70 gms. BaO:. 





About 0.2 gm. of the substance is weighed into a 400 e.c. 
beaker, covered with 300 c.c. of cold water, and treated, under 
constant stirring, with 20-30 c.c. of hydrochloric acid (1:5). 
When all the BaQOg has dissolved, the solution is titrated with 
0.1 N. KMnO,4. The addition of H2SOx, is not advisable, as the 
precipitated BaSO. is likely to enclose some BaQOez which will 
then escape the titration. 

Another method for the analysis of BaOz has been proposed 
by Kassner.* 


12. Analysis of Potassium Percarbonate. 
1000 c.c. N. KMn0,="" == =99.10 gms. K2C20.¢. 

0.25 gm. potassium percarbonate is weighed out into 300 c.c. 
of cold, dilute sulphuric acid (1:30), in which it dissolves with 





* Arch. Pharm., 228, 432. 


ANALYSIS OF PERSULPHATES. 629 


~ violent evolution of carbon dioxide and formation of an equiva- 
lent amount of hydrogen peroxide: 


K2C20¢g+2H2804= 2KHS04+2C02+ H202, 


and the latter is titrated with potassium permanganate. 


13. Analysis of Persulphates (Persulphuric Acid, H2S20s). 


97.08 gms. H2S.0s 
111.4 ‘ (NH4)2S8.0; 
1385.2 “ K.S.0, 


1000 c.c. N. KMnO,= R805 


A solution of persulphuric acid does not reduce permanga- 
nate, nor does it react with titanic acid; on the other hand it oxi- 
dizes ferrous salts immediately in the cold to ferric salts, and by 
means of this behavior it can be easily determined. The ammo- 
nium and potassium salts are now commercial products, and are 
analyzed as follows: About 0.3 gm. of the salt is weighed out into 
a flask fitted with a Bunsen valve, the air is replaced by carbon 
dioxide, 30 c.c. of a freshly titrated solution of ferrous sulphate 
are added and then 200 c.c. of hot water; the flask is closed and 
its contents rotated. The salt dissolves without difficulty, and 
the ferrous sulphate is oxidized: 


HSo0g-+2FeSO.4=Feo(SO4)3-+H2S0.. 


After all of the salt has dissolved, the contents of the flask 
are cooled by placing the flask in cold water, and the excess of 


ferrous salt is titrated with x KMn0O,.* 





- * The ferrous sulphate must be added to the persulphate, and then the 
hot water. If the hot water is added first, the persulphate is souisse ities 
somewhat and the results obtained will be low. 


630 VOLUMETRIC ANALYSIS. 


In this way it is found that: 


30 c.c. ferrous sulphate solution require T’ c.c. I KMn0O, solution. 


30 c.c. ferrous sulphate+ a gm. persulphate require ¢ ¢.c. 2 KMnO, 


10 
solution. 
Consequently a gm. of persulphate correspond to (7'—t) ¢.c. 
N° 
10 KMn0O,. * 


In the case of the potassium salt, since 1000 c.c. N. KMnQ, 


135.2 oms. K2S20s, and 1 e.c. 7 KMn04=0.01352 em. KoS2Q0xg, 


we have: (7’—t) X0.01352 gm. KeSeO0g in a gm. of the commer- 
cial salt, or in per cent.: 


a: (T7'—1t)0.01352 = 100: 


Ee : ge ts =per cent. K2S2Osg. 





With the ammonium salt the factor becomes 0.01141 instead of 
0.01352. 

The ferrous sulphate necessary for this determination is pre- 
pared by roughly weighing out 30 gms. of crystallized ferrous 
sulphate (FeSO4+7H20O), dissolving it in 900 c.c. of water, 
and diluting to 1000 c.c. with pure concentrated sulphuric 
acid. | 

Persulphates may also be analyzed very satisfactorily by means 
of oxalic acid.* When a sulphuric acid solution of a persulphate 
is treated with oxalic acid alone, there is no perceptible reaction. 
On adding a small amount of silver sulphate as catalyzer, however, 
a lively evolution of carbon dioxide takes place, and at the water- 
bath temperature the reaction is soon completed. 


ae H2S20s = 5 H2C204 = 2H2SO04 + 2COvs. 





* R. Kempf, Ber., 38, 3965 (1905). 


DETERMINATION OF HYDROXYLAMINE. 031 


The excess of the oxalic acid can be titrated with perman- 
ganate. — . 

Procedure.—About 0.5 gm. of the persulphate is placed in a 
400-c.c. Erlenmeyer flask, 50 c.c. of tenth-normal oxalic acid solu- 
tion, and a solution of 0.2 gm. silver sulphate in 20 c.c. of 10 per 
cent. sulphuric acid are added, and the mixture is heated on the 
water bath until the evolution of carbon dioxide ceases; this 
requires not more than 15 or 20 minutes. The solution is then 
diluted to about 100 c.c. with water at about 40° and titrated with 
tenth-normal permanganate. 


14. Determination of Hydroxylamine (Raschig).* 


NH,OH _ 33.03 


1000 ¢.c. N. KMnO,= 
2 2 





=16.52 gms. NH,OH. 


Principle-—Hydroxylamine is oxidized in hot acid solutions 
by means of ferric salts to nitrous oxide and water: 


2NH20H + O2= N20+3H20, 
and an equivalent amount of ferrous salt is formed: 
2NH20H +2Feo (SOx) 3=4FeS0,+2H2S0,+ H.O+ N2O. 


The amount of ferrous salt is determined by titration with 
a potassium permanganate. 

Procedure.—About 0.1 gm, of the hydroxylamine salt is placed 
in a 500-c.c. flask and dissolved in a little water, 30 c.c. of a cold 
saturated solution of ferric-ammonium alum are added, and 10c.c. of 
dilute sulphuric acid (1:4). The contents of the flask are heated 





* Ann. d. Chem. und Pharm., 241, p. 190. 


632 VOLUMETRIC ANALYSIS. 


to boiling and kept at this temperature for five minutes, after 
which the solution is diluted with distilled water to a volume of 
about 300 c.c. and immediately titrated with permanganate 
solution. 

Remark.—If only slightly more than the theoretical amount 
of the ferric salt is added, the oxidation of the hydroxylamine 
does not take place entirely in accordance with the above equa- 
tion, but part of the substance is oxidized to nitric oxide: 


2NH,OH+30=3H,0+2NO, 


so that it is then impossible to obtain exact results, 


15. Determination of Hydroferrocyanic Acid (de Haén).* 
1000 e.c. N. KMnO,=1 mol. K,Fe(CN),= 368.3 gms. K,Fe(CN),. 


Principle—By oxidation in acid solution, hydroferricyanic 
acid is formed from hydroferrocyanic acid: 


2H,Fe(CN),+ O=H,0+ 2H,Fe(CN),. 


This procedure is chiefly used for the analysis of potassium 
ferrocyanide (yellow prussiate of potash), so that the concentra- 
tion of the permanganate solution is expressed in terms of this 
salt. 

Procedure.—0.9 gm. of the salt to be analyzed is dissolved in 
100 c.c. of water, 10 c.c. of dilute sulphuric acid are added, and 
this solution is titrated. in a porcelain dish with permanganate 
until a permanent pink color is obtained. It is not easy 
to determine the end-point. On acidifying, the solution of the 
ferrocyanide becomes milky with a bluish tinge, and on the addi- 
tion of permanganate at first a yellow shade is obtained, after- 
wards becoming green, and finally on the addition of more perman- 
ganate the color changes to pink. On account of the difficulty 
in determining this point, de Haén recommends that the perman- 
ganate be standardized against pure potassium ferrocyanide solu- 
tion (K,Fe(CN),+38H,0). 


* Ann, d. Chem. und Pharm., 90, p. 160. 





DETERMINATION OF HYDROFERRICYANIC ACID, ETC. 633 


16. Determination of Hydroferricyanic Acid. 
1000 c.c. N. KMnO,=1 mol, K,Fe(CN),=329.2 gms. K,Fe(CN),. 


Principle.—The potassium ferricyanide is reduced in alkaline 
solution to potassium ferrocyanide, and the latter is titrated with 
permanganate. 

Procedure.—In a 300-c.c. flask, 6.0 gms. of the ferricyanide are 
dissolved in water, the-solution made alkaline with potassium 
hydroxide, heated to- boiling, and an excess of a concentrated 
ferrous sulphate solution is added. At first yellowish-brown 
ferric hydroxide is precipitated, later black ferrous-ferric hydrox- 
ide is formed, and this shows the completion of the reaction. 
After cooling, the contents of the flask are diluted with water up 
to the mark, filtered through a dry filter (after thoroughly mix- 
ing), and 50 c.c. of the filtrate * (=1 gm. of the substance) are taken 


for the titration with a KMn0, solution. 


17. Determination of Chloric Acid. 


RCIO, _ cay gms. KCIO, 


1000 c.c. N. KMn0O,= 6 17.74 “ NaCio, 





About 5 gms. of potassium chlorate, or 4 gms. of the sodium 
salt, are dissolved in water, and the solution diluted to 1 liter. 
After thoroughly mixing, 10 c.c. are placed in a flask fitted with a 
Bunsen valve and the air expelled from the flask by a current 
of carbon dioxide. After this 50 c.c. of a freshly-standardized 
solution of ferrous sulphate (prepared as described on p. 630) 
are added, and the solution boiled ten minutes. The following 
reaction takes place: ) 


KCIO,+ 6FeSO,+ 3H,S0, = KCl+ 3Fe,(SO,),+ 3H,0. 


After cooling the solution is diluted with cold distilled water, 
10 c.c. of manganous sulphate solution are added (cf. p. 607), 
and the excess of the ferrous sulphate is titrated with potassium . 
permanganate. We find that: 





* The first ten or fifteen cubic centimeters of the filtrate should be 
discarded. 


634 VOLUMETRIC ANALYSIS. 


50 c.c. ferrous sulphate...... FO Oe required T' ¢.c. a KMn0O, sol. 
506.0. “ 1410 ¢.c. chlorate sol. “ —t ce. “ = 2 





10 c.c. chlorate solution =7— aa gm. substance = (7'—2)c.c. bd Toh Mn0o," 


For the analysis of potassium chlorate a gm. of ie nice 
contain (7'—t)x 0.2044 gm. KCIO,, and the per cent. present is 
20.44 x (T'—t) 

nie ; 

The calculation for sodium chlorate is analogous. 





= per cent. 


18. Determination of Nitric Acid (Pelouze-Fresenius). 


21.01 gms. HNO 
RNO, -| 28.34 “ NaNO, 
33.70 “ KNO, 


This method depends upon the fact that on heating a nitrate 
in the presence of considerable hydrochloric acid and ferrous 
chloride the latter is oxidized to ferric chloride and the nitric acid 
is reduced to nitric oxide: 

2KNO,+ 6FeCl,+ 8SHC]=2KCl+ 2NO+ 4H,0+ 6FeCl,. 

As a measure for the amount of nitrate reduced ¥ we have: 

1. The excess of ferrous salt. 

2. The ferric salt produced. 

3. The nitric oxide formed. 


1000 c.c. N. KMnO,= 





The method of Schlésing-Grandeau described on p. 456 is 
based upon the measurement of the nitric oxide formed. C. D. 
Braun * estimates the amount of ferric salt formed, while Pelouze 
and Fresenius determine the amount of ferrous salt not used up 
in the reduction of the nitric acid. 

Procedure.—A weighed amount of iron wire (about 1.5 gms.) 
is placed in a long-necked flask, and the air expelled by passing 
a current of pure carbon dioxide through it for two or three 
minutes. After this 30 to 40 c.c. of pure, concentrated hydro- 
chloric acid are added and the flask is placed in an inclined posi- 
tion and closed by means of a rubber stopper through which 
tubes pass so that a current of carbon dioxide can be conducted 





* Journ. f. prakt. Chem., 81 (1860), p. 421. 


DETERMINATION OF NITRIC ACID. 635 


through the flask. The solution is heated on the water-bath 
in this atmosphere of carbon dioxide until the iron has com- 
pletely dissolved, when the solution is allowed to cool in a cur- 
rent of the gas. Meanwhile about 0.25 to 0.3 gm. of the nitrate 
is weighed out in a small glass tube closed at one end; this is 
thrown into the acid solution of the ferrous sulphate and the flask 
quickly closed again. The flask is then once more placed in its 
inclined position upon the water-bath and heated for fifteen 
minutes, while the current of carbon dioxide is continually 
passed through it. The tube through which the gas leaves 
the flask, during the whole operation, dips into a beaker filled 
with water so that there is no chance of any air getting back into 
the flask. After this the solution is heated to boiling and kept 
there until its dark color disappears and the yellow color of the 
ferric chloride becomes apparent. In order to make sure that the 
nitric oxide is entirely removed, the contents of the flask are boiled 
five minutes longer and then allowed to cool in the atmosphere 
of carbon dioxide. When cold the solution is poured into a 
beaker, the flask washed out with a little boiled water, the solu- 
tion is diluted to a volume of about 400 to 500 e.c., 10 c.c. of 


manganese sulphate solution are added, and the unoxidized iron 


is titrated with e KMn0O, solution. 


The amount of pure iron present in the wire used is deter- 
mined under the same conditions as prevailed during the pre- 
vious operation, using a smaller portion of wire but the same 
amount of acid, manganese sulphate, etc. 

The calculation is as follows: 

If a gm. of potassium nitrate and p gm. of the wire were taken for 


the analysis, t c.c. of 2 KMn0O, were required to oxidize the excess 


of iron, and further p gm. of the wire require 7’ c.c. of > KMn0O, 
solution, we have, then: z 





WOM, WOR... FET Cee ee een require 7’ c.c. > KMn0O, solution 
-p gm, iron +a gm. saltpeter...... Coe RSS = BMa):. 
and a gm. saltpeter = (T —t) c.c. A KMn0O,, 


2 


636 VOLUMETRIC ANALYSIS. 


so that a gm. of saltpeter contain (7’—t) x0.01685 gm. KNOs, 
and in per cent. 
(T —t) X 1.685 
a 

Remark.—This method gives results just as accurate as those 
obtained by the method of Devarda, but the latter determination 
is much easier to carry out. 

The determination becomes simpler if the contents of the 
iron wire is assumed to be 99.7 per cent. Fe and the second titra- 
tion thus done away with. It does not take long to make the 
analysis of the wire, however, and it is advisable to do it. Instead 
of titrating the excess of the ferrous salt with potassium perman- 
ganate solution, a solution of potassium dichromate may be used. 
For the determination of the ferric salt formed, cf. p. 681. 





=per cent. KNO,.* 


19. Determination of Vanadium. 


1000 c.c. a FCM, a ae LE =9.12 gms. V,O,. 

Sulphur dioxide is conducted into the boiling solution of an 
alkali vanadate containing sulphuric acid until the solution 
appears a pure blue; by this means the vanadic acid is reduced 
to vanady] salt: 

V,0,+ SO, =SO,+ V,0,. 

The boiling is continued and a current of carbon dioxide is 
passed through the solution until the escaping gas will no longer 
decolorize a solution of potassium permanganate, showing that 
the excess of the sulphur dioxide has been expelled. The hot 
solution is then titrated with potassium permanganate until a 
permanent pink color is obtained. The end-point is easily recog- 
nized only when the solution is hot. This accurate determina- 
tion is used for the analysis of vanadium in iron and steel, or in 
ores. (Cf. p. 310.) 


* Of course the calculation can be made from the amount of iron oxi- 
ized. In that case: 





Fe : 4KNO,=(p—tX0.02793) :z 
(p —tX 0.02793) -KNO, 
r= 


3Fe 





gms. KNO, in a gm. of substance, 


and in per cent. 
100(p —t xX 0.02793) KNO, 


3Fe-a 





=per cent KNO,. 


DETERMINING PHOSPHORUS IN IRON AND STEEL, 637 


20. Blair Method for Determining Phosphorus in Iron and Steel.* 


Principle.—The substance is dissolved in nitric acid, all 
carbonaceous matter is destroyed by the action of strong per- 
manganate solution, any precipitated manganese dioxide is 
































(Te % 
~‘S 


Fig. 91. 








th 


redisssolved, and the phosphorus is precipitated in slightly acid 
solution as ammonium phosphomolybdate. The precipitate is 
dissolved in ammonia, the solution acidified with sulphuric acid 





* Andrew Blair. The Chemical Analysis of Iron, 


638 | VOLUMETRIC ANALYSIS. 


and the molybdenum reduced by means of a so-called Jones 
reductor. The reduced solution is titrated with perman- 
ganate. 

The Jones reductor (Fig. 91) is made by placing a platinum 
spiral (or glass beads) in the bottom of a glass-stoppered tube 
which is 30 cm. long and has an inside diameter of 18 mm. Upon 
the spiral, or beads, is placed a plug of glass wool and then a thin 
layer of asbestos such as is used for*Gooch crucibles. The tube is 
then filled with amalgamated zine to within 5 cm. of the top. 
This zine can be prepared by taking some 20- to 30-mesh zine, 
cleaning it with a little hydrochloric acid, and adding mercuric 
chloride until hydrogen ceases to be evolved. In this condition 
the zine is scarcely acted upon at all by hydrochloric acid, but is 
capable of reducing an iron or molybdenum solution just as 
effectively as if it were not amalgamated. On top of the column 
of zinc is placed a little glass wool to serve as filter. 

Procedure.—A 2 gm.-sample is taken in the case of steels and 
1 gm. in the case of cast irons. The metal is weighed into a 250- 
c.c. Erlenmeyer flask and dissolved in 100 c.c. of nitric acid (sp. gr. 
1.13) which is prepared by mixing one volume of nitric acid 
(sp. gr. 1.42) with 3 volumes of water and then testing the gravity. 
A small funnel is placed in the neck of the flask and the solution 
heated until all the iron has dissolved and the nitrous fumes 
expelled. Ten c.c. of strong permanganate solution (15 gms. to 
the liter) are added and the boiling continued until the pink color 
of the permanganate disappears. The slight precipitate of 
manganese dioxide is dissolved by the addition of a little sodium 
sulphite solution. After filtering, 40 -c.c. of ammonia (sp. gr. 
0.96) are added, the solution is brought to a temperature of about | 
40° and treated with 40 c.c. of a freshly-prepared solution of. 
ammonium molybdate.* The flask is then closed with a 





* 100 gms. of pure molybdic acid (MoO,) is stirred into 400 c.c. of cold 
distilled water, and 80 c.c. of concentrated ammonia added. The solution is 
filtered, and the filtrate slowly poured, with constant stirring, into a solution 
of 400 ¢c.c. nitric acid (sp. gr. 1.42) in 600 ¢.c. of water. After the addition 
ef 0.05-gm: of microcosmic salt, the solution is allowed to stand 24 hours 
and is then filtered. 


DETERMINING PHOSPHORUS IN IRON AND STEEL. 639 


solid rubber stopper and shaken vigorously for five minutes. 
After allowing the precipitate to settle for a few minutes, it is 
filtered and washed promptly with acid ammonium sulphate . 
solution (1000 ¢.c. water, 25 c.c. concentrated H2SOx, and 15 c.c. 
strong ammonia), until the washings give no test for molybdenum 
when treated with a drop of yellow ammonium sulphide solution. 
The color obtained is compared with a similar amount of the wash 
water itself which has been treated with the same ammonium 
sulphide. | 

The ammonium phosphomolybdate precipitate is dissolved 
in a mixture of 5 ¢.c. concentrated ammonia (sp. gr. 0.90) and 20 
e.c. of water, the filter washed with water and the filtrate treated 
with 10 c.c. of concentrated sulphuric acid. It is then run through 
the Jones reductor. <A blank is run with the reductor before each 
series of determinations, using the same quantity of reagents. 
After a reductor has stood for some time, it should be well 
washed with dilute sulphuric acid, before even running a blank 
test. : 

In making blanks and in all determinations, the procedure is 
as follows: 100 c.c. of dilute sulphuric acid (25 c.c. concentrated 
acid to 1 liter of water) are run into the funnel, B, and the stop- 
cock C is opened, using a little suction. When only a little of the 
dilute acid remains in the funnel, the hot solution to be reduced is 
added and when this has nearly passed out of the funnel, it is 
followed by 250 c.c. of hot dilute sulphuric acid, washing out the 
original beaker with this acid and adding it in small portions. 
Finally 100 c.c. of water are passed through the reductor. At no 
time, however, should any air be allowed to enter, as it forms 
hydrogen peroxide and vitiates the result. 

The reduced solution is titrated with tenth-normal per- 
manganate. 

The ammonium phosphomolybdate precipitate, (NH4)3PO4- 
12Mo00s3, contains 12 molecules of MoO3 to 1 atom of phosphorus. 
Although zine reduces MoO3 to Mo2O3, there is a slight oxidation 
by the air in the flask, so that correct results are obtained by 
assuming a reduction to Mozg4037 and subsequent oxidation by the 
permanganate to MoO 3 again. Therefore during the titration 
the following reaction takes place: _ 


640 VOLUMETRIC ANALYSIS, — 


Mo24037 +350 = 24Mo003 
and 
1P = 12Mo003=35H. 


To illustrate the computation, let it be assumed that the 
ammonium phosphomolybdate precipitate from 2 gms. of a sample 
of steel requires by the above metltod 12 c.c. of permanganate 
solution, of which 1 c.c.=0.00392 gm. Fe. The blank on the 
reductor was 0.18 c.c. The phosphorus present in the steel is 
then: 


(12—0.18) x 0.00392 x P x 100 
 35Fe X 2 





=per cent. phosphorus. 


Remarks.—The Jones reductor may be used to advantage for 
reducing sulphuric acid iron solutions which are to be titrated 
with permanganate. The blank experiment must always be 
made, as the zinc invariably contains a little iron. If in the above 
determination the reductor tube is prolonged so that it reaches 
nearly to the bottom of the flask, and dips into 50 c.c. of ferric 
alum solution (100 gms. ferric alum, 25 c.c. concentrated H2SO4 
and 40 c.c. glacial H3PO4) the molybdenum comes in contact with 
this solution while it is entirely reduced to MogO3._ The ferric alum 
at once oxidizes the Mo203 to MoOgs and an equivalent amount of 
iron is reduced to the ferrous condition. The titration with 
permanganate can then be carried out, and in the computation 
the molecular weight of Mo2O3 is used instead of Mo24037 as above 
and 1P=36H. The blank determination should be carried out 
with the ferric alum solution in the flask. 

Concordant results can be obtained by both methods, but the 
latter has the advantage that there is no danger of some of the 
molybdenum being oxidized while shaking the flask during the 
titration. | 

In the case of steels containing tungsten and vanadium, the 


phosphorus may be left in the residue insoluble in nitric acid. 


POTASSiUM DICHROMATE METHODS. 641 


B. Potassium Dichromate Methods. 
Determination of Iron according to the Method of Penny. 
1000 c.c. N. K,Cr,O0,=1 gm.-at. Fe =55.85 gms Fe. 


Principle.—If a solution of a ferrous salt, in either hydrochloric 
or sulphuric acid, is treated with an alkali chromate solution, the 
chromate is at once reduced in the cold and the ferrous salt is 
oxidized quantitatively: 


K,Cr,0,+ 6FeSO,+ 8H,SO,=2KHSO,+ 
+ Cr,(SO,),+3Fe,(SO,),+ 7H,O 
or. 
K,Cr,0,-+ 6FeCl,+ 14HCl=2KCl+ 2CrCl, + 6F eC], +7H,0. 


On account of the formation of the chromic salt the solution 
becomes emerald-green in color. 

The end-point of the reaction is determined by removing a 
drop of the solution and testing it with a freshly-prepared solu- 
tion of potassium ferricyanide; if no blue coloration is formed, 
the ferrous salt has been completely oxidized. 


The . potassium dichromate solution necessary for this titra- 


1 

tion may be prepared: by dissolving ALHOr — 4.903 ems. of the 
salt, purified as described on p. 36, and dried at 130° C. It is not 
advisable to remove the last traces of moisture by melting 
_ the salt, for, either by overheating or by means of the dust of the 
air, there is some reduction of the chromate, so that subsequently 
a turbid solution will be obtained, containing small amounts of 
suspended Cr,O,. 

Method of Titration.—To the acid solution of the ferrous salt 
contained in a beaker (with about 0.1 to 0.15 gm. iron in each 


190 c.c.) the solution of 59 a! ok 2Cr,O, is added, preferably from a 


glass-stoppered burette. 

From time to time a drop of the solution is removed on the end 
of a glass stirring-rod to a white porcelain plate, and placed beside 
a drop of a not more than 2 per cent. solution of potassium ferri- 


642 VOLUMETRIC ANALYSIS. 


* cyanide.* By means of a stirring-rod one solution is made to run 
into the other. If considerable ferrous salt remains in the solu- 
tion, the blue color will be formed immediately, but in propor- 
tion as the ferrous salt is replaced by ferric salt, a bluish-green 
color is obtained, perceptible at the junction of the two solutions. 
As soon as no more bluish-green coloration is to be detected the 
reaction is complete. In all cases the analysis is made in dupli- 
cate, and, other things being equal, the second determination 
should be the more accurate. This time it is possible to add 
almost the whole of the required amount of bichromate at once, 
and for the testing not more than two or three drops of the dilute 
solution of ferrous salt. The loss of ferrous solution will then be 
inappreciable. 

Remark.—The dichromate method is slightly less accurate 
than the permanganate method, but it possesses the advantage 
that a solution of a ferrous salt containing hydrochloric acid can 
be titrated without the addition of manganese sulphate, even 
when the solution is turbid with suspended insoluble salts, fibres » 
of filter-paper, etc. In turbid solutions it is difficult to recognize 
the permanganate end-point. A further advantage lies in the 
fact that the normal dichromate solution can be readily pre- 
pared by simply weighing out the required amount of the pure, 
dry salt, and diluting the aqueous solution to a volume of 1 liter. 
It is then unnecessary to test the concentration in any other way. 


Determination of Manganese in Iron and Steel. Method of 
J. Pattinson.7 


Principle.—If a solution containing iron, manganese, and cal- 
cium salts is treated with “chloride of lime” solution, all of the 1ron 
and manganese are precipitated, the latter in the form of its 
hydrated dioxide. The whole precipitate is dissolved in an acid 
ferrous sulphate solution of known strength, and the excess of the 
latter is titrated with dichromate solution: 





* The potassium ferricyanide must be absolutely free from ferrocyanide; 
and as the former is readily reduced by the dust of the air, the surface of 
the salt should be washed off several times with water before dissolving it 
for the test solution. 

+ Journ. of the Chem. Soc. (1879), p. 365. 


DETERMINATION OF MANGANESE IN IRON AND STEEL. 643 


MnO,+ 2FeO = Fe,O,+ MnO. 


1000 c.c. N. K,Cr,0, = 1Fe=* = aes = 27.47 gms. Mn. 


Procedure.—5 gms. of the iron or steel (or 1 gm. of ferromanga- 
nese) are dissolved in hydrochloric acid, the solution oxidized with 
nitric acid, evaporated to a small volume, poured in a 100-c.c. 
measuring-flask, and diluted up to the mark with water. After 
thoroughiy mixing, 20 c.c. of the solution are placed in a large 
beaker (of about 1 liter capacity) and neutralized with pure cal- 
cium carbonate. The carbonate is added in small portions until 
the solution finally becomes a dark brown but still remains clear. 
After this 50 c.c. of “chloride of lime”’ solution* are added, and 
more calcium carbonate with constant stirring until finally a little 
of the latter remains undissolved. To the slimy contents of the 
beaker 700 c.c. of hot water are added, and after stirring, the in- 
soluble residue is allowed to settle, which requires but two or three 
minutes. If the supernatant liquid is violet, on account of the 
. formation of calcium permanganate, one or two drops of alco- 
hol are added, the liquid boiled, and the precipitate again allowed 
to settle; in this case the upper liquid should be colorless. If, per- 
chance, it should be still colored, the treatment with the alcohol 
must be repeated. The clear solution is then decanted through 
a filter which is placed in a funnel containing a platinum cone and 
conneeted with a suction flask. The precipitate is decanted with 
300 c.c. of hot water four times, then transferred to the filter 
without making any attempt to remove the last portions of the 
precipitate from the sides of the beaker, and washed with the 
aid of suction until the filtrate will no longer turn iodo-starch 
paper blue. ‘The precipitate together with the filter is then placed 
in the original beaker in which the precipitation took place, 50 c.c. 
of a freshly-standardized ferrous sulphate solution containing sul- 
phuric acid are added, and the liquid is stirred until the precipitate 
has entirely dissolved,t leaving behind the filter-paper and some- 
times small amounts of undissolved calcium sulphate. The ex- 





* Prepared by shaking 15 gms. of fresh b!eaching powder with 1 liter of water. 
and allowing the mixture to stand until the supernatant solution is clear. 

+ If the precipitate should not completely dissolve, a little sulpharic 
acid (1:1) is added until the brown color entirely disappears. 


‘ 


644 VOLUMETRIC ANALYSIS. 


cess of the ferrous sulphate is titrated with. potassium dichro- 
mate solution. In order to compensate any error that may arise 
from the presence of the filter-paper, an equally large filter is 
placed in the ferrous sulphate solution, when it is standardized. 
The calculation is simple: 
Assume that a gms. of steel are dissolved in 100 c.c. of the 


solution and of this 20 e.c. (= 5 ems. steel) were taken for the 


analysis; further, 50 c.c. of ferrous sulphate solution=T7' c.c, 
= K,Cr,O, and 50 cc. ferrous sulphate+ = gms. substance=f c¢.c, 


N a N 

(ieee Consequently gms. substance = (7'— 1?) 0.0.) KaCraO7. 
Since 1000 c.c. N. KeCr207 solution =27.47 gms. Mn, then 1 c.c. 

is 

~ KaCr207 will correspond to 0.002747 gm. Mn, and we have 


a 


(T—t) X0.002747 gm. Mn in = gms. steel and in per cent. 


(T—t) X1.374 
a 
Remark.—According to the author’s experience, this method 
is one of the best for the determination of manganese in iron and 
steel. As regards the time required, four determinations can be 
carried out together within four hours. It is not particularly 
suited to the analysis of alloys rich in manganese. | 





=per cent. Mn. 


C. Iodimetry. 

The fundamental reaction of iodimetry is the following: 

2Na28203 +I, =2Nal+ Nag2S40g. 

If to a solution containing an unknown amount of ‘iodine a 
little starch solution is added, and sodium thiosulphate solution 
is run in from a burette, the blue color will disappear from the 
solution as soon as the iodine has all been reduced to hydriodie 
acid (sodium iodide) in accordance with the above equation. This 
reaction is one cf the most sensitive reactions used in analytical 
chemistry. If, therefore, a sodium thiosulphate solution of known 
strength is at hand, we have a means of determining not only iodine 
itself, but all of those substances (oxidizing agents) which when 
treated with potassium iodide set free iodine, or evolve chlorine 
when acted upon by hydrochloric acid. Consequently, iodimetric 
processes are not only accurate but capable of most general appli- 


PREPARATION OF SODIUM THIOSULPHATE SOLUTION. 645 


cation. For most analyses a oa sodium thiosulphate solution and 


a . iodine solution are required, and starch solution as indica- 


tor. In some few cases a solutions are used. 


Preparation of Sodium Thiosulphate Solution. 


From the above equation it is evident that 1 gm.-at. I=1 gm.- 
mol. Na,S,0,=1 gm.-at. H. Hence, exactly ; gm.-mol. of crys- 
tallized sodium thiosulphate (Na,$,0,+5H,O) must be taken for 1 
liter of tenth-normal solution. Such a solution, however, would 
rapidly change in concentration, some of the salt being decomposed 
by the action of the carbon dioxide in the distilled water: 


1. Na,S,0,+2H,CO,=2NaHCO,+ H,8,0,, 
9 418.0,—HS0,+8. 


and the solution would become stronger, for the sulphurous acid 
formed reacts with more iodine than the corresponding amount 
of thiosulphate: 


H,S0,+ 21+ H,O =2HI+ H,S0,. 


Ajter all the carbonic acid in the distilled water has been used up, 
the solution can be kept for months without suffering an appreciable 
change in concentration (see p. 649). 

A large amount of the thiosulphate solution (about 5 liters) 
is prepared by roughly weighing out the required amount of the 
commercial salt * and after standing for from eight to fourteen 
days, the solution is standardized by one of the following methods. 


Standardization of Sodium Thiosulphate Solution. 


1. With Pure Iodine. 


Commercial iodine is contaminated with chlorine, bromine, 
water, and sometimes cyanogen; it must be purified. For this 





* The molecular weight of Na,S,O,+5H,O is 248.32. To prepare 1 liter 


of = solution 24.832 gms. of the salt are necessary, or, in round numbers, 


25¢gms_ For 5 liters, 125 gms. should be weighed out. 


646 VOLUMETRIC ANALYSIS. 


purpose 5 or 6 gms. of the commercial product are ground up Wivs 
2 gms. potassium iodide, and any chlorine or bromine present 
forms with this potassium chloride or bromide, setting free an 
equivalent amount of iodine. The mixture is placed in a dry 
beaker, B (Fig. 92), of about 300 c.c. capacity and upon the beaker 
is placed the bulb-tube K, which is closed at one 
end. The latter is filled with water at the room 
temperature and the glass is surrouhded with an 
asbestos cylinder (not shown in the illustration). 
The beaker is then placed on wire gauze and heated 
over a small flame. The iodine sublimes rapidly 
and collects as a crystalline crust on the bottom 
of the bulb-tube, and practically none of it is lost. 
As soon as the evolution of violet vapors from the — 
bottom of the beaker has practically ceased, the sublimation is 
complete. The flame is removed, and after allowing to cool, the bulb- 
tube K is removed with the iodine adhering to it. In order to remove 
the latter, a current of cold water is conducted through the tube a 
into the bulb and out at b. This causes the glass to contract 
somewhat. and the whole of the iodine crust can: be removed by 
lightly pushing it with a clean glass rod. It is caught upon a 
watch-glass, broken up into large pieces, and the sublimation is 
repeated without the addition of potassium iodide at as low a tem- 
perature as possible: in this way a product free from potassium 
iodide is obtained. The iodine thus prepared is ground somewhat 
in an agate mortar and dried in a desiccator containing calcium 
chloride. If dried over sulphuric acid, some of the latter is likely 
to be present in the iodine. Furthermore, the cover of the desicca- 
tor must not be greased, for grease is attacked by iodine vapors, 
forming hydriodic acid, which might cause contamination. 

The Weighing Out of the Iodine—In each of two or three 
smal! weighing-tubes with tightly-fitting glass stoppers are placed 
2 to 24 gms. of pure potassium iodide free from iodate and } e.c.. 
of water (not more), the tubes are stoppered and accurately 
weighed by the method of swings. The tubes are then opened, 
0.4—-0.5 gm. of pure iodine is added to each, the tubes are quickly 
_stoppered and again weighed; the difference shows the amount 
of iodine. The iodine dissolves almost instantly in the concen- 





STANDARDIZATION OF SODIUM TH:OSULPHATE SOLUTION. 647 


trated potassium iodide solution. One of the tubes is then placed 
in the neck of a 500-c.c. Erlenmeyer flask which is held in an 
inclined position and contains 200 c.c. of water and about 1 gm. 
of potassium iodide. The tube is dropped to the bottom of the 
flask, but just as it begins to fall the stopper is removed and allowed 
to follow it. In this way there is no iodine lost, which will be 
the case if the contents of a tube are washed into the water.* A 
solution is thus prepared: containing a known amount of iodine 
and to it the sodium thiosulphate solution to be standardized 
is added from a Mohr burette until the liquid is pale yellow 
in color. Now, 2 or 3 ce. of starch solution are added and the 
solution carefully titrated until it becomes colorless. From the 
mean of two or three determinations, the strength of the thio- 
sulphate solution is calculated. For example, it was found that 


(a) 0.5839 gm. iodine required 50.07 c.c. Na,S,O, solution, 
or 1 c.c.==9.011661 gm. iodine. 





(b) 0.5774gm. “ 49.42 ¢.c. Na,S,O, solution, 
or 1 c.c.=0.011683 gm. iodine. 
The mean value is 1 c.c.=0.011672 gm. iodine. 


Tf this number is divided hy the amount of iodine which would 
be contained in 1 c.c. of normal iodine solution, the normality 
of the sodium thiosulphate solution will be obtained. Thus, in 

0.011672 


this case the solution is 0.012688 = 0-0920! normal. 


2. With Potassium Biiodute (C. Than).t 


If a solution of potassium biiodate is added tu a solution of 
potassium iodide containing hydrochloric acid, the following 
reaction takes place: 


KI0,-H1O,+10K1+11HC1=11KC1+6H,O+ 121 


eee | aa 
389.95 , 1523.0 
389.95 5 ‘: 
If, therefore, 3.2496 gms Gene 120 *) of pure potassium biiodate 


are contained in one liter of the aqueous solution, 10 ¢.c. of such a 





= ~ * Wagner first called attention to this fact, and it mee been confirmed in 
the author’s laboratory. 
{ Zeitschr. f. anal. Chem., XVI (1877), p. 477. 


648 VOLUMETRIC ANALYSIS. 


solution on being treated with an excess of potassium iodide and hy- 
drochlorie acid will set free exactly as much iodine as would be con- 
tained in 10 c.c. of x iodine solution. By means of such a solution 
a known amount of iodine may be obtained at any time and in 
this way the solution of sodium thiosulphate may be standardized. 
At presens it is possible to obtain commercially very pure potas- 
sium biiodate, but the product is Seldom pure enough for the 


preparation of a a solution. It is better to prepare a solution 


by weighing out 3.2496 ems. for 1 liter and determining the 
concentration accurately by titrating it against a solution of 
thiosulphate which has been freshly standardized against pure 
iodine. In this way a solution is obtained which can be con- 
veniently used from time to time for testing the concentration 
of the thiosulphate solution. 

Method ef Titrating—One or two grams of pure potassium 
iodide are placed in a beaker, dissolved in as little water as possible, 
and to this 5 c.c. of hydrochloric acid (1 : 5), and then 20-25 c.c. 
of the biiodate solution are added (never in the reverse order). 
Iodine is liberated, immediately and quantitatively. After dilut- 
ing with about 200 c.c. of distilled water, the iodine is titrated as 
under 1. 


3. With Potassium Permanganate (Volhard) * 


On adding potassium permanganate solution to an acid solu- 
tion containing potassium iodide, the permanganate is reduced 
to manganous salt, while an equivalent amount of iodine is set 
free from the iodide: 


2KMnO,+ 10KI + 16HCl=12KCl+2MnCl, + 8H,0+ 101. 


If an accurately-standardized solution of potassium perman- 
ganate is at hand, it can, therefore, be used advantageously for 
the standardization of the sodium thiosulphate solution. The pro- 
cedure is the same as was described with the potassium biiodate 
solution. 





* Ann. d. Chem. u. Pharm., 242, p. 98, 


’ 


PERMANENCE OF A SODIUM THIOSULPHATE SOLUTION. 649° 


4. With Potassium Dichromate. 


Similarly, an acid solution of potassium iodide (1:10) will, 
in the cold, quantitatively reduce chromic acid to green chromic 
salt, setting free an equivalent amount of iodine :>* 


K,Cr,0,+ 6KI+ 14HCl= 8KCl+ 2CrCl, + 7H,0 + 61. 


By weighing out 4.903 gms. of pure, dry potassium dichro- 
mate a tenth-normal solution is prepared and a measured amount 
of it is added to the acid solution containing about 3 gms. ‘of 
potassium iodide and 10 c.c. strong hydrochloric acid. In 
this case, however, the solution is diluted with 500-600 e.c. of 
water, for here the color change is not from blue to colorless but 
from blue to light green.f| With too concentrated solutions the end- 
point is indistinct, so that a considerable dilution is necessary. 


Permanence of . Sodium Thiosulphate Solutions. 


A two-months-old sodium thiosulphate solution was stand- 
ardized against pure iodine in June, 1899, and its concentration 


found to be 
1 c.c.=0.011672 gm. I. 


In March, 1900, or about eight months later, the same solution 
of thiosulphate was again standardized and found to be 


1 c.c. =0.011667. 


At the end of eight months, therefore, the concentration of 
the solution was practically unchanged. Frequently the addition 
of ammonium carbonate is recommended in order to obtain a more 
permanent solution; it has the opposite effect. 


Preparation of = Iodine Solution. 


There is no advantage to be obtained by dissolving the theo- 
retical amount of sublimed iodine in a definite volume of solution, 





* The solution should be quite acid with hydrochloric acid. In very dilute 
sulphuric acid solutions the chromic acid is reduced very slowly if at all. 
+ In all these methods starch solution is added toward the end of the reac- 


tion. See p. 647. 


650 VOLUMETRIC ANALYSIS. 


for the latter cannot be kept very long unchanged. It is more 
practical to prepare the iodine solution by placing 20-25 gms. of 
pure potassium iodide in a liter flask dissolving it in as little water 
as possible and then adding about 12.7 gms. of commercial iodine, 
weighed out roughly on a watch-glass. The contents of the flask — 
are shaken until the iodine is all dissolved. When this is accom- 
plished, the solution is diluted up to the mark with water and 
standardized according to one of thedollowing methods. 


1. With af Sodium Thiosulphate Solution. 


Of the thoroughly mixed iodine solution, 25 ¢.c. are titrated 
with the standard sodium thiosulphate solution. 


r 


If 25 ¢.c. of iodine solution require 25.16 c.c. of * Na,S,0, 


solution, 1 c.c. of the former=1.0064 c.c. of a solution, or, in 


other words, the solution is 0.10064 normal. 


2. With a Arsenious Acid. 


If iodine is allowed to act upon a solution of arsenious acid 

the reaction which takes place may be expressed as follows: 
H3AsO3+1.+ H20@ H3As04+4+2H 1, 

This reaction can be made to go completely in either directions 

according to the conditions.* 

If the hydriodic acid is immediately removed from the solu- 
tion as fast as it is formed, the reaction will proceed quantita- 
tively in the direction from left to right, In the presence of 
sufficient hydrochloric acid, however, the reaction will take 
place completely in the opposite direction. When it is desired 
_ to make the oxidation of the arsenious acid quantitative, there- 
fore, there should be but very little hydrogen ions in solution 
at any time; in other words, the solution must remain as nearly 
neutral as possible. The presence of free alkali is not permis- 
sible because any appreciable concentration of the hydroxyl 
ion reacts with iodine to form iodide, hypoiodite, and eventually 
iodate. | = 

Alkali bicarbonates are without action upon iodine, so that 





*. W. Washburn, J. Am. Chem. Soc., 30, 21 (1908). 


PREPARATION OF A STANDARD IODINE SOLUTION 651 


sodium bicarbonate is used for the neutralization of the hydriodie 


acid formed by the above reaction. 


Siew her ee As,O, 198 
From the equation, it is evident that 1 gm.-at. I= ait Sok en 


49.5 gms. As,O,,. and = om.-at. I corresponds, therefore, to 4.95 


gms. As,O,=the amount necessary for 1000 c.c. of solution. 


For the preparation of the a arsenious acid solution, the 
vitreous form of commercial As,O, is sublimed from a porcelain dish 
upon a watch-glass. If arsenic trisulphide is present (shown by a 
yellow sublimate being first formed) the preparation must be previ- 
ously purified. For this purpose it is dissolved in hot hydrochloric 
acid (1:2), the insoluble sulphide filtered off, and the arsenic 
trioxide caused to deposit by cooling the filtrate. After pour- 
ing off the mother-liquor, the crystals are washed several times 
with water, dried on the water-bath, and the pure substance 
‘obtained by sublimation. After standing for twelve hours in 
a desiccator over calcium chloride, 4.95 gms. of the oxide are 
accurately weighed out into a porcelain dish and dissolved by 
warming with a little concentrated sodium: hydroxide solution. 
After two or three minutes all will be dissolved. The solution is 
now poured through a funnel into a graduated liter flask, and 
the dish carefully washed out with water. A drop of phenol- 
phthalein is added to the contents of the flask and pure dilute 
sulphuric acid until the solution is decolorized. About 20 gms, 
of sodium bicarbonate are dissolved in 500 e.c. of water and the 
filtered solution is added to the barely-acid contents of the flask. 
If the mixture reacts alkaline (shown by the red color of the phe- 
nolphthalein), a few drops of sulphuric acid are added until it be- 
comes colorless, after which the solution is diluted up to the mark 
with water. After thoroughly mixing, a burette is filled with it and 
titrated against a measured amount of iodine solution as under 1. 


3. With Anhydrous Sodium Thiosulphate.* 


Anhydrous sodium thiosulphate may be prepared in a state of 
sufficient purity to permit its use for standardizing iodine solutions. 
A saturated solution of the commercial salt is prepared at 30° to 





*S. W. Young, J. Am. Chem. Soc., 26, 1028 (1904). 


652 VOLUMETRIC ANALYSIS. 


35° and then cooled while stirring constantly. The salt thus 
obtained is dehydrated over sulphuric acid until it has fallen to a 
powder, and a little of it in a test-tube shows no sign of fusion 
when heated to 50°. The final dehydration is effected by 
heating at 80° with repeated stirring of the powder. 

Young standardized a solution of iodine by this method and 
obtained the same value as by titrating against a thiosulphate 
solution which had been standardized against pure iodine. 


The Starch Solution. 

About 5 gms. of powdered starch are rubbed into a paste with 
a little cold water, and the paste is slowly added to a liter of boiling 
water contained in a porcelain dish. The boiling is continued 
for one or two minutes so that an almost clear solution is obtained. 
The liquid is cooled by placing the dish in cold water, and after 
standing overnight the clear liquid is filtered into small 50-c.e. 
medicine bottles. These are placed in a water-bath and filled 
up to the neck with the starch solution, heated two hours, and 
closed by means of soft stoppers before removing from the hot- 
water bath. The solution thus sterilized can be kept almost 
indefinitely without the slightest trace of mould formation. Such 
a solution prepared according to the above directions by H. N. 
Stokes remained perfectly clear after standing 14 years and was 
as sensitive then as when first made up. After opening the bottle, 
mould begins to form within one week, which explains why the 
solution is poured into small bottles; it may then be used before 
it becomes spoiled. 

It is nowadays much more convenient to use the Zulkowsky 
“soluble starch,’’ which is obtained commercially in the form 
of a paste. The reagent is prepared by dissolving a little of the 
paste in cold water. 


Sensitiveness of the Iodo-Starch Reaction. 


As already mentioned in Vol. I, p. 267, iodine produces a 
blue color with starch only when hydriodic acid or a soluble iodide 
is present, and further the formation of the blue color depends 
not only upon the presence of iodide but is largely influenced 
by the concentration of the iodide solution. With the same 
amount of iodide and different volumes of liquid quite different 


SENSITIVENESS OF THE IODO-STARCH REACTION. 653 


amounts of iodine are necessary to produce the blue color. From 
this it is evident that in any iodimetric analysis about the same 
concentration should be maintained as in the case of the stant 
ardization of the solutions used for the analysis. When + solutions 
are used, the error produced by not following this rule is a small 
one and for most purposes can be neglected. On the other hand, 
| N 


when an analysis is made with 700 solutions, a large error may 


be introduced. © 3 
‘fo show what the error can amount to, the following results 
will be given. To each of the following amounts of water, 1.5 c¢.c. 


of starch solution were added and then Ee iodine solution until 


a barely-visible coloration was obtained. 


c.c. Water. a3 Todine Solution. 
Ne reg ee ee ee ees 0.15 ¢.¢ 
MUR re ee eke saa hss pao 
ae he eg ee ee re ia oss (ae 
DR es te vs Oo gb eka he ke be 0.64 * 


These experiments were repeated using 3 ¢.c. of the starch 
solution with almost the same results. But when to each 1 
gm. of potassium iodide was added, the following results were 
obtained : 


Water. a Iodine Solution, 
Ne SES 0 6 i reer ge eee ere ene 0.04 e.c. 
Tig Ces bseekecse 0.04 “ 
bE Aiea is 8 Sh aA a 0.04 
DO MR eee fe we ie ig SOAS: O.14 * 
SO ob eee eae Bai G32“ 
DUO. POR sates oe ewes Gs Ogee" 
EU 3 drain: bo eeenien Saran svar RAS iad 


The results show that the amount of iodine solution necessary 


654 VOLUMETRIC ANALYSIS. 


to produce the blue color in the absence of potassium iodide* is 
directly proportioned to the dilution. If the solution contains 
1 gm. of potassium iodide, a blue color will be produced by the 
same amount of ivdine solution as long as not more than 159 c.c. 
of solution are present, but with a greater volume than that, more 
iodine is necessary independent of whether the solution contains 
1 gm. or more of potassium iodide. 

In order to show the action of “the iodide more distinctly, a 
very dilute iodine solution was added to 50 ¢.c. of water containing 
starch solution and in the absence of ‘iodide, 15 ¢.c. were added 
before the blue color was permanent. After adding 1 gm. of 
potassium iodide, it was only necessary to add 1.5 e.c. of the 
dilute iodine. 

When solutions. were used without the addition of potassium 
iodide, the same amount of iodine solution (0.03 ¢.c.}) was 
necessary when not more than 300 c.c. of water were present. 
With 600 ¢.c. of water, 0.06 c.c. of iodine was necessary, and with 
1000 c.c. it was found that 0.15 ¢.c. of iodine solution was re- 
quired. On the other hand, when the solution contained 1 gm. 
of potassium iodide, only 0.06 c.c. of iodine was necessary in 
1000 e.c. of liquid.t 


ANALYSES BY IODIMETRIC PROCESSES, 
1. Determination of Free Iodine. 


N 
— 1000 c.c. 10 iodine solution = 12.692 gm. I. 


The iodine is dissolved in a solution of potassium iodide. The 
solution is titrated either with sodium thiosulphate or with arse- 
nious acid exactly as described under the standardization of an 
iodine solution. 


2. Determination of Chlorine in Chlorine Water. 


1000 c.c. = iodine solution = 3.546 gm. Cl. 

* With the exception of the potassium iodide contained in the iodine 
solution itself. 

+ 0.03 ¢.c.=1 drop. 

t The temperature of the solution also exerts an influence. Other things 
being equal, the end-point is best obtained in a cold solution._{Translator. } 





DETERMINATION OF HYPOCHLOROUS ACID. 655 


A measured amount of chlorine water is added to a solution 
containing an excess of potassium iodide. The point of the 
pipette should be held just above the surface of the iodide solution 
and the latter should be contained in a glass-stoppered bottle. 
After the chlorine water has been added, the contents of the 
bottle are vigorously shaken, and the iodine set free is titrated 
with sodium thiosuiphate as above: 


KI+Cl=KCI+1. 


3. Determination of Bromine in Bromine Water. 


1000 c.c. as iodine solution =7.992 gm. Br. - 


The procedure is the same as under 2: 
KI+Br=KBr+l. 


4. Determination of Hypochlorous Acid in the Presence of 
Chlorine. © 


The determination is based upon the following reactions: 


HOCI]+2KI-KCl+ KOH +L; 
Cl,+2KI=2KC1+ 1, 


1 gm.-mol. of hypochlorous acid sets free 1 gm.-mol. of iodine, 
but produces at the same time 1 gm.-mol..of potassium hydroxide, 
while the chlorine simply sets free an equivalent amount of iodine. 
After neutralizing the alkali by means of an excess of hydrochloric 
acid and determining the iodine by titration with sodium thio- 
sulphate, the excess of hydrochloric acid is titrated with standard 
alkali solution. 


Procedure-—A measured volume of = hydrochloric acid is 


added to a potassium iodide solution, to this a known amount 
of the mixture of chlorine and hypochlorous acid is added, and the 


iodine set free is titrated with = thiosulphate solution. The now 


colorless solution is treated with methyl crange and the excess of 


, hydrochloric acid is titrated with NaOH. The KOH produced 


by the action of the hypochlorous acid tpon the iodide requires 


656 VOLUMETRIC ANALYSIS. 


half as much - acid for neutralization as are required of = Na,S,0, 


solution to react with the iodine set free by the action of the hypo- 
chlorous acid. : 
Example.—lf V c.c. of chlorine+hypochlorous acid were taken 


for analysis, ¢ c¢.c. ~ HCl present at the start, 7 cc. ~ Na,8,0, 


po, sites N eect 
used for titrating the iodine, and ?¢, Cc. To NaOH for titrating the 


r 


excess of acid, then t—i, ¢.c. 10 acid were required to neutralize 


the potassium hydroxide and 2(t—#,) c¢.c. . Na,S,0, to react 


with the iodine formed from the hypochlorite. 
Hence (t—t,) 0.005247 *=gm. HOC! in V e.e. solution 


and 
T —2(t—t,) 0.003546 = gm. Clin V e.c. solution. 


5. Determination of Iodine in Soluble Iodides.} 


(a) By Decomposition with Ferric Salts. 


If a solution of a soluble iodide is treated with an excess of 
iron-ammonium alum and acidified with sulphuric acid, the 
ferric salt will be reduced to ferrous salt with separation of iodine: 


Fe,(SO,), + 2HI=H,SO,+2Fe80, +L, 


If the solution is heated to boiling, the iodine escapes with 
the steam and can be collected in a solution of potassium iodide 
and then titrated with sodium thiosulphate or arsenious acid. 
This method is suited for separating iodine from bromine, for 
bromides do not reduce ferric salts. The bromide will be found 
in the residue obtained after the distillation, and is best deter- 
mined gravimetrically. 





N 
* HOCI= 52.47; 1 c.c. +h solution=0.005247 gm. HOC] (against NaOH). 


+ In the case of insoluble iodides, the metal must first be removed if the 
iodine is to be determined volumetrically. This can be accomplished by 
the method of Mensel (Z. anal. Chem., 12, 137). It may be said, however, 
that the volumetric method offers no advantages over the gravimetric one. 


DETERMINATION OF IODINE IN SOLUBLE JIODIDES. 657 


(b) By Decomposition with Nitrous Acid (Fresenius). 


This excellent method, which is especially suited for deter- 
mining small amounts of iodine in the presence of bromine and 
chlorine in mineral waters, depends upon the easy oxidation of 
hydriodic acid by means of nitrous acid: 


2HI +2HNO,=2H,0+2NO0+42I. 


Hydrochloric and hydrobromic acids are not attacked by 
nitrous acid. 

Procedure.—In the small apparatus shown in Fig. 93 the 
neutral or slightly alkaline solution of the iodide is placed; it is 
slightly acidified with dilute sulphuric acid, and 
a little freshly-distilled, colorless carbon bisulphide 
(or chloroform) is added, so that it does not quite 
reach to the stop-cock, near the bottom of the tube. 
Then two, or at the most three, drops of “nitrose’’* 
are added, the tube stoppered and vigorously 
shaken, after which the carbon bisulphide is 
allowed to settle once more. The small amount 
of the latter which at first adheres to the glass 
sides is made to run to the bottom by revolving 
and inclining the tube. On the upper surface of 
the liquid there will still remain a few tiny drops 
of carbon bisulphide. To obtain these a funnel 
containing a filter-moistened with water is placed 
under the glass stop-cock, the stopper is removed 
from the tube and the aqueous solution is 
allowed to run through the filter, but the carbon 
bisulphide will remain behind on the paper. 
The carbon bisulphide remaining in the tube is 
shaken three times with successive portions of 
distilled water, and each time the latter is allowed 
to run off through the same filter. The funnel is 
then placed at the top of the tube, punctured with 

Fie 93. a pointed glass rod, and the carbon bisulphide 
washed into the tube by means of about 0.5 ¢.c. of water. After 


1 * Cf. Vol. I, p. 285. 








658 VOLUMETRIC ANALYSIS. 


this one or two drops of sodium bicarbonate solution are added 
and thoroughly shaken with the carbon bisulphide, then standard 
sodium thiosulphate solution is added until the reddish-violet 
carbon bisulphide solution becomes colorless. 

The value of the sodium thiosulphate solution is not determined 
as ordinarily, but by means of a potassium iodide soluticn treated 
as above described. 

Remark.—This method is useful for determining small amounts 
of iodine in the presence of relatively large amounts of chlorine 
and bromine, as in the analysis of mineral waters. For the stand- 
ardization of the sodium thiosulphate solution, as nearly as possible 
the same amount of potassium iodide is used as is present in the 
unknown solution; this is determined by the color of the carbon 
bisulphide. Pure potassium iodide must be used for this purpose, 
and its purity tested by means of a gravimetric determination of 
the iodine present in the salt after it has been dried at 170°-180° C. 

The reason the sodium thiosulphate solution must be stand- 
ardized in this way is as follows: 

When an aqueous solution containing iodine is chaki with 
carbon bisulphide, not all of the iodine but the greater part ‘of 
it will pass into the latter solvent.* The error is compensated, 
however, by standardizing the solution in the same way. 





* If the solution of a substance is shaken with another solvent in which 
the former does not mix, the original amount of the substance divides itself 
between the two solvents, and in fact the concentration of one solution 
(amount of the dissolved substance present per cubic centimeter) always 
bears a constant relation to that of the other. 

Thus if x) gms. of iodine are diss lved in V c.c. of water, and the solution 
is shaken with V, c.c. of carbon bisulphide, then x, gms. of iodine will remain 
in the aqueous solution and 2)—z, gms. will pass into the carbon bisul- 
phide. 

The amount z is found by the following equation: 


Ty _%—% DPE 2 iN 
(1) 7 V, k, and 2, 7 VE 


2 and ap +1 are the concentrations in each of the solutions and k is the dis- 
5 . 

tribution coefficient, which is <4}, for iodine.’ If the aqueous solution is now 

shaken with the same amount of fresh. carbon bisulphide, then z, gms. oi 





\ Berthelot and Jungfleisch, Comptes rend., 69, p 338. 


DETERMINATION OF BROMINE IN SOLUBLE BROMIDES. 659 


If, after shaking with carbon bisulphide, the aqueous solution 
still appears yellow, it must be treated a second, and perhaps 
a third, time with fresh amounts of carbon bisulphide. 


6. Determination of Bromine in Soluble Bromides (Bunsen). 


If chlorine water is added to a colorless bromide solution in 
a porcelain dish, the solution becomes yellow: 


KBr+Cl=KCl+ Br. 


If it is heated to boiling, the bromine is expelled and the solu- 
tion becomes colorless again. The addition of the chlorine water 
is continued until finally no yellow coloration is produced. 


Preparation and Standardization of the Chlorine Water. 


100 c.c. of a saturated chlorine water are diluted to 500 c.c. 
and titrated against a weighed amount of pure potassium bromide 
which has been dried at 170° C., the same amount of bromide being 
taken for the standardization as is supposed to be present in 
the solution to be analyzed. During the titration, the burette 
containing the chlorine water is enveloped in black paper to pro- 
tect its contents from the light, and the tip of the burette is held 





iodine will remain in the water and x,—2, will be extracted by the carbon 
bisulphide. In this case, however, 


Sa Spe 
(2) X= (sar) gms. iodine, 


so that after shaking » times with fresh portions of carbon bisulphide, the 
amount of iodine remaining in the water would be: 


kV \n ae 
(3). La By (95) gms. iodine. 


Assuming that in the analysis 0.005 gm. of iodine was dissolved in 
10 ¢.c. of water and that this solution was shaken once with 1 c.c. of carbon 
bisulphide, then according to equation (1 


1 
r. =0.005-—— 79 = 0.0057; =0.0001 gm. iodine 





ta 


would ror-arr ssorved in the water, or an amount that can be neglected. 


660 » VOLUMETRIC ANALYSIS. 


just above the surface of the hot bromide solution, so that ° as 
little chlorine as possible is lost by evaporation. 


7- Determination of Iodine and Bromine in Mineral Waters. — 


According to the amount of halogen present, from 5 to 60 liters 
of water are taken for the analysis. 

The amount of bromine and i@dine present is usually small 
compared with the chlorine, so that the residue obtained by the 
evaporation of a large amount of water cannot be used directly 
for the analysis, but by partial crystallization a mother-liquor 
rich in bromide and iodide must first be obtained. | 

Procedure.—The water is placed in a large porcelain evaporating- 
dish, a liter at a time, and if not already alkaline,* enough pure 
sodium carbonate solution is added to make it distinctly so, and 
the water is evaporated to about one-fourth of its original volume. 
This causes the separation of some calcium and magnesium car- 
bonates in the presence of hydroxides of iron and manganese, 
while all of the halogen salts remain in solution. The residue 
is filtered off and thoroughly washed with waiter. The filtrate 
is further concentrated until salts begin to crystallize out, and 
the hot solution is then poured into three times its volume of 
absolute alcohol; this causes the greater part of the sodium chloride 
and other undesired salts to precipitate. After standing twelve 
hours, the alcoholic liquid is filtered and the residue washed five 
or six times with 95 per cent. alcohol. 

The alcoholic solution, which contains all of the iodine and 
bromine with considerable chlorine in the form of the alkaline 
salts, is treated with five drops of concentrated potassium hydroxide 
solution and almost all of the alcohol distilled off, while a current of 
air is passed through the solution by means of a capillary tube 
reaching to the bottom of the liquid in the distilling-flask. 
~The residue from the distillation is further concentrated until 
salts again begin to crystallize out and the precipitation with alco- 
hol is repeated. The alcohol is again distilled off, but this time 
with the addition of only one or two drops of potassium hydroxide 





* The solution is alkaline if after the addition of phenolphthalein the 
solution turns red on boiling. 


DETERMINATION OF IODINE AND BROMINE. 661 


solution. According to the amount of salts present in solution 
this operation is repeated from three to six times. The final 
filtrate, after the alcohol has been distilled off, is placed in a plati- 
num dish, evaperated to dryness, the dish covered with a watch- 
glass, and the residue gently ignited to destroy organic matter. 
The residue from the ignition is dissolved in a little water, the 
carbonaceous material filtered off,* the solution slightly acidified 
with dilute sulphuric acid, the iodine liberated by the addition of 
one or two drops of ‘‘nitrose,” and titrated with sodium thio- 
sulphate, after shaking with chloroform, as described on p. 657.+ 
‘The bromine is determined in the aqueous solution obtained after 
the extraction of the iodine with chloroform. ‘The acid solution is 
made alkaline by the addition of sodium carbonate solution, two 
drops of a saturated sugar solution are added, and the solution 
evaporated to dryness in a platinum dish. With a watch-glass 
upon the dish, the residue is gentiy ignited in order to destroy the 
sugar and the excess of nitrite.— After this has been accomplished 
the residue is dissolved in water, filtered, acidified slightly with 
sulphuric acid, and the bromine titrated with chlorine water as 
described on p. 659. 

Remark.—lf sufficient mineral water is available it is better to 
divide the mother-liquor containing the bromide and iodide into 
two portions; in one portion the iodine is determined as before, 
while in the other the bromine and iodine are determined by 
titration with chlorine water.§ 


8. Analysis of Peroxides (Bunsen). 


All peroxides of the heavy metals, which evolve chlorine on 
treatment with hydrochloric acid, can be determined with great 





*Tf the filtrate is not completely colorless, it is evaporated and again 
ignited. 

+ Lecco determines the iodine colorimetrically (Zeitschr. f. anal. Chem., 
XXXV, p. 318). 

t The addition of the sugar causes the nitrite to be destroyed at a lower 
temperature than would otherwise be the case, and the danger of losing 
bromine by volatilization is avoided. 

§ As the chlorine water was standardized against bromine, an amount 
of the latter equivalent to the iodine present is deducted from the amount 
‘represented by the chlorine water used; the difference shows the broming 
present. 


662 VOLUMETRIC ANALYSIS, 


accuracy by conducting the chlorine into potassium iodide solu- 
tion and titrating the deposited iodine with sodium thiosulphate 
or arsenious acid solution. It is only necessary to make sure 
that the chlorine is allowed to act upon the potassium iodide 
without loss. For all such determinations, Bunsen employed the 
apparatus shown in Fig. 94. The small decomposition-flask of about 
40 c.c. capacity has a ground-glass, connection with the delivery- 











Fig. 94. 


tube * and is held firmly in place by means of rubber rings, as at a. 
The lower end of the bent delivery-tube is drawn out into a not- 
too-small capillary. 

Procedure.—The finely-powdered substance is placed in the 
small glass-stoppered weighing-tube (Fig. 94 B), which has a 
small piece of glass fused on the end, and weighed. The tube is 
then taken hold of by means of the glass at the bottom,y intro- 
duced into the neck of an absolutely dry decomposition-flask, 
and the required amount of the substance is allowed to fall into 
it by carefully revolving the weighing-tube. On again weighing 
the tube, the amount of substance taken is determined. Hydro- 
chloric acid is now added (its concentration depends upon the 
nature of the substance), the delivery tubing is at once connected 
with the flask and introduced into the retort containing potassium 
iodide solution. By means of a tiny flame, the contents of the 
flask are heated to boiling and from half to two-thirds, of the liquid 





* Instead of the ground-glass connection, Bunsen used a tube of the 
same size as the neck of the flask and eoniieated them with rubber tubing, 
the two glass tubes being against one another. 

‘+ By holding the tube in this way, deviations of weight, due to pare 
‘warming, are avoided, 


DETERMINATION OF MANGANESE DIOXIDE. 664 


is distilled over into the retort.. In order to prevent ‘the. iodide 
solution from sucking back into the flask, the delivery-tube is 
taken out of the retort before removing the flame; the contents 
of the tube are then washed into the retort. 
The potassium iodide solution is poured into a large beaker, 
the retort washed out several times with a little water, and then 
with potassium iodide solution in order to remove any iodine 
which may remain adhering to the glass. The iodine is titrated with 
as sodium thiosulphate solution. In this way pyrolusite, chro- 
mates, lead peroxide, minium, ceric oxide, selenic, telluric, and 
molybdie acids may be analyzed. 


(a) Determination of Manganese Dioxide in Pyrolusite. 


_Mn0, 86.93 
20 Meee = 4,347 gms. Mn0Q,. e 








How much pyrolusite shall be taken for the analysis? * » 

If possible, an amount should be taken for analysis which will 
not require more than one buretteful of the a Na.S,O0, solution. 
We assume that the sample contains 100 per cent. of MnO,, and 
calculate how much of the latter would correspond to 50 c.c. of 
N Na§,0,: 
10 a2 3:5 


L é.c. i solution :0.004347 gm. MnO,=50:2; 


x =5v X 0.004347 = 0.2173 gm. Mn0O,,. 


Consequently for the analysis about 0.2 gm. of the substance 
is taken, which has been dried at 100°C. To this 25 ¢.c. of hydro- 


chloric acid (1:2) are added and the analysis is made as described 
above. 





* This is applicable to almost every volumetric analysis. To insure the 
most accurate results, the concentration of the standard solution and the 
weight of substance taken for analysis should be so chosen that between 35 
and 50 c.c. of the reagent are used in the final titration. In this way the errors 
in determining the end-point, reading the burette, etc., with not influence the 
result appreciably,—{Translator. } 


664 VOLUMETRIC ANALYSIS, 


The calculation is based upon the following equations: 
MnO, + 4HCl=2H,0+ MnCl, + CL, 
2Cl=2I=1Mn0O,, 
1Cl=11=4Mn0O, = 48.47 ems. 


The amount of substance taken for analysis=a gms., and the 


x Na,S,0, solution used for the titration of the iodine=tc.c. Then 


a:tX 0.004347 = 100: 2: 





pa ST = per cent. MnOz. 


The determination of chromates, lead peroxide, and selenic 
acid is carried out in the same way, except that concentrated 
hydrochloric acid is used for the decomposition. 


(b) Determination of Telluric Acid. 


If the telluric acid is present as the hydrous acid (H,TeO,+2H,0) 
or as tellurate, the analysis is performed in the same way as with 
selenic and chromic acids. If, however, the tellurium is present 
as the anhydrous acid or as the anhydride, the method must be 
modified, for these substances are scarcely attacked by concen- 
trated hydrochloric acid. They are placed in the decomposition- 
flask, dissolved in a little concentrated potassium hydroxide,* and to 
the tellurate solution thus obtained the concentrated hydrochloric 
acid is added; the reduction then is accomplished without difficulty: 


K,TeO,+4HCl=2KCl+ H,TeO,+ H,O+Cl,. 
According to this equation 


1Cl= =5= 2 = 63.75 ems. Te: 





* The solution could not be effected by using sodiwm hydroxide. 


DETERMINATION OF CERIC OXIDE AND VANADIC ACID. 665 


.c) Determination of Ceric Oxide. 


Nee ere pr te 
1000 c.c. io iodine solution = oan ean 17.225 gms. CeQ,. 
Ceric oxide when mixed with considerable lanthanum and di- 
dymium oxides is reduced by distillation with concentrated hydro- 


chloric acid: 


2CeO, + 8SHC1=4H,0 + 2CeCl, + Cl. 


If, however, the mixture contains but little of the two last 
substances, or if it is pure ceric oxide, the heating with concen- 
trated hydrochloric acid is of no avail; the ceric oxide will not 


dissolve. 


In the presence of hydriodic acid, however, the reduction 
takes place readily, so that it is only necessary to add 2 gms, 
of potassium iodide to a weighed amount of the substance 
(0.67—0.68 gm.) in the decomposition-flask, and then, after the 
addition of hydrochloric acid, violet vapors of iodine can be dis- 
tilled from the solution: 


2CeO, + 2KI-+8HC1=2KCl + 2CeCl, + 4H,0 + 21. 


Often there will be so much iodine given off that the solid is 
likely to stop up the tube and the flask will often explode. To 
prevent this, the end of the delivery-tube is not drawn out into 
a capillary, but at the bottom an opening of about 4 mm. in diame- 
ter is left. During the operation, the flame must be protected 
from air-currents, for otherwise there is danger of liquid sucking 
back from the retort. 


(d) Determination of Vanadic Acid.t{ 


ie 2 VO, 1624 
1000 c.c. 55 iodine solution = 50 7 99 79-12 ems. V0, 





By boiling vanadic acid, or one of its salts, with concentrated 
hydrochloric acid, the vanadium is reduced with evolution of 
chlorine. Unfortunately, this reaction cannot be used for the 
determination of vanadic acid, for the amount of chlorine evolved 


666 ' VOLUMETRIC ANALYSIS. 


depends upon the concentration of the vanadium solution: the 
vanadium is not reduced to a definite oxide. On the other hand, 
by means of hydrobromic acid,* vanadic .acid is reduced to a blue 
vanadyl! salt: 


V,0,+2HBr=V,0,+H,0+ Br. 


| If the free bromine is absorbed in potassium iodide, and the 

liberated iodine titrated with sodiunf thiosulphate, a sharp deter- 
mination of the vanadium will be obtained. To carry out this 
analysis, about 0.3-0.5 gm. of the vanadate, together with 1.5 
to 2 gms. of potassium bromide, is placed in the decomposition- 
flask of the Bunsen apparatus (Fig. 94, p. 662), 30 ¢.c. of con- 
centrated hydrochloric acid are added, and distillation is effected 
as before. The decomposition is always complete when the 
liquid in the flask is a pure blue. : 

If hydriodie acid is used instead of hydrobromic acid, the 
vanadie acid is reduced still further, almost to V,O,.+ In faet, 
a complete reduction to the latter oxide can be accomplished if 
potassium iodide, concentrated hydrochloric acid, and 1 or 2 c.e. 
of syrupy phosphoric acid are added and the liquid distilled 
until no more vapors of iodine are evolved. According to 
Steffan, this will always be the case when the liquid is reduced to 
one-third of its vriginal volume. 


(e) Determination of Molybdic Acid. 


The determination depends upon the fact that molybdie acid 
is reduced to molybdenum pentoxide by means of hydriodic acid 
with liberation of iodine: 


-2Mo0,+2HI=H,0+ Mo,0,+I,. 


Remark.—This method finds no practical application on account 
of the fact that it is difficult to obtain a quantitative reduction in 





* Holverscheidt, Dissertation, Berlin, 1890. 
+ Friedheim and Euler, Berichte, 28 (1895), 2067. 
t Ibid., 28 (1895), 2067, and 29 (1896), 2981. 


DETERMINATION OF VANADIC AND MOLYBDIC ACID. 667 


accordance with the above equation. Gooch and Fairbanks * found 
that if a solution containing molybdie acid is distilled in the 
Bunsen apparatus with potassium iodide and hydrochloric acid, 
until iodine vapors are no longer visible and the solution is a light 
green, too little iodine is obtained. On the other hand, if the 
distillation is continued still further, they found that the reduction 
goes on and more iodine is obtained than corresponds to the above 
equation. Steffan,; who tested the method in the author’s labo- 
ratory, obtained results agreeing with those published by Gooch 
and Fairbanks. By means of hydrobromic acid, molybdiec acid 
is not reduced. 


({) Determination of Vanadic and Molybdic Acids in the Presence 
of One Another. 


According to Steffan, these two acids may be determined 
very accurately when present together. The vanadic acid is 
determined, according to MHolverscheidt, by distillation with 
potassium bromide and concentrated hydrochloric acid, absorp- 
tion of the bromine in potassium iodide solution, and titration 
of the liberated iodine (cf. p. 666). The contents of the distilla- 
tion flask, in which the vanadium is present as vanady] salt and 
the molybdenum as molybdic acid, are treated with hydrogen 
sulphide in a pressure-flask, and the precipitated molybdenum 
sulphide is filtered through a Gooch crucible, and weighed as MoO,, 
as described on p. 286. The results obtained by this method are 
perfectly satisfactory. 

As molybdie acid is unattacked by hydrobromic acid, but 
is reduced to Mo,O, with separation of iodine by means of hydri- 
odic acid, Friedheim and Euler proposed the following method for 
the determination of vanadic and molybdic acids ped present 

together: | 

The mixture =e the two acids is distilled as before with potassium 





* Gooch and Fairbanks, Zeitschr. f. auOry, Chem., xm (1897), 101, and 


ed Be hy a 
+ Steffan, Inaug. Dissertation, Zurich, 1902. — 


668 VOLUMETRIC ANALYSIS, 


bromide and hydrochloric acid and the vanadium thereby reduced 
to the tetroxide compound ; 


V,0;,+2HBr=H,0+ V.0,+ Br,, 


with separation of two atoms of bromine which are determined 
iodimetrically. To the cold solution remaining in the distilling- 
flask, potassium iodide, hydrochlorie acid, and syrupy phosphoric 
acid are added, and the distillation continued until no more iodine 
is given off and the solution is a light green. 

By means of this second reduction the vanadium tetroxide is 
supposed to be reduced to V.Os, 


V,0,+2HI=H,0+V,0,+1L, 


and consequently more iodine is liberated by the vanadium. 
Furthermore, according to Friedheim and Euler, the molybdenum 
is reduced to Mo,0;: 


2Mo0,+2HI= H,0 + Mo,0,+ I,. 


If, therefore, the amount of iodine corresponding to the first 
titration’ is deducted from the amount obtained in the second, 
the difference should correspond to the amount of molybdenum 
present. But Gooch and Fairbanks have shown that this is 
not the case.* 

The error in the method lies in the fact that the vanadic acid 
is only reduced completely to V,O; when the solution is distilled 
to one-third of its original volume. In this case, however, the 
molybdenum is reduced further than corresponds to the formation 
of Mo,0,;; too much iodine is liberated and too high a value is 
obtained for the molybdic acid present. On the other hand, 
if after the addition of the potassium iodide the liquid is 
only distilled until the iodine vapors cease to appear and the 
solution is a light green, the vanadium is not completely 
reduced to V,O,, and then a too low value for the molybdenum 
is obtained. 








* The results of Gooch and Fairbanks have been confirmed in every ree 
spect by Steffan. 


DETERMINATION OF HYPOCHLOROUS ACID. 669 


9. Analysis of Chlorates. 


This is carried out the same way as the analysis of pyrolusite 
(ef. p. 663): 


KCIO, + 6HCl= KCl1+3H,0+3CL, 
i m.=at. iodine= 1000 C.C N Na,S,O, solution 
i9 S@- E €. 75 Na,8,05 


“RCO, - 122.6 
pecs eg 





= 2.043 gms. KCIO,. 


Many oxidizing agents can be determined iodimetrically with- 
out previous distillation with hydrochloric acid. 

For other methods of analyzing chlorates iodimetrically, 
consult H. Ditz, Chem.-Ztg. 1901, 727 and Luther and Rutter, 
Z. anal. Chem. 46, 521 (1907). 


10. Determination of Hypochlorous Acid. 


This determination is made use of in the analysis of chloride 
of lime. 

Procedure.—Into atared weighing-tube about 5 gms. of ‘‘chloride 
of lime ” are introduced, and the stoppered tube is weighed. Its 
contents are washed into a porcelain dish, rubbed to a paste by 
means of a pestle, and then transferred without loss to a 500-c.c. 
measuring-flask, diluted up to the mark with water and well shaken. 
Of this turbid solution, 20 c.c. are run into 10 ¢.c. of 10 per cent. 
potassium iodide solution, and after acidifying with hydrochloric 
acid the iodine set free is titrated with 2 Na,S,0,;. The result 
is expressed in per cent. of chlorine. 

Remark.—lf the “ chloride of lime” contained calcium chlo- 
rate it will be partially reduced by hydrochloric acid and 
potassium iodide with liberation of iodine, and consequently 
the results obtained for hypochlorite chlorine (bleaching chlorine) 
will be too high. In this case the hypochlorite is best determined 
by a chlorimetric process with arsenious acid (see p. 701). 


670 3 VOLUMETRIC ANALYSIS. 


11. The Analysis of Iodates. 


N HI1O,. 175.93 
1000 c.c. 70 Na.S,0,= 27 wile ar ae 2.932 gms. HIO,. 


The solution of the iodate is allowed to run into an acid 
solution containing an excess of potassium iodide. Iodine is — 


set free according to the equation | 


KIO,+5KI+6HCl=6KC1+3H,0+31L, 


and the iodine is titrated with thiosulphate solution as described 
on p. 647. 
| ¥. 12. The Analysis of Periodates. 


HIO0, __ 191.93 
80 SO 


The haa se of periodates is carried out ae as with 
iodates; the reaction that takes place is 


1000 c.c. ; ip NaS: 0O,= 





= 2.399 gms. HIO,. 


tae eee 


KIO, +7KI+S8HCl=8KCl+44H,O +41. 


13. Analysis of a Mixture of Iodate and Periodate.* 


If a neutral or slightly alkaline solution of an alkali periodate 
is treated with a solution of potassium iodide, the following 
reaction takes place: 


KIO, +2KI+ H,O=2KOH+KIO,+1.. 


The liberated iodine is titrated with tenth-normal arsenious 
acid (not with sodium thiosulphate); in a neutral solution the 
iodate does not react with potassium iodide. For the analysis 
‘of a mixture of iodate and sepegenstes the following procedure is is 
used : 

' In one sample the iodate 4 eitiddatb3 is determined by adding 
‘the solution of the substance to an acid solution containing. an 
‘excess of potassium iodide and. _ liberated iodine is titrated 
‘with sodium thiosulphate solution;:: ) ‘i Fe 
A second sample of the substance: is dissolved in water, a drop 


© *E, Mullér and’O. Friedberger, Berichte, 1902, 2655. 





ANALYSIS OF IODIDES. 67 rT) 


of phenolphthalein added, and the solution is made just alkaline’ 
enough to give the pink color with phenolphthalein, adding alkali 
if the solution is acid and hydrochloric acid if the. solution is 
strongly alkaline. To the barely alkaline solution, 10 c.c. of a 
cold, saturated solution of sodium bicarbonate are added and then 
an excess of potassium iodide; the liberated iodine is. at once 
titrated with tenth-normal arsenious acid.* 2 
| Example. —In a mixture of KIO, and KIO, weighing a grams, 
the iodine liberated on treatment eth an acid solution of KI 
reacts with 7’ c.c. of 0.1 N Na,S,O, and the same weight of sample 
liberates in alkaline solution only enough iodine-to react with 
t c.c. of 0.1N As,O, solution. By comparing the equations given 
under 12 and 13, it is evident that the periodate alone would 
react with 4t c.c. of 0.1N Na,§,O, in acid solution. The amount — 
of KIO, and KIO, present will be ; 


—. 150 





0.01150 gm % m0," 


_@- a0 0.3567 


(T—4t) X0.003567 gm % KIO,. 





14. Analysis of Iodides. + 
Method of H. Dietz and B. M. Margosches. 


1000 ¢.c. x 9 KIO <2 = 52 1A8-2 


0 60 3 10.58 gm. iodine. 


The solution of the iodide is treated with an excess of tenth- 
normal potassium iodate solution, acidified with hydrochloric 
acid, a piece of calcite added, as suggested by Prince,t and boiled 
until all the iodine is expelled. The solution is allowed to cool, 
then an excess of potassium iodide is added, and the iodine now 





*The iodine cannot be titrated in the alkaline solution with sodium 
thiosulphate, and the iodine in the acid solution cannot be titrated with the 
arsenious acid. 

+ Chem. Ztg., 1904, II, 1191. 

t Inaug. Dissert. Ziirich, 1910. 


672 VOLUMETRIC ANALYSIS. 


liberated, which corresponds to the excess of potassium iodate 
used, is titrated with tenth-normal Na,§,0, solution. 
From the equation 


KIO, + 5K1 +6HCl=6KC1+3H,0 +31, 


it is evident that five-sixths of the iodine liberated comes from 
the iodide. If, therefore, 7 c.c. of 0.1N KIO, solution were 
added and ¢ c.c. of 0. IN Na,S,0 “were used ae titrating the 
excess of KIO,, then there is present 


(T —t) X0.01058em. iodine as iodide. 


15. Determination of Copper with Potassium Iodate. * 


Potassium iodate in dilute hydrochloric acid solution is 
reduced by potassium iodide to free iodine (cf. p. 670): 


KIO, +5KI+6HCl=6KC1+31,+3H,0, 


but if the solution is strongly acid with hydrochloric acid and an 
excess of the iodate is added, the iodine is oxidized to ICI: 


21, + K10,+6HCl =KCl+ 51C1+3H,0, 


and in this case the whole reaction may be expressed by the 
equation: 
KIO,+2KI+6HCl=3KCl+ 3ICl+3H,0. 


The ICI is not very stable, and is at once reduced to free iodine 
in the presence of any oxidizable substance. 

L. W. Andrew + has shown that quite a number of reducing 
substances, such as free iodine, iodides, arsenites, and antimonites, 
can be titrated with potassium iodate very exactly, by taking 
advantage of the fact that when the reducing agent is present 
in excess free iodine is formed, which is oxidized quantitatively 
by more iodate, provided the proper amount of hydrochloric acid 
is present. Copper solutions are precipitated quantitatively 





* Jamieson, Levy, and Wells, Jour. Am. Chem. Soc., 30, 760 (1908). 
} Ibid., 25, 756 (1903). 


DETERMINATION OF COPPER WI1H POTASSIUM IODATE 673 


by potassium thiocyanate and sulphurous acid as cuprous thio- 
cyanate, CuSCN, and Parr* has estimated copper quantita- 
tively by titrating this precipitate with permanganate. The 
oxidation is, however, simpler and more accurate when the 
titration is effected by potassium iodate, or biiodate. The 
reaction goes through the stage in which iodine is set free, but 
the latter is oxidized completely to iodine chloride upon the 
addition of more iodate: | 


(a) 2CuSCN+3KI0,+4HCl=2CuS0,+I,+ICl+2HCN+3KCI 
+H,0. 


() 21,+K10,+6HCl= KCl+51C1+3H,0, 
and the whole reaction is (multiplying (a) by 2 and adding (6)), 


(c) 4CuSCN +7KIO3 + 14HCl=4CuSO4+ 71C1+ 4HCN + 7KC1 
+5H20. 


The potassium iodate solution is very stable and can be 
preserved for years if protected from evaporation. The standard 
solution used can be prepared by weighing out a known amount 
of the pure salt and dissolving to a definite volume, or the solution 
may be standardized against pure copper, carrying out the process 
as in an analysis. A convenient concentration is one-fifth of the 
formula weight. | 

Procedure.—To 0.5 gm. of the ore in a 200 c.c. flask, add 6 to 
10 c.c. of strong nitric acid, and boil gently, best over a free flame, 
keeping the flask in constant motion and inclined at an angle 
of about 45°, until the larger part of the acid has been removed. 
If this does not completely decompose the ore, add 5 e.c. of 
strong hydrochloric acid and continue the boiling until the 
volume of liquid is about 2.¢.c. Now add gradually and carefully, 
best after cooling somewhat, 12 c.c. of sulphuric acid (1:11), 
and continue the boiling until sulphuric acid fumes are evolved: 
copiously. Allow to cool, add 25 c.c. of cold water, heat to’ 





* J. Am. Chem. Soc., 22, 685 (1900). 


O74 4. _ VOLUMETRIC ANALYSIS. 


boiling, and keep hot until the soluble sulphates have dissolved. 
Filter into a beaker, and wash the flask and filter thoroughly 
with cold water.* Nearly neutralize the filtrate with ammonia 
and add 10 to 15 c.c. of strong sulphur dioxide water. Heat 
just to boiling and add 5 to 10 c.c. of a 10 per cent. solution of 
ammonium thiocyanate, according to the amount of copper | 
present. Stir thoroughly, allow the precipitate to settle for 5 
or 10 minutes, filter on paper, and. wash with hot water until 
the ammonium thiocyanate is completely removed. 

Place the filter with its contents in a glass-stoppered bottle 
of about 250 c.c. capacity, and by means of a piece of moist 
filter-paper transfer into the bottle also any precipitate adhering 
to the stirring-rod and beaker. Add to the bottle about 5 c.e. 
of chloroform, 20 ¢.c. of water and 30 c.c. of concentrated hydro- 
chloric acid (the two latter liquids may be previously mixed). 
Now run in standard potassium iodate solution, inserting the 
stopper and shaking vigorously between additions. A violet 
color appears in the chloroform, at first increasing and then 
diminishing, until it disappears with great sharpness. The 
rapidity with which the iodate solution may be added can be 
judged from the color changes of the chloroform. 

In order to make another titration it is not necessary to wash 
the bottle or throw away the chloroform. Pour off two-thirds 
or three-fourths of the liquid in order to remove most of the 
pulped paper, too much of which interferes with the settling of 
the chloroform globules after agitation, add enough properly 
diluted acid to make about 50 ¢c.c. and proceed as before. In 
this case, where iodine monochloride is present at the outset, 
the chloroform becomes strongly colored with iodine as soon as 
the cuprous thiocyanate is added, but this makes no difference 
with the results of the titration. 





* With substances containing appreciable amounts of silver a few drops of 
hydrochloric acid should be added before making this filtration, but not 
enough to dissolve any considerable amounts of the lead sulphate or antimonie 
oxide that may be present. 


DETERMINATION OF CHROMIUM IN CHROMITE, 675 


16. Analysis of Soluble Chromates. 


A concentrated, acid solution of potassium iodide is treated 
with a weighed amount of the chromate, diluted with water, 
and the liberated iodine titrated. (Cf. standardization of sodium 
thiosulphate against potassium dichromate, p. 649.) 


16a. Determination of Chromium in Chromite. 

About 0.2 gm. of the finely powdered chromite is intimately 
mixed with 2 gms. of sodium peroxide in a porcelain crucible. 
This crucible is placed inside a larger porcelain crucible and 
heated for fifteen or twenty minutes over a small flame.* At the 
end of this time, all the chromium will be converted into soluble 
sodium chromate. The crucible and its contents are placed in 
100-200 c.c. of water, which is heated to boiling and kept at this 
temperature until the melt is completely disintegrated. The 
ferric oxide is then filtered off, the filtrate evaporated in a porce- 
lain dish nearly to dryness,t the residue taken up in as little 
water as possible, one or two grams of potassium iodide added, 
the solution diluted to about 400 c.c., and the free iodine titrated 
with tenth-normal thiosulphate solution: 


I¢,¢: af Na,S,0,= 0.001733 gm. Cr. 


17. Determination of Lead Peroxide. 
Method of Diehl, modified by Topf.t 
The analysis depends upon the fact that lead peroxide is re- 


duced by means of potassium iodide in acetic acid solution when 
considerable alkali acetate is present: 


PbO, +4HI= PbI,+2H,0O+1,. 


After diluting with water the iodine is titrated with 55 = g NaS. O, 
solution. 





* If the crucible is heated too wry it is likely to be strongly attacked by 
the sodium peroxide. With care, a single crucible may be used for four or 
six determinations. 

+ The evaporation to dryness is necessary to remove the last traces of per- 
oxide. 

t Diehl, Dingl. polyt. Journ., 246, P: 196, and Topf, Zeitschr. f. analyt. 
Chem., XX VI (1887), p. 296. 


676 VOLUMETRIC ANALYSIS. 


Procedure.—About 0.5 gms. of the substance are dissolved with 
1.2 gms. of potassium iodide and 10 gms. of sodium acetate in 
5 c.c. of 5 per cent. acetic acid. The solution is diluted with 
water to a volume of 25 c.c. and titrated with sodium thiosulphate. 

Remark.—Moist lead peroxide reacts almost instantly on 
undergoing the above treatment; thoroughly dried material, on 
the other hand, dissolves after a few minutes provided it is finely 
ground. If, however, the dry perexide is in the form of coarse 
grains, it may be several hours before the reaction is finished, 
or the decomposition may be incomplete. 

Furthermore, too much potassium iodide should not be used, 
as otherwise lead iodide will separate out. In that case from 3 to 
5 gms. more of sodium acetate are added and a few cubic centi- 
meters of water. The mixture is shaken until the lead iodide 
has dissolved completely and not till then diluted to a volume of 
25 c.c. The solution must remain perfectly clear and there 
should not be a trace of lead iodide precipitate. 

This excellent method may also be used by the analysis of 
minium (red lead). 


18. Determination of Ozone in Ozonized Oxygen. 


N 
1000 c.c. 1G Na,S8,0, =— =— =2.4 gms. O,. 


(a) Schénbein’s Method. 


The most accurate method for estimating ozone consists in 
allowing the ozonized oxygen to act upon potassium iodide solution 
whereby free iodine is formed: 


2K1+ 03+ H20=2KOH+12+ Og, 


and the iodine may be titrated, after acidifying the solution with 
dilute sulphuric acid, by means of N/10 sodium thiosulphate. 

It is not, however, immaterial whether the ozone reacts with 
a neutral or with an acid solution. In the latter case far too much 
iodine is liberated, although in the former case exactly the right 
amount is set free. Sir B. C. Brodie * called attention ‘to this 





+ Phil. Trans., 162, 435-484 (1872). 


’ 


DETERMINATION OF OZONE IN OZONIZED OXYGEN. 677 


fact in his classic researches on ozone. Brodie confirmed the 
results obtained of his titrations by weighing the amount of ozone 
used in the experiments. This work of Brodie’s appears to have 
been forgotten,* for many other chemists have since that time 
attempted to work out an iodimetric method for estimating 
ozone, some using acid solutions of potassium and iodide and some 
neutral solutions to absorb the gas, although for a leng time it 
occurred to no one else that the results could be checked by weighing 
out a definite amount of ozone for test experiments. In 1901, 
- however, this was done in a very simple way by R. Ladenburg 
and R. Quasig,t who were without knowledge of Brodie’s work. 
Their method consisted in weighing a glass bulb of known capacity 
which was provided with glass stop-cocks, filling it with oxygen 
and then weighing. The oxygen was then replaced by ozone, so 
that the gain in weight multiplied by three represented the amount 
of ozone present. 

In order, now, to titrate the ozone, Ladenburg and Quasig — 
expelled the gas from the bulb by distilled water, and conducted 
it slowly through a neutral solution of potassium iodide which 
was subsequently treated with an equivalent amount of sul- 
phuric acid and the liberated iodine titrated with N sodium thio- 
sulphate. 

The results of Ladenburg and Quasig have been carefully 
tested in the author’s laboratory{ and the method improved 
somewhat by absorbing the ozonized oxygen by potassium iodide 
solution in the glass bulb itself rather than expelling the gas from 
the bulb and passing it into the iodide solution. 

The estimation of ozone by weighing is a much too round- 
about process to permit a practical application, particularly on 
account of the fact that the measurement and weighing of the gas 
must take place in a room at constant temperature, a condition 
which cannot in many cases be readily fulfilled. Consequently 
the volumetric titration of the gas is far more practical. 

Procedure—A glass bulk of about 300 to 400 c.c. capacity, of 
the form shown in Fig. 95, is procured and its volume accurately 





* Luther and Inglis, Z. phys. Chem., 48, 208 (1903). 
+ Ber. 34, 1184 (1901). 
t Treadwell and Anneler, Z. anorg. Chem., 48, 86 (1905). 


678 VOLUMETRIC ANALYSIS. 


determined by weighing it empty and then filled with water, 
applying the correction for temperature as described on p. 517 et seq. 
The bulb is then connected with a gas delivery tube, making use 
of Babo flanged joints (Fig. 95, c and d) which are pressed together 
by means of a steel clamp, lined with cork. The delivery tube 
is connected with the supply of ozone and oxygen, with which 
the water in the bulb in replaced. During the filling of the bulb, 
but little of the ozone is absorbedeby the water. When the 























ener 


Fie. 95. Fia. 96. xa. O75 a. 


tube is filled, the lower stop-cock is closed first and the upper one 
a few seconds later. The bulb is then disconnected with the gas 
delivery tube, inverted, the upper-stop cock opened quickly for 
an instant in order to establish atmospheric pressure in the bulb, 
«od then connected by means of rubber tubing with the gas 
reservoir N which is filled with double-normal potassium iodide 
solution (Fig. 97). The air imprisoned in the rubber tubing is 
allowed to escape through the three-way stop-cock 6 and after 
properly setting the cock, about 20 to 30 c.c. of the iodide solution 


DETERMINATION OF OZONE IN OZONIZED OXYGEN. 679 


are introduced into the bulb. Finally the stop-cock 6 is closed 
and the rubber tubing disconnected, The contents of the bulb 
are vigorously shaken and allowed to stand for half an hour; 
at the end of this time the absorption of the ozone will be 
complete. 

An Erlenmeyer flask is then placed under the stop-cock 6’; 
this is opened and immediately afterwards the upper stop-cock also. 
The bulb is washed out first by introducing some potassium 
iodide solution through a and finally with pure water. The 
contents of the flask are then acidified with dilute sulphuric 
acid and the liberated iodine titrated with tenth-normal sodium 
thiosulphate. 

The computation takes place as follows: 


Contents of the bulb=V c.e. 
Ozone found by titration=p gms. 


Temperature=t, barometer reading=B, aqueous tension 
= Ww. } 
The volume of the bulb at 0° and 760 mm. pressure is 


_V(B—w)273 
~ 760(273-+1) ’ 





Vo 


When filled with oxygen this would weigh: 


32.Vo 
22,391 





ems. 


‘Therefore the weight of oxygen and ozone in the bulb is 


92391 3! 





and the per cent. of ozone in the mixture is 


—— 100-p ___6,717,300-p 
32-Vo, p 96Vo+22,391 


22,391 ' 3 





=per cent. ozone. 


680 VOLUMETRIC ANALYSIS. 


(b) Method of Soret-Thenard.* 


Ozone is absorbed quantitatively by means of sodium arsenite 
solution in accordance with the following equation: 


NagAsOz3 + 03 = NazAsO4+ Oo, 


although A. Ladenburg { finds that the absorption takes place 
much more slowly than by means of potassium iodide. When, 
therefore, the ozone is passed through the arsenite solution, there 
is danger of getting too low results. If the absorption takes place 
in a glass bulb, however, the results are good. © 

Ozone is also absorbed by alkali bisulphite t solutions and may 
be estimated in this way, by titrating the excess of bisulphite 
with iodine. Ladenburg,§ however, has shown that the method 
is not as accurate as the potassium iodide one, so that it will not 
be considered further here. 


19. Determination of Hydrogen Peroxide. Kingzett’s Method.|| 


1000 c.c. as Na.8.0, solution = Ha “ 

The hydrogen peroxide solution is diluted until its H,O, 
content corresponds to about 0.6 per cent. by weight and of this 
solution 10 ¢.c. are used in the analysis. | 

Procedure.—About 2 gms. of potassium iodide are placed in an 
Erlenmeyer flask and dissolved in 200 c.c. of water, 30 c.c. of 
sulphuric acid (1:2) are added, and then, with constant stirring, 
10 c.c. of the hydrogen peroxide solution are added from a pipette. 
After standing five minutes, the iodine liberated in accordance 
with the equation 


H,0,+2KI+H,80,=K,SO,+2H,0 +I, 


is titrated by means of tenth-normal thiosulphate solution. 





= 1.7008 gms. H,0O:. 





* Compt. rend., 38, 445 (1854), 75, 174 (1872). 

+ Ber., 36, 115 (1903). 5 

{ Neutral alkali sulphite is not suitable here, because it is not oxidized 
quickly by pure oxygen alone. 

§ Loc. cit. 

|| J. Chem. Soc., 1880, 792. 


DETERMINATION OF IRON. 681 


Remark.—This method is rather better than that described 
on p. 627 because the titration can take place in the presence of 
glycerol, salicylic acid, etc., which are sometimes used as pre- 
servatives in commercial hydrogen peroxide preparations. These 
substances will render the results obtained by the permanganate 
titration less accurate. 


20. Determination of Iron. 


This method was first proposed by Carl Mohr * and is based 
upon the following reaction: 


FeCl, +HI = HCl+ FeCl, +I. 


As the reaction is reversible, it is necessary to have an excess 
of hydriodic acid present in order that it may take place quanti- 
tatively in the direction from left to right. 

Procedure.—The hydrochloric acid solution containing a 
weighed amount of the ferric salt is placed in a 300-c.c. glass- 
stoppered bottle, the greater part of the acid is neutralized by means 
of sodium hydroxide, and the air removed by means of a 
current of carbon dioxide. After this about 5 gms. of potas- 
sium iodide are added, the bottle closed, shaken, and allowed 
to stand in the cold for twenty minutes. The liberated iodine 


is then titrated with = sodium thiosulphate solution. As soon 


as the blue color has disappeared | more carbon dioxide is con- 
ducted through the solution, the bottle is stoppered and allowed 
to stand for a few minutes to see whether the blue color will! re- 
appear. Should this be the case, more thiosulphate is added, 
the flask again stoppered and allowed to stand. If a blue 
color again appears, the solution contains too little potassium 
iodide, so that it is necessary to repeat the entire analysis, 
using 1-2 gms. more of it. With sufficient potassium iodide 
and only little free hydrochloric acid, the reaction is always com- 
plete at the end of twenty minutes. The results obtained are 
satisfactory. 





* Ann. d. Chem. u. Pharm., 105, p. 53. 
¢ Starch is added in all these titrations. 


682 VOLUMETRIC ANALYSIS. 


21. Determination of Copper. Method of Haen*-Low.+ 


1000 c.c. = Na,8.O3 solution=[=6. 357 gms, Cu. 


Principle.—If an acid solution of a cupric salt is treated with 
an excess of potassium iodide, all the eee is precipitated as 
cuprous lodide, 

2Cuso, +4KT= Cu,I,- +2K,50,+L, 


and there is liberated one atom of iodine for each atom of copper 
present. The iodine is titrated with sodium thiosulphate solution. 

This method has found extensive application, especially at 
the copper mines in the United States and, if the proper conditions 
are fulfilled, the results are just as accurate as those obtained by 
electrolysis. The method has been studied by Gooch and Heath, { 
who find that. the above reaction takes place preferably in a 
solution containing not over 3 c¢.c. concentrated sulphuric acid, 
hydrochloric acid, nitric acid free from lower oxides, or 25 ¢.c. 
of 50 per cent. acetic acid. Obviously the solution must not 
contain ferric iron or any other oxidizing agents which will react 
with potassium iodide under the same conditions. 


Standardization of the Thiosulphate Solution. 


In technical work it is customary to standardize the thio- 
sulphate solution against pure copper. About 0.2 gm. of pure 
copper is weighed into a 200-c.c. Erlenmeyer flask and dissolved 
in 5 c.c. of a mixture of equal parts nitric acid (sp. gr. 1.42) and 
water. The solution is diluted with 25 ¢.c. water and boiled a 
few minutes to. get rid of the greater part of the reduced oxides 
of nitrogen. To remove the last of the nitrous oxides, 5 c.c. 
of bromine water are added and the solution boiled until the 
excess bromine is expelled. The flask is then removed from 
the flame and strong ammonia added to its contents until a 
slight excess is present. After boiling off the excess of ammonia, 





* Ann. Chem. Pharm., 91, 237 (1854). 
nee = +Peehnieal Methods-of Ore Analysis:----—- 
{ Z. Anorg. Chem., 55, 129 (1907). 


DETERMINATION OF COPPER IN ORES. 683 


7 c.c. of strong acetic acid are added, which dissolves any copper 
oxide that has deposited. After cooling to room temperature, 
3 gms. of potassium iodide are added and the brown solution 
titrated with sodium thiosulphate until nearly colorless. Then 
starch solution is added and the titration finished. In making 
the titration for the first time, one is bothered somewhat by the 
fact thatthe cuprous iodide is of a light-brown color but after 
a little practice there is no difficulty in getting the correct end- 
point. If t c.c. of the solution were used in titrating a gms. of 


copper then | c.c. of thiosulphate == gm, Cu. 


Determination of Copper in Ores. Low’s Method. 


Principle.—The ore is dissolved in acid, the copper separated 
from iron, etc., by precipitating it upon metallic aluminium, the 
deposit dissolved in nitric acid and treated as in the standardiza- 
tion. 

Procedure.—To 0.25-0.50 gms. of finely ground ore weighed 
into a 250-c.c. Erlenmeyer flask, add 6 ¢.c. of nitric acid (sp. gr. 
1.42), and boil gently until nearly to dryness. Add 5 e.c. of 
- strong hydrochloric acid and heat again. As soon as the in- 
crusted matter has dissolved add 7 c.c. of concentrated sulphuric 
acid and heat until the sulphuric acid fumes freely. Cool and 
add 25 c.c. of water. Then heat until any anhydrous ferric 
sulphate is dissolved, and filter to remove insoluble sulphates 
and silica. Wash the flask and filter-paper until the volume of 
the filtrate amounts to about 75 c.c., receiving it in a No. 2 
beaker. Place on its edge in the beaker a piece of aluminium- 
foil bent into triangular shape. Cover the beaker and boil 
gently for seven to ten minutes, which will be sufficient to pre- 
cipitate all the copper, provided the solution does not much 
exceed 75 ¢.c. Avoid boiling to very small bulk. The alu- 
minium should now appear clean, the copper being detached 
or loosely adhering. Remove from the heat and wash down 
the cover and sides of the beaker with cold water. There is 
danger of finely-divided copper being oxidized and dissolved. 
To prevent this, and at the same time to remove any traces of 
copper remaining in solution, add 15 c.c. of strong hydrogen- 


684 VOLUMETRIC ANALYSIS. 


sulphide water. The next step depends upon the amount of 
copper in the ore. 

(a) When there is apparently less than 20 per cent. Cu 
present, decant the liquid through a filter and then without delay 
transfer by means of a jet of hydrogen-sulphide water from a 
wash-bottle, the copper to the filter leaving the foil as clean as 
possible in the beaker. Wash the copper and filter thoroughly 
with this hydrogen-sulphide watér, being careful not to allow 
the filter to stand empty until the washing is finished. (The 
filtrate should not show a brown tinge of copper sulphide.) Now 
place the original flask under the funnel. Pour over the alu- 
minium in the beaker 5 c.c. of a mixture of equal parts concen- 
trated nitric acid and water. Heat just to boiling and pour 
the hot acid very slowly upon the filter, lifting the fold if neces- 
sary. Now, before washing, pour 5 c.c. of bromine water into 
the filter and wash the beaker and filter with hot water. Finally 
remove the filter and wash any residue upon it into the flask. 
If the bromine was not sufficient to give a slight tinge to the 
filtrate more of it must be added. Boil the filtrate, which does 
not exceed 75 c.c., to expel the excess of bromine, but do not 
concentrate to small volume. Remove from the heat and add 
a slight excess of strong ammonia (usually 7 ¢.c.). Boil off the 
excess of ammonia and add 3 or 4 c.c. of strong acetic acid. Cool 
to room temperature, add 3 gms. of potassium iodide, and titrate 
with thiosulphate, adding starch toward the end of the reaction. 

(b) With high percentages of copper it is better to wash the 
copper by decantation instead of on the filter. Transfer the 
liquid.and copper to the original flask, and set the beaker and 
aluminium aside temporarily. Allow the liquid in the flask to 
settle, decant through a filter and wash three or four times by 
decantation with hydrogen-sulphide water, using about 20 c.c. 
each time. Now place the flask containing the copper under the 
funnel. Heat the 5 c.c. of nitric acid (1 vol. conc. HNOs to 1 vol. 
water) in the beaker with the aluminium-foil and pour it through 
the filter. Remove the flask, place the beaker in its place 
under the funnel, and heat the acid until all the copper has 
dissolved and the red fumes are mostly expelled. Now return 
the flask under the funnel, add the bromine as in the above 
method of analysis (a), and continue as described above. 


DETERMINATION OF ANTIMONY TRIOXIDE COMPOUNDS. 685 


22. Analysis of Arsenious Acid. 


The titration is effected in the same way as in the standard- 


ization of the 2 iodine solution, described on p. 650. 


23. Determination of Antimony Trioxide Compounds. 


1000 c.c. . iodine solution = "720? — 7.21 g. Sb,O,;=6.01 g. Sb. 


The titration is carried out exactly as in the case of arsenious 
acid (cf. p. 650) except that tartaric acid, or Rochelle salt, must 
be added to the solution in order to prevént the precipitation of 
antimonous acid, or antimony oxychloride, as a _ result of 
hydrolysis. 

Examples: 


(a) Determination of Antimony in Tartar Emetic. 


If an aqueous solution of tartar emetic be treated with iodine in 
the presence of starch, the first few drops of reagent will impart a 
permanent blue color to the solution. If, however, a little sodium 
bicarbonate is added to the solution, the trivalent antimony is 
oxidized quantitatively to the pentavalent condition. 


K(SbO) C4H40¢ + 6NaHCOs + I> = 
= NasSbO,4,+2NalI + KNaC,H4,0¢ +3H20 + 6CO>. 





1 
1000 c.c. bs iodine solution= K(SbO)C4H406 + 3H20 sa 
10 20 
332.34 


8.309 gms. of tartar emetic are dissolved in water, the solution 
diluted to exactly 500 c.c. and well mixed. Of this solution, 20 
c.c. are removed by a pipette, diluted to 100 c.c., treated with 
20 c.c, of 2 per cent. sodium bicarbonate solution, and titrated with 


686, VOLUMETRIC ANALYSIS. 


tenth-normal isodine solution, using starch as an indicator. If 
ic.c. are used for the titration, the salt contains: 


1.6617 X25 xt 
8.309 





=5-t=per cent. tartar emetic, 

or | 

= 1.809-t=per cent. antimony. 
> 


(b) Determination of Antimony in Stibnite. 


Not over 0.5 gm. stibnite is dissolved in a small covered beaker 
by means of 10 c.c. concentrated hydrochloric acid (sp. gr. 1.2). 
The acid is allowed to act in the cold for about ten minutes, after 
which the contents of the covered beaker are heated gently on the 
water bath for ten or fifteen minutes. Three gms. of powdered 
tartaric acid are then added and the heating is continued for ten 
minutes longer, but care is taken not to allow the liquid to evapor- 
ate sufficiently to expose any part of the bottom of the beaker. 
When this precaution is taken, there is no volatilization of the 
antimony, and all of the hydrogen sulphide is expelled. 


Sb283 + 6HCl] = 2SbCl3 +3H2S8. 


The solution is now removed from the water bath, allowed to cool 
to the room temperature, and very cautiously diluted with water, 
which is added at first drop by drop, until a volume of about 100 
c.c. is obtained. If, in the meantime, a red coloration due to 
antimony sulphide appears during the dilution, the solution should 
be at once heated until it disappears, and the diluting then 
continued. | 
The diluted solution is nearly neutralized with ammonia, but 
is left slightly acid. The cold, slightly acid solution is poured into 
a 700 c.c. beaker containing 3 gms. of sodium bicarbonate dissolved 
in 200 c.c. of water, starch paste is added, and the solution titrated 
with iodine to the appearance of a permanent blue. 
- Remarks.—Antimony chloride is volatile with steam from its 
concentrated solutions, so that the solution should not be boiled 
until it has been diluted. The heating on the water-bath can be 


DETERMINATION OF ANTIMONY PENTOXIDE COMPOUNDS. 687 


carried out, however, without fear of losing antimony provided 
the acid is not allowed to evaporate to any extent. This heating 
serves to remove all the hydrogen sulphide which would otherwise 
precipitate the antimony as trisulphide upon diluting the solution. 
If insufficient tartaric acid is present, antimony oxychloride, 
SbOCI, precipitates and if the solution is titrated in this condition 
it is impossible to obtain a permanent end-point. Such a pre- 
cipitate may be filtered off, dissolved in concentrated hydrochloric 
acid and the solution treated by itself as above described. The 
value of the iodine solution in terms of Sb is given in the previous 
process (a). 


24. Determination of Antimony Pentoxide Compounds 
(A. Weller).* 


By heating a pentavalent antimony compound with concen- 
trated hydrochloric acid and potassium iodide in the Bunsen 
apparatus (Fig. 94, p. 662), the antimonic acid is reduced to. 
antimonous acid with separation of iodine: 


Sb205+ 4HI=Sb2034+2H20 4 2ls. 
The iodine is distilled over into potassium iodide solution and 


titrated with as Na2S2Q0z solution. The results are a little low. 


25. Determination of Hydrogen Sulphide. 


N » HS 34.09. 
1000 c.c. i0 Na,S,O,; solution =55 mee =1.704 gms. HS. 


If a solution of hydrogen sulphide is treated with iodine, it 
is oxidized with separation of sulphur: 
H,S+2I1=2HI+5S. 
For the determination of the atrount of the gas present in 
hydrogen sulphide water, a measured amount is transferred by 
* Annal., 213, 364. 





688 VOLUMETRIC ANALYSIS. 


N 
Bele 
the excess of the latter is titrated with thiosulphate solution.* 

If the amount of hydrogen sulphide present is not very large, 
correct results are obtained without difficulty. With consider- 
able hydrogen sulphide, on the other hand, the deposited sulphur 
is likely to enclose some of the iodine solution, as shown by its 
brown color; this iodine escapes the titration with thiosulphate. 
In such a case, the film of sulphur floating on the surface of the 
liquid is removed with a glass rod after the completion of the 
thiosulphate titration, transferred to a glass-stoppered cylinder, 
and shaken with 1-2 c.c. of carbon bisulphide. The latter dis- 
solves the iodine with a violet color and the color is discharged 
by the addition of sodium thiosulphate ‘solution.t In this way 
the total amount of the iodine that remains can be titrated. 

Remark.—This method can be used to advantage for deter- 
mining the sulphur present in soluble sulphides. The sulphides 
are decomposed as described on p. 350 by means of acid, and 
the hydrogen sulphide evolved is conducted into a definite amount 


means of a pipette to a known amount of — iodine solution and 


of af iodine solution. The excess of the latter is titrated as 


above with sodium thiosulphate solution. 


Determination of Hydrogen Sulphide in Mineral Waters. 


A measured amount of BF iodine solution and 2 gms. of potas- 


sium iodide are placed in a tall liter cylinder, 1000 c.c. of the water 
to be analyzed are added, and after thoroughly shaking, the excess 


of the iodine is titrated with a thiosulphate. The standardiza- 


* Correct results cannot be obtained by titrating directly with iodine, ef, 
O. Brunck, Z. Anal. Chem., 45, 541 (1906). 

+ The separation of the sulphur into a coherent film can be prevented 
by sufficiently diluting the solution with boiled water. O. Brunck (Z. anal. 
Chem., 45, 541) therefore, recommends using hundredth-normal iodine instead 
of tenth-normal solution, and this is certainly advisable in the case of small 
quantities of hydrogen sulphide as, for example, in a mineral water. On 
the other hand, when a relatively large volume of hydrogen sulphide is 
. liberated from a sulphide by means of acid (see page 350) it is advisable 
to use tenth-normal iodine, as otherwise the volume of solution will be too 
large unless a very small weight of substance is used in the analysis. 





ANALYSES OF ALKALI SULPHIDES. 689 


tion of the iodine solution used is accomplished by measuring 
off 10 c.c. of the solution, adding 2 gms. of potassium iodide, 


diluting to 1 liter with boiled water, and titrating with Ze thio- 


sulphate solution. 


26. Analysis of Alkali Sulphides. 


eee! ae aor. 

1000 c.c. i0 iodine solution= 30° 20 = 30 1.604 gms. S. 
_ A measured volume of the alkali sulphide solution is allowed 
to run slowly, with constant stirring, into a very dilute solution 
of iodine which is acid with hydrochloric acid. The excess of 
iodine is titrated with sodium thiosulphate solution. 


27. Analysis of a Mixture of Alkali Sulphide and Alkali 
Sulphydrate. 


If a solution of alkali sulphide and alkali sulphydrate is treated 
with an acid solution of iodine, the following reactions take place: 


(a) Na,S+2HC1=2NaCl+H,S,, 
(b) NaSH +HCl=NaCl+H,§, 
(c) H,S+L=2HI+S. 


It is evident from these equations that in the case of the 
sulphide, the quantity of hydriodic acid formed by the oxida- 
tion of the hydrogen sulphide is equivalent to the quantity of 
hydrochloric acid required to decompose the sulphide; in this 
case, therefore, the acidity of the solution remains unchanged. 
In the case of the sulphydrate, however, which is the acid salt 
of hydrosulphuric acid, the quantity of hydriodic acid formed is 
equivalent to twice the quantity of hydrochloric acid required to 
decompose the sulphydrate. Thus the acidity of the solution is 
a measure of the quantity of sulphydrate present. Moreover, 
if t c.c. of tenth-normal alkali is required to titrate this acid then 


690 VOLUMETRIC ANALYSIS. 


2t c.c. of tenth-normal iodine must have been required to oxidize 
the hydrogen sulphide from the sulphydrate. 

Procedure.—A known volume of tenth-normal iodine together 
with a known volume of tenth-normal hydrochloric acid * is 
diluted in a beaker to a volume of about 400 c.c. and the solution 
containing the sulphide and sulphydrate is added slowly from a 
burette, with constant stirring, until the solution becomes pale 
yellow. Starch indicator is addéd and the excess of iodine 
titrated with tenth-normal thiosulphate solution. Finally, the 
acid in the solution is titrated with tenth-normal sodium hydroxide 
solution. 

Computation.—In the analysis, the volumes of standard 
solutions used were: 7’ c.c. tenth-normal iodine; ¢, ¢.c. tenth- 
normal thiosulphate;, ¢, ¢.c. tenth-normal hydrochloric acid; 
t, ¢.c. of tenth-normal sodium hydroxide: and V c.c. of the 
sulphide mixture. 

Then (7'—t,) c.c.=total volume of iodine required, and 
(t,—t,) c.c.=the volume of sodium hydroxide required to neu- 
tralize the acid formed by the reaction with the sulphydrate. 
Then 


NaSH 
and 
, NaS 
[(T—t) -2(,—t) 1p =[(T—t) —2(4,—t)] 0.003903 gms, Na,S 


are present in V c.c. of solution. 





* The quantity of hydrochloric acid present must be sufficient to decompose 
all the sulphide and sulphydrate; an excess does no harm. 


DETERMINATION OF SULPHUROUS ACID. 691 


28. Analysis of a Mixture of Hydrogen Sulphide and Alkali 
Sulphydrate. 


The analysis is carried out exactly as in the case just described 
_and the computation is similar. 


Let T =e,c. > iodine, t,=c.¢. a thiosulphate, t,=c.c. 


acid, 
N ee | 
and t,=c.c. io alkali. Then 


10 


[(7—t,) — (.—#,)] 0.005608 gm. NaSH 
and 
[¢, +2t,— (7 +2t,)]<0.001701 gm. HS. 


Remark.—The last two methods of analysis are applicable 
‘only to solutions containing no other compounds decomposable 
by hydrochloric acid than sulphide and sulphydrate, and no 
other substance that will react with iodine. The analysis may 
be carried out without the addition of any hydrochloric acid. 
In this case the solution of sulphides is diluted to about 400 c.c., 
starch added, and the titration with iodine carried out directly. 
The hydriodic acid formed is titrated with caustic soda, using 
lacmoid * as indicator. The reactions are 


NaSH +2I,=NaI+HI+S, 
H.S+1,=2HI+S. 


29. Determination of Thiosulphate in the Presence of Sulphide 
and Sulphydrate. 


A measured volume of the solution is treated in a 200-c.c. 
graduated flask with an excess of freshly-precipitated cadmium 
carbonate. After shaking thoroughly, the liquid is diluted to 
the mark, filtered through a dry filter and 100 c.c. of the filtrate 





* Methyl orange can be used, but it is not so easy to distinguish the end- 
point. Phenolphthalein works well but no better than the lacmoid. Care 
should be taken to titrate the acid: against the alkali during the standardiza- 
tion at the same dilution as to be used in the analysis and an appreciable 
amount of carbonate should not be present. 


692 VOLUMETRIC ANALYSIS. 


titrated with iodine solution. By shaking with cadmium car- 
bonate, the sulphide. and sulphydrate are removed and the 
thiosulphate remains in solution. 


30. Determination of Sulphurous Acid. 


Wes : SO, 64.07 
1000 c.c. 10 iodine solution = 55° aT =3.203 gms. SO,, 
. 


The determination is based upon the following reaction: 
SO,+H,0+ 21 =2HI+S0,, 


the sulphurous acid being oxidized to sulphuric acid. If starch 
is added to a solution of sulphurous acid, and a titrated iodine 
solution is run into it from a burette, the blue color will not be 
obtained until all of the sulphurous acid has been acted upon. 
Bunsen, however, in 1854 showed that this sensitive reaction, 
which was first used by Dupasquier, will only take place quanti- 
tatively according to the above equation when the solution does 
not contain more than 0.04 per cent. by weight of SO,. With 
greater concentrations uniform results are not obtained. This 
irregularity was ascribed to the reversibility of the reaction, so 
that it was suggested that the titration be performed in alkaline 
solution,* thus removing the hydriodic acid as fast as it is formed. 
But the results then obtained are still inaccurate.t Finkener,t 
an the other hand, states that correct values will be obtained if 
the sulphurous acid is allowed to run into the iodine solution. 

J. Volhard § has confirmed the results of Finkener and shown 
that the anomalous results obtained on titrating sulphurous acid 
with iodine are not due to the reversibility of the reaction, for 
the direct addition of 20 per cent. sulphuric acid is without 

* Addition of MgCO,; or NaHCO, (Fordos and Gelis). 

+ E. Rupp, Ber., 35, 3694 (1902), states that it is possible to obtain good 
results by the method of Fordos and Gelis if the sulphurous acid is allowed 
to act for at least half an hour upon an excess of iodine in the presence of 
sodium bicarbonate. The solution is then titrated with sodium thiosulphate. 
According to E. Miller and O. Diefenthiler, however, this is theoretically in- 
correct, for the iodine tends to form a little hypoiodite: I,+ H,O@HI+HIO, 
and the latter reacts with sodium thiosulphate: Na,S,0,;+4HIO+H,0= 
Na,SO,+H,S0O,+4HI. ~ 

t Finkener-Rose, Quantitative Analyse (1871), p. 937. 

§ Ann. d. Chem. u. Pharm., 242, 94. 





" 


- DETERMINATION OF SULPHUROUS ACID. 693 


influence. The incomplete oxidation of. the sulphurous acid is 
caused by the fact that the hydriodic acid reduces a part of the 
sulphurous acid to free sulphur: * 


(1) SO,+4HI=41+42H,0+48. 


If sulphurous acid, whether dilute or concentrated, is allowed 
to run into a solution of iodine with constant stirring, there is 
complete oxidation of the SO,: 


(2) $O,+21+2H,0 =2HI+H,S0,. 


If, on the contrary, iodine solution is run into the solution 
of sulphurous acid, both reactions will take place: 


(3) 380,+4HI+2H,0=2H,S0,+4HI ++8. 


According to Raschig,t however, Volhard’s explanation is 
also incorrect, for he finds that no separation of free sulphur takes 
place if the iodine is allowed to act upon sulphur dioxide in a 
dilute solution. Raschig believes that the error that results 
when iodine is added to the sulphurous acid solution is due to a 
loss of SO, by evaporation. 

Correct results are always obtained if the sulphurous acid is 
added slowly, with constant stirring, to the iodine solution until 
the latter is decolorized. 

In the analysis of sulphites, the sulphite solution is added from 
a burette to the solution of iodine and hydrochloric acid. 





* If iodine solution is added slowly to a not too-dilute sulphurous acid 
solution, a distinct separation of sulphur is soon apparent. : 
+ The HI acts as a catalyser according to Volhard. 

t Z. Angew. Chem., 1904, 580. 


694, VOLUMETRIC ANALYSIS. 


31. Determination of Formaldehyde (Formalin). Method of 
G. Romijn.* 


HCHO 30.02 


5 5 =15.01 gms. formaldehyde. 





1000 c.c. N. iodine solution = 


Principle—Formaldehyde is quantitatively oxidized to formic 
acid by remaining in contact with iodine for a short time in 
alkaline solution: 


-HCHO+H,0+1,=2HI+HCOOH. 


Procedure.—The aqueous solution of formaldehyde, known com- 
mercially as ‘‘formaline,”’ contains about 40 per cent. of for- 
maldehyde. For analysis, 10 ¢.c. of the formaldehyde soiution are 
diluted to 400 c.c., and of this 1 per cent. solution, 5 c.c. (=0.125 e.e. 
of the original solution) are taken for analysis. 40 c.c. of a 
iodine solution are added, and immediately afterwards strong 
sodium hydroxide solution, drop by drop, until the color of the 
solution is a light yellow; it is then placed one side for 
ten minutes. The solution is then acidified with hydro- 
chloric acid, and the unused iodine is titrated back with = sodium 
thiosulphate solution. ) 


1.6. - iodine solution=0.001501 gm. formaldehyde. 


32. Determination of Hydroferricyanic Acid.+ 


De es, . _ _K,Fe(CN), 329.2 
1000 c.c. i0 iodine solution= 10 © sae 


Principle.—If a neutral solution of potassium ferricyanide is 
treated with an excess of potassium iodide, the ferricyanide ion 
is reduced to ferrocyanide ion with separation of free iodine: 

2. Fe(CN),+2KI22K,Fe(CN),+1,. . 


* Zeitschr. f. anal. Chem., 36 (1897), p. 19. 
+ Lenssen, Ann. Chem., 91, 240. Mohr., /bid., 105, 60. 





= 32.92 gms. K;Fe(CN).. 





DETERMINATION OF PHENOL, | 695 


Lenssen titrates the liberated iodine with sodium thiosulphate, 
but the results are not concordant, because the reaction is a 
reversible one. The reaction is quantitative, however, as Mohr 
first showed, if the ferrocyanide is removed from the solution as 
fast as it is formed. This is accomplished, according to Mohr, 
by adding an excess of zinc sulphate, free from iron, to the 
solution. According to the experiments of E. Miller and O. | 
Diefenthiler,* the titration should take place in a solution 
which is as nearly neutral as possible, but not in one made alkaline 
by the addition of sodium bicarbonate (see p. 692). 

Miller and Diefenthdler’s Procedure.—About 0.7 gm. of the 
_ ferrocyanide is weighed into a glass-stoppered flask, dissolved in 
about 50 c.c. water, and treated with 3 gms. potassium iodide 
and 1.5 gms. of zinc sulphate free from iron. If an acid solution 
of ferricyanide is to be analyzed, it is carefully neutralized with 
caustic soda until barely alkaline and then just acidified with 
a drop of sulphuric acid. Alkaline solutions must always be 
neutralized with acid. 


33- Determination of Phenol. Method of W. Koppeschaar.t 


N _C,H;OH 94.05 _ 
1000 c.c. io Na,S,0; = 66-7. Te 1.567 gms. C,H,OH. 





Principle.—If an aqueous solution of phenol is treated with 
an excess of bromine, the phenol is converted quantitatively into 
_tribromophenol: 


C,H,0H+3Br,=3HBr+C,H,Br,(OH). 


The tribromophenol is a pale yellow, crystalline substance which 
is quite insoluble in water (43,700 parts of water dissolve 1 part 
of tribromophenol). If, after the reaction has taken place, 
potassium iodide is added to the solution, iodine is liberated 
corresponding to the excess of bromine, and by titrating this 





* Z. anorg. Chem., 1910, 418. 
+ Z. anal. Chem., 15, 233 (1876). 


696 : VOLUMETRIC ANALYSIS. 


iodine with sodium thiosulphate solution, it is easy to find how 
much bromine reacted with the phenol. 

Requirements.—A tenth-normal solution of bromine and a 
tenth-normal solution of sodium thiosulphate. 

On account of the volatility of free bromine, Koppescharr 
uses a solution of potassium bromate and bromide which, upon 
being acidified, gives a known amount of bromine in accordance 
with the equation: 2 


KBrO, +5KBr + 6HCl=6KC1+3H,0 + 3Br,. 


Thus, to obtain a tenth-normal solution of bromine which will 
keep indefinitely, exactly 2.784 gms. of pure potassium bromate 
(dried at 100°) and about 10 gms. of potassium bromide are 
dissolved in water and the solution diluted to one liter. An 
excess of bromide does no harm: 


KBrO, _ 167.02 








ts ap 72-784, 
5KBr_ 5X119.02_ 
aan yet als 


Procedure.—About 0.5 gm. of phenol is weighed out in a 
weighing beaker, dissolved in a little water, the solution rinsed 
into a liter flask and well shaken. Of this solution, 100 c.c. are 
withdrawn in a pipette, transferred to a second liter flask, diluted 
with water up to the mark, mixed and 170 c.c. this solution 
transferred to a stoppered bottle of about 250-c.c. capacity, 
treated with 50 c.c. of the bromate solution, shaken, acidified 
with 5 c.c. concentrated hydrochloric acid, shaken again, and 
allowed to stand fifteen minutes. At the end of this time, 2 gms. 
of potassium iodide are added and the liberated iodine, corre- 
sponding to the excess of bromine, is titrated with tenth-normal 
thiosulphate solution, using starch as indicator. Then if ¢ ¢.c. 
of the last solution are used and the weight of phenol was a gms.: 


(50 —t) X 0.1567 


2 = % phenol. 





REDUCTION METHODS. 697 


Remark.—Before making an analysis, a blank experiment 
should always be made with the bromate solution to make sure 
that its strength corresponds to the theoretical value. 

This method is suitable for the analysis of pure preparations 
of phenol (carbolic acid) but not for crude phenol, creosote oil, 
etc.* 


B. REDUCTION METHODS. 


1. Determination of Ferric Iron (Fresenius).+ 


In the case of all methods previously discussed, it was 
necessary to reduce the iron to the ferrous condition before it 
could be determined volumetrically. In the following method, 
first suggested by Penny and Wallace,t but improved by Fresenius, 
the iron in the ferric condition may be determined with accuracy 
and rapidity. , 

The hydrochloric acid solution containing ferric chloride is 
titrated hot with stannous chloride solution until the former 
becomes colorless. By this means the ferric salt will be reduced 
to ferrous salt: 


2FeCl, + SnCl, = SnCl, + 2FeCl,. 


Inasmuch as it is not very easy to determine the end-point 
with accuracy, because the last part of the iron is reduced very 
slowly, it is customary to run over the end-point and to titrate 
the excess of the stannous chloride. with iodine solution. 

Solutions Required. 1. A Ferric Chloride Solution Containing a 
Known Amount of Iron.—It is prepared by dissolving exactly 
10.03 gms. of bright iron wire in hydrochloric acid within a long- 
necked flask held in an inclined position; the iron is oxidized 
with potassium chlorate and the excess of chlorine is completely 





* J. Toth, Z. anal. Chem., 25, 160 (1886). 
t Z. anal. Chem., 1, p. 26. 
t Dingl. polyt. J., 149, 440. 


698 VOLUMETRIC ANALYSIS. 


expelled by boiling. The solution of ferric chloride is washed 
into a liter flask and diluted up to the mark with water; 50 c.c. of 
this solution contain 0.5 gm. of pure iron.* 

2. A Stannous Chloride Solution.—25 gms. of tin-foil are heated 
for two hours on the water-bath with 50 c.c. of hydrochloric acid 
of specific gravity 1.134 and a few drops of chloroplatinic acid 
_ in a porcelain dish which is covered with a watch-glass. After this, 
150 ¢.c. of hydrochloric acid and afi equal volume of water are 
added, the solution filtered and diluted up to 1 liter. As stan- 
nous chloride is oxidized by contact with the air, it is placed 
in a flask which on one side is connected with the burette as shown 
in Fig. 87, p. 556 and on the other side with a Kipp carbon di- 
oxide generator. 

3. An Iodine Solution Approximately Tenth-normal. 

Procedure.—(a) Standardization of the Solutions. 

First of all, the stannous chloride and iodine solutions are 
titrated against one another. About 2 c.c. of the former are 
measured from the burette, diluted to about 60 c.c., a little starch 
solution added, and the mixture titrated with iodine until a 
blue color is obtained. 

Next, 50 c.c. of the acid ferric chloride solution containing 
a known amount of iron are titrated against the stannous chloride 
solution. 

(b) Determination of Iron in Hematite. 5 gms. of the 
finely-divided ore are ignited in order to destroy any organic 
matter which may be present, then placed in a long-necked flask 
and boiled with concentrated hydrochloric acid and a little potas- 
sium chlorate until the iron oxide is all dissolved, leaving behind 
nothing but a white sandy residue. After this 20 ¢.c. more of 
hydrochloric acid are added and the boiling is continued while 
a current of air is passed through the solution, until all the excess 
of chlorine is completely removed and the escaping vapors will 
no longer set free iodine when passed into a potassium iodide 
solution. The solution thus obtained is diluted to exactly 500 c.c. 
and 50 c.c. of it are taken for the analysis. 





* The assumption being made that the iron wire contained 99.7 per cent 
pure iron. 


DETERMINATION OF HYPOCHLOROUS ACID. Ogy 


Example. 


1. Standardization of the reagents: 

2 c.c. of stannous chloride solution require 

7.2 c.c. of iodine solution. 1 c.c. iodine solution =0.278 c.c. SnCl, 
50 ¢.¢. ferric chloride solution (=-0.5 gm. iron) 








Poquire tor Gecolorization - oe ee nee 30.34 ¢.c. SnCl, 
and for the titration of the excess 0.51 c.c. of 

lodine solution = 0.51 0.28... sis. hes ean. 0.14 c.c. SnCl, 
Consequently, 50 ¢.c. ferric chloride solution : 

Moen BEE 6 as. Fo, Re a =30.20 c.c. SnCl, 

and 1 ¢.e. SnCl,= 37>, =0.01656 em. Fe. 

2. Trtration of the solution to be analyzed: 
50 ¢.c. (=0.5 gm. of iron ore) require......... 18.96 c.c. SnCl, 
and for the titration of the excess, 0.64 c.c. of 

posting OBA O28 6 ioral s Laeins ch seen = 0.18 c.c. SnCl, 
so that 0.5 gm. of ore corresponds to.......... 18.78 c.c. SnCl, 


and contain, therefore, 18.78 x 0.01656 =0.3110 gm. Fe, 
and in per cent.: 
0.5:0.3110=100:2z 
x= 62.20 per cent. Fe. 


2. Determination of Ferric Iron by Means of Titanous Chloride 
(Knecht and Hibbert).* 


N F 
1000 c.¢. 10 Ticl, solution =. =0.8 g. oxygen =~ =5.585 g. Fe. 


Principle.—lf an acid solution of a ferric salt is treated with 
titanous chloride, the iron is immediately reduced in the cold to 
the ferrous condition: 


FeCl; + TiCls = TiCl, + FeCl. 
Preparation of Titanous Chloride Solution.—A concentrated 


solution of titanous chloride, prepared by the electrolysis of TiCl,, 
can now be obtained on the market. Such a solution is treated 





*Ber. 36, 1551 (1903). 


JOO VOLUMETRIC ANALYSIS. 


with an equal volume of concertrated hydrochloric acid, boiled,* 
and. then diluted with ten times as much boiled water. 

The solution is maintained in contact with an atmosphere of 
hydrogen, or carbon dioxide, and kept in a bottle such as is shown 
in Fig. 87, p. 556 which is connected with a burette, and in this 
case with a Kipp hydrogen, or carbon dioxide, generator instead of 
the soda lime tube. 

Standardization of the Titanou¥ Chloride Solution.—A ferric 
chloride solution known of strength is prepared as described on 
p. 697, and of this solution 50 c.c. are measured out into a beaker, 
and the titanium trichloride is slowly added with constant stirring, 
while a current of carbon dioxide is constantly being passed into 
the beaker. After the solution is early decolorized, a drop of 
potassium sulphocyanate solution is introduced, and the addition 
of titanous chloride is continued to the disappearance of the 
red color. 

The analysis proper is carried out in exactly the same manner. 


3. Determination of Ferrous and Ferric Iron by the Titanium 
Method. 


The ferrous iron is first titrated by means of permanganate in 
the presence of manganous sulphate (cf. p. 607) and then the total 
iron is determined as above with titanous chloride. 

The method can be carried out very rapidly and the results are 
accurate. | 

4. Determination of Hydrogen Peroxide.} 

If titanous chloride is run into an acid solution of hydrogen 
peroxide, the latter is colored first yellow, then a deep orange, and 
as soon as the maximum depth of color is produced, it begins 
to fade upon the further addition of titanous chloride until finally 
the solution becomes colorless, which is taken as the end-point. 

The reaction takes place in two stages: 

Tig03 +3H202=2Ti03+3H2O 
2Ti03 + 2Ti203=6Ti02 
or combining the two equations: 
2TiCl3 + H2O2 + 2HCl=2TiCl, + 2H,20. 


* The boiling serves to expel any hydrogen sulphide that is present. 
¢ Knecht and Hibbert, Ber., 38, 3324 (1905). 





DETERMINATION OF FERRIC IRON BY TiC\. 7O1 


On account of the fact that the value of the titanous chloride 
solution is not very permanent, it is standardized against ferric 
chloride before each series of experiments. 

If ¢ c.c. of titanous chloride solution of which 1 c.c.=a gms. Fe 
were required for the reduction of 1 ¢.c. of hydrogen peroxide, then 
the amount of the latter is 

2Fe:H20o=at:2x 
ne - oie gms. HO 
and in per cent. 
30. 46at=per cent. H20. 


If it is desired to express the per cent. in per cent. by volume 
of active oxygen (cf. p. 628) the following proportion holds: 


10023-at=per cent. oxygen by volume. 


According to Knecht and Hibbert,* persulphuric acid may 
likewise be estimated by titration with titanous chloride. The 
solution of the persulphate is treated with titanous chloride 
solution and the excess of the latter is titrated with ferric chloride 
in an atmosphere of carbon dioxide. 


5. Determination of Hypochlorous Acid by Means of Arsenious 
: Acid. 


1000 c.c. = As,O,= 3.546 gms. chlorine. 


On adding arsenious acid to a solution of a hypochlorite, the 
former is oxidized to arsenic acid, while the latter is reduced to 
chloride: 

2NaOCl-+ As,O,=As,O,; + 2NaCl. 


The end-point is reached when a drop of the solution added 
to a piece of iodo-starch paper will cause no blue coloration. 

Alkali hypochlorites and chloride of lime may be analyzed 
by this method and the results obtained are more reliable than 
in the case of those obtained by the iodimetric method described 
on p. 669, for the presence of chlorate has no effect in this case. 


* Knecht and Hibbert, Ber. 38, 3324 (1905). 





702 VOLUMETRIC ANALYSIS. 


III, PRECIPITATION ANALYSES. 
1. Determination of Silver. Method of Gay-Lussac. 


This exceedingly accurate detewmination, which is extensively 
used for testing silver alloys, depends upon the precipitation of 
silver chloride from nitric acid solution. Common salt is used 
as the precipitant. 

Solutions Required. 1. Sodium Chloride Solution of Known 
Concentration.—For convenience, it is customary to make the 
solution of such a strength that 1000 c¢.c. correspond to exactly 5 
gms. of silver. It is more practical, however, to use a some- 
what weaker solution, consequently 2.700 gms. of chemically pure 
salt are dissolved in distilled water and diluted to 1 liter. 

. 2. Decimal Solution of Sodium Chloride.—100 c.c. of the above 
solution are diluted with distilled water to 1 liter. 

In laboratories where silver determinations are frequently 
made, the above solutions are made up in much larger quanti- 
ties and kept in bottles similar to the one shown in Fig. 87, p. 556. 
The stronger solution is connected with a 100-c.c. pipette and 
the decimal solution with a burette. 

Standardization of the Sodium Chloride Solution. — Exactly 
0.5 gm. of chemically pure silver is weighed into a 
200-c.c. flask provided with a well-ground glass stopper, and 
dissolved in 10 c.c. of nitric acid of specific gravity 1.2, free 
from chlorine. The solution is hastened by heating on a sand- 
bath. When the silver has dissolved, the solution is heated to 
boiling in order to expel the nitrous acid formed. The brown 
vapors collecting in the flask are removed by blowing in air. 
As soon as no more of these are formed, the flask is 
removed from the sand-bath, and allowed to cool. To the 
silver sclution exactly 100 c.c. of the stronger salt solution are 
added, the flask stoppered, and vigorously shaken until the pre- 
cipitated silver chloride collects together, and the supernatant 
liquid appears clear. ; 


.-RMINATION OF ‘SILVER. 793 


As the salt solution was made up a little weak, the precipita- 
tion of the silver is not quite complete and consequently more. 
sodium chloride must be added. For this purpose half a cubic 
centimeter of the decimal salt solution is added from the burette, 
so that the solution runs down the sides of the flask upon the 
surface of the liquid, causing a distinct cloud of silver chloride 
to be formed. The liquid is shaken, allowed to settle, again 
treated with half a cubic centimeter of the decimal salt solution, 
and the process repeated until finally the addition of the salt 
solution fails to produce any further turbidity; the last half cubic 
centimeter is not used in the calculation. 

Example.—0.5 gm. of chemically pure silver (490° fine) re- 
quired 100 c.c. of the standard salt solution+1 c.c. of the decimal 
solution, i.e., 100.1 ¢.c. of the salt solution correspond to 1000 
silver; * this is the value of the salt solution. 

Silver Determination.—In order to obtain absolutely accurate 
results it is necessary to employ the same amount of silver for 
the analysis as was used in the standardization of the solution, 
consequently the approximate amount of silver present in the alloy 
must be determined. This can be accomplished by cupellation, 
or volumetrically by the method of Volhard, described further on. 

Example.—It was found by cupellation that an alloy contained 
about 82° fine silver; for the titration an amount must be taken 
which will contain 0.5 gm. of silver; we have then 


1:0:8=2:0.5 
x=0.625 gm, 


We weigh out, therefore, 0.625 gm. (=1250 f) of the alloy and 
proceed exactly as in the standardization. 

1250 of alloy require for the precipitation of the silver 100 c.c. 
of the standard salt solution+3 c.c. of the decimal solution, Le., 
1250 parts of the alloy require 100.3 ¢.c. of the standard salt 





* For convenience in calculation, 0.5 gm. of pure silver is designated 
by 1000, 0.25 gm. by 500, and 0.1 gm. by 250, etc. 
¢ If 0.5 gm.=1000, then 0.5:1000=0.625:2; x=1250. 


704 _ VOLUMETRIC ANALYSIS. 


solution. Since 100.1 c.c. of this salt solution correspond to 
1000 parts of pure silver, we have 


100.1: 1000=100.3:2; 


=e a 1002 parts silver in 1250 parts of alloy; 


so that in 1000 parts of the alloy there will be 





1250: 1002 = 1000: 
x=801.6 parts fine of silver. 


This procedure is designated as the French method in contrast 
to the German or Dutch method. In the latter case, 0.5 gm, 
of the alloy (=1000) is weighed out and the same amount of 
silver is added which the alloy lacks in fineness. In this way 
one more weighing is necessary, but the calculation is somewhat 
simpler. 

Example.—By cupellation an alloy is found to contain 8,%% 
silver. In order to make the silver equal 1000, 200 parts of 
fine silver must be added. For the analysis, therefore, 0.5 gm. 
of the alloy and 0.1 gm. of pure silver (=200) are taken, dis- 
solved in nitric acid, and titrated with sodium chloride. 

For the titration of the alloy, 100.25 ¢.c. of the stronger 
salt solution were required, or of the decimal solution, 1002.5 c.c. 
and for the titration of 1000 fine silver (0.5 gm.)....... 1001.0 c.e. 





Differenoe. ¢ 25. 2205684 5s Se cee = Lbés4: 


As 1 c.c. of the decimal solution corresponds to zjy7* silver, 
it is evident that 1.5 ¢.c. are equivalent to =1,8, silver. If this 
amount is added to the assumed silver contents (in this case 800), 
the true fineness of the silver alloy will be obtained; i.e. 801.5 
parts fine silver. 





* 100.1 ¢.c. of the stronger salt solution=5 gms. }9°° silver, then 1001 c.c. 
of the decimal solution correspond to the same amount, and 1 c.c.=y 5, 
silver. 


DETERMINATION OF SILVER. 795 


2. Determination of Silver (Volhard). 


N _ Ag _ 107.88 | 
1000 c.c. io KCNS="5 = Jo = 10-788 gms. Ag. 

If to a silver solution containing iron ammonium elum, free 
from chloride but containing enough nitric acid to discharge the 
- brown color of the iron salt, a solution of alkali sulphocyanate is 
added, white insoluble silver sulphocyanate is precipitated: 


AgNO,+ KCNS=KNO,+ AgCNS. 


When all the silver is precipitated, the next drop of the sulpho- 
cyanate solution will cause a permanent red coloration due to the 
formation of ferric sulphocyanate. 

Requirements. 1. Tenth-normal Silver Solution.—10.788 gms. 
of chemically pure silver are dissolved in nitric acid free from chlo- 
ride, boiled until the nitrous acid is all removed, and diluted 
with distilled water to a volume of 1 liter. 

2. Tenth-normal Potassium (or Ammonium) Sulphocyanate Solu- 
tion.—As both of these salts are hygroscopic and cannot be dried 
without decomposition, an exactly tenth-normal solution cannot 
be prepared by weighing out the solid salt. Approximately, the 
right amount (about 10 gms. KCNS or 9 gms. NH,CNS) is dis- 
solved in a liter of water and the solution standardized against 
the silver solution. 

3. Iron-ammonium Alum Solution.—A cold, saturated solution 
of ferric alum to which enough nitric acid is added to cause 
the disappearance of the brown color. Of this indicator the 
same amount is used for all titrations, about 1 or 2 e.c. for 100 c.e. 
of the silver solution. 

For the standardization of the sulphocyanate solution, 20 c.c. 
of the silver solution are placed in a beaker, diluted with about 
50 c.c. of water, and 1 ¢c.c. of the indicator added. The sulpho- 
cyanate solution is then added from a burette, with constant 
stirring, until a permanent red color is obtained. 


706 7 VOLUMETRIC ANALYSIS. 


Determination of Silver in Silver Alloys. 


About 0.5 gm. of the brightly polished metal is dissolved in 
nitric acid of specific gravity 1.2, the solution boiled to expel 
the nitrous acid, diluted with cold water to about 50 e.c., and 
after the addition of 1 ¢.c. of the ferric alum solution it is titrated 
with the sulphocyanate solution as@in the standardization of the 
latter. The presence of metals whose salts are colorless does 
not influence the accuracy of this determination, except tha: 
mercury must be absent because its sulphocyanates are insol- 
uble. Nickel and cobalt must not be present to any extent, 
because their salts are colored, and not more than 60 per cent. 
of copper in an alloy is permissible. In case more copper is present 
the following procedure must be used: The silver is precipitated 
by means of an excess of alkali sulphocyanate, washed completely 
with water, the funnel placed over an Erlenmeyer flask, the apex 
of the filter broken, its contents washed into a flask by means of 
concentrated nitric acid (sp. gr. 1.4), and the liquid heated to gentle 
boiling for three-quarters of an hour. As the sulphuric acid 
formed will have some influence upon the subsequent titration, 
the solution is diluted with water to about 100 c.c., and a con- 
centrated barium nitrate solution is added drop by drop until 
the sulphuric acid is all precipitated, after which the silver is 
titrated with sulphocyanate solution without filtering off the 
barium sulphate. 

Remark.—From experiments in his laboratory carried out by 
Osann, the author concludes that the Volhard method is less 
reliable than that of Gay-Lussac. Apparently the experiments 
of Hoitsema * indicate that the precipitate adsorbs potassium 
thiocyanate. If, however, the solution is standardized against 
very nearly the same quantity of silver (or the equivalent amount 
of silver nitrate) as is taken for analysis, this error is compensa 
and the results are very exact.—(TRANSLATOR.) 





* Z. angew. Chem., 1904, 647. 


Fa 


DETERMINATION OF CHLORINE. 197 


ae Determination of Chlorine. 
(a) Volhard’s Method. 


1000 c.c. a AgNO, solution =$1 =3.546 gms. chlorine. 


_ According to Volhard’s original directions, the chloride solution 
was treated with tenth-normal silver nitrate solution and then, 
without filtering off the precipitate, 5 c.c. of the ferric-ammonium 
alum solution were added and the excess of silver titrated 
with tenth-normal potassium or ammonium thiocyanate (see 
p. 705). 

The results are satisfactory with large quantities of chloride, 
but in the titration of small quantities of chloride too high 
results are obtained, as was first shown by G. Drechsel * and 
later confirmed by M. A. Rosanoff and A. E. Hill.t Drechsel 
showed that it was impossible to get the true end-point of the 
reaction, as the red coloration gradually disappeared on stirring, 
remaining permanent only after a considerable excess of thio- 
cyanate had been added. The reason for this is that silver 
chloride is more soluble than silver thiocyanate. Thus the 
precipitate gradually reacts with the red ferric thiocyanate, as 
follows: | 

3A¢gCl + Fe(CNS),=3AgCNS + FeCl,. 


To avoid this error Drechsel proceeds as follows: 

The chloride solution is placed in a 200-c.c. graduated flask, 
‘an excess of 0.1N AgNO, solution added, the solution acidified 
with nitric acid, and the stoppered flask shaken until the pre- 
cipitate coagulates enough to give a clear supernatant liquid. 
The solution is then diluted up to the mark, thoroughly mixed 
and filtered through a dry filter, rejecting the first 10 c.c. of 
filtrate. Of the filtrate, 50 or 100 c.c. are taken, the ferric alum 
indicator added, and the excess of silver titrated with 0.1N 
thiocyanate solution. The results thus obtained are excellent. 





* Z. anal. Chem., 16, 351 (1877). 
+ J. Am. Chem., Soc. 29, 269. 


708 VOLUMETRIC ANALYSIS. . 


Remark.—V. Rothmund and A. Burgstaller * find that it is 
possible to obtain correct results without filtering off the silver 
chloride precipitate. They heat the solution after the addition 
of the excess of silver nitrate, until the precipitate coagulates 
thoroughly, in which form it reacts less readily with a soluble 
thiocyanate. After cooling, the ferric alum indicator is added 
and the titration finished. Rothmund and Burgstaller also find 
that the coagulation of the silver thloride precipitate by ether Tt 
suffices to make the filtration unnecessary. The chloride solution 
is placed in a flask with tightly fitting glass stopper, 5 ¢.c. of 
ether added, and an excess of silver nitrate solution. After 
shaking a few minutes, the supernatant solution becomes clear 
and the titration can be finished with accuracy. 


(b) Fr. Mohr’s Method. 


If the neutral solution of an alkaline or alkaline-earth chloride 
containing a few drops of potassium chromate solution * is treated 
with silver nitrate solution, added from a burette, a red precipi- 
tate of silver chromate is formed which, on stirring, disappears on 
account of its being decomposed by the alkali chloride to silver — 
chloride and alkali chromate: 


Ag,CrO,+2NaCl=2AgCl+ Na,CrO,. 


When all of the chlorine is changed to insoluble silver chloride, 
the next drop of the silver solution will impart a permanent 
reddish color to the liquid. For small amounts of chloride in 
concentrated solutions this method gives very sharp results. If, 
however, the volume of the solution is too large, the results are 
not very accurate. In all cases, a blank experiment must be 
made to see how much of the silver solution is necessary to 
produce the red shade used in the titration when no chloride is 
present, and this amount: must be deducted from that used in 
the analysis. 





* 7. anorg. Chem., 68, 330 (1909). 

+ Cf. E. Alefeld, Z. anal. Chem., 48, 79 (1909). 

+ Lunge uses sodium arseniate as indicator, and this is to be reeommended 
on account of the change from colorless to brown being very easy to detect. 


DETERMINATION OF BROMINE AND IODINE. 709 


Remark.—lf it is desired to titrate free hydrochloric acid, 
the solution is first neutralized with ammonia. In the case of 
colorless chlorides having an acid reaction (AICI,) the solution is 
treated with an excess of neutral sodium acetate solution and 
then titrated. With colored metal chlorides, the metal is pre- 
cipitated with caustic potash or sodium carbonate, filtered, 
washed, the filtrate acidified faintly with acetic acid, and the 
titration then made. 


4. Determination of Bromine. 


(a) Volhard’s Method. 


Br | 


1000 c.c. = AgNO, solution = 


10 7.992. gms. bromine. 


The solution of the bromide is treated with an excess of 
0.1N silver solution and the solution titrated with ammonium 
thiocyanate, using ferric alum as indicator. From the required 
volume of silver nitrate, the quantity of bromine is com- 
puted. 

Remark.—lt is not necessary to filter off the silver bromide, 
because, unlike the chloride, silver bromide is more insoluble 
than is silver thiocyanate. 


(b) Fr. Mohr’s Method. 


The procedure is the same as in the case of the chloride deter- 
mination. 


5. Determination of Iodine. 


Volhard’s Method. 


1000 c.c. a3 AgNO, solution = = 12.692 gms. iodine. 


If silver iodide is produced in a solution of an iodide by the 
addition of silver nitrate, the precipitate will usually enclose a 


710 : VOLUMETRIC ANALYSIS. 


measurable amount of either the soluble iodide or the silver nitrate, 
so that the analysis cannot be accomplished in the same way as 
in the analysis of chlorides and bromides. 

The solution is placed in a glass-stoppered flask, diluted to 
200-300 c.c., and the silver solution is added with vigorous shak- 
ing until the yellow precipitate col ects together and the superna- 
tant liquid appears colorless. As ‘ong ds the solution appears milky 
the precipitation is not complete. little more silver nitrate is 
final'y added and the solution again shaken in order to precipitate 
any iodide in the pores of the silver iodide. Then ferric alum 
solution * is added, the excess of silver titrated with potassium 
sulphocyanate, and the iodine calculated from the amount of 
silver used. In this way Volhard obtained exact results. 


6. Determination of Cyanogen. 


(a) Volhard’s Method. 


@; 


) 


N 
5 =6.511 gms. KCN. 





K 
1000 c.c. as AgNO, solution = 


If an excess of silver nitrate is added to a solution containing 
potassium cyanide and we attempt to titrate the excess of the 
forner by means of potassium sulphocyanate, using a ferric salt 
as an indicator, there will be no distinct end-point, because the 
silver cyanide reacts with the ferric sulphocyanate: 


3AcCN-+ Fe(CNS), + 3HNO,=3AgCNS + 3HCN + Fe(NO,),. 


fhe red color obtained in the titration will disappear on stirring, 
If, however, the neutral cyanide solution is treated with an excess 
of the silver solution, then slightly acidified with nitrie acid, di- 
luted up to a definite volume in a measuring-flask and filtered 
through a dry filter, the excess of silver can then be titrated in 
an al quot part of the filtrate. 





* The ferric solution must not be added before the iodine is completely 
precipitated, because in acid solution it oxidizes the hydriodic acid with 
separation of iodine. Silver iodide, however, is without action on ferric 
saits. 


aed 


DETERMINATION OF CHLORINE AND CYANOGEN. 7I1 


(b) Liebig’s Method.* 


N 
1000 c.c. 1AeNOs solution = 





=13.022 gns. KCN. 


On adding silver nitrate solution drop by drop to a neutral 
or alkaline alkali cyanide, a white precipitate is formed when 
the two liquids first come in contact with one another, but on 
stirring it redissolves owing to the formation of potassium silver 
cyanide: 

AgCN + KCN = Ag(CN),K. 


As soon as all of the cyanogen is transformed into potassium 
silver cyanide, the next drop of the silver solution will produce 
a permanent turbidity: | 


Ao(CN),K+AgNO,=KNO,+2AgCN. 
The total reaction is, therefore, 
2KCN + AgNO, =KNO,+Ag(CN),K. 


1 Ag corresponds to 2 CN and the end-point of the reaction 
is shown by the formation of a permanent precipitate. 

The alkali cyanide solution is placed in a beaker, a little potas- 
sium hydroxide is added, and the solution diluted to a volume 
of about 100 c.c. The beaker is placed upon a piece of black 
glazed paper and titrated with constant stirring until the tur- 
bidity is obtained. 

For the analysis of free hydrocyanic acid, the solution is satu- 
rated with potassium hydroxide and treated as above. 


Determination of Chlorine and Cyanogen in the Presence 
of One Another. 


First, the cyanogen is determined by the method of Liebig, and 
then enough silver solution is added to convert all of the cyanogen 
and chlorine into their silver salts. The solution is acidified 
with nitric acid, diluted with water to a definite volume, filtered 





_—— 


* Ann. d. Chem. und Pharm., 77, p. 102. 


712 VOLUMETRIC ANALYSIS. 


through a dry filter, and an aliquot part of the filtrate used. for 
the titration of the excess of silver by means of potassium sulpho- 
cyanate, according to Volhard. The calculation of the cyanogen 
and chlorine is illustrated by the following example: 


10 c.c. of the solution required for the production of a per- 
manent turbidity ¢ ¢.c. - silver solution. Then an excess of af 
silver solution is added (7 ¢.c. being*the total amount used), the 
solution acidified with nitric acid, diluted to exactly 200 c.c.,* 
filtered through a dry filter, and the excess of the silver titrated 
in 100 c.c. of the filtrate; this required ¢, c.c. al potassium sulpho- 
cyanate solution. Consequently the amount of cyanogen pres- 
ent is ¢ 0.005202 gm., and the chlorine present amounts to 
[7’— 2(¢+t,)]0.003546 gm. 


7. Determination of Sulphocyanic Acid. Volhard’s Method. 


1000 c.c. ju AgNO, solution FT id 


i0 =5.909 gms. HCNS. 





This is the reverse of the silver determination (p. 705). An 


et foe : ws 
excess of 0 silver solution is added to the solution containing 


0 
the sulphocyanate, and the excess of silver is titrated with potas- 
sium sulphocyanate solution, using ferric alum as an indicator. 


Determination of Sulphocyanic and Hydrocyanic Acids in the 
Presence of One Another. 


A little potassium hydroxide is added to the solution, and 
after diluting to about 100 c.c., the cyanogen is titrated by the 
method of Liebig (p. 711). Then, after adding an excess of si'ver 
solution, nitric acid is added to acid reaction, and the excess of 
the silver is titrated with potassium sulphocyanate in an aliquot 
part of the filtrate. 





* The operation is performed in a measuring-flask. After the addition 
of the acid, the flask is filled up to the mark with water, thoroughly mixed, 
and then filtered. 


DETERMINATION OF HYDROCHLORIC ACID, ETC. 713 


Determination of Hydrochloric, Hydrocyanic, and Sulphocyanic 
Acids in the Presence of One Another. 


In one portion the cyanogen is determined according to Liebig. 
A second portion is treated with an excess of = silver solution, 
acidified with nitric acid, filtered, the precipitate washed with 
water, and the excess of silver in the filtrate determined according 
to Volhard. The filter containing the precipitate is washed by 
means of concentrated nitric acid into a flask and boiled for 
three-quarters of an hour. By this means the cyanide and sulpho- 
cyanate of silver go into solution, while the silver chloride 
remains undissolved. The solution is diluted to about 100 e.c., 
a sufficient amount of barium nitrate is added to precipitate the 
sulphuric acid formed, and the silver corresponding to the cyanide 
and sulphocyanate is titrated with potassium sulphocyanate with- 
out filtering off the silver chloride or barium sulphate. 

The calculation is accomplished as follows: 

1. For the titration of the cyanide in alkaline solution, t ¢.c. » 
silver solution were necessary, and for the precipitation of the 
same amount of cyanogen in acid solution 2 ¢ c.c. 2 silver solu- 
tion were required. 

2. For the precipitation of the chlorine+cyanogen-+sulpho- 


ites. sage 
cyanogen in acid solution, 7’ c.c. of — silver solution were used. 


10 
3. Finally, ¢, c.c. * KCNS solution were used for the precipi- 
tation of the silver cyanide+sulphocyanide. 


Then 

1. Cyanogen =¢<0.005202 gm. CN. 

2. Sulphocyanogen = (¢; — 2t) X 0.005808 gm. CNS. 
3. Chlorine =(T—t,) X0.003546 gm. Cl. 


714 VOLUMETRIC ANALYSIS. 


8. Determination of Sulphuric Acid by Benzidine Hydrochloride.* 


H,SO, 98.086 


a 30 = 4.904 gms. H,SO,,. 





N 
.c. — NaOH= 
1000 c.c 10 a 


Benzidine, Cj2gHg(NHe)2, is a weak organic base. It forms 
stable salts with strong mineral acids, of which the sulphate is 
characterized by its slight solubility, particularly in water con- 
taining hydrochloric acid. The base itself is neutral toward 
phenolphthalein. On account of being such a weak base, there- 
fore, the aqueous solutions of its salts undergo hydrolysis. Thus 
benzidine hydrochloride is decomposed according to the equation: 


Cy2Hg(N He) 2° 2HCl+ 2H,0@2HCl + Ci2Hs (N Hoe) 2 (HOH). 


into hydrochloric acid and benzidine hydroxide, and the latter 
breaks down further into benzidine and water: 


C12Hs(NH2)2(HOH)2—>Cy2Hg(NH>)> +2H.0. 


In other words, an aqueous solution of benzidine hydrochloride 
behaves like a mixture of hydrochloric acid and benzidine, and 
the amount of acid present may be titrated with alkali, using 
phenolphthalein as indicator. 

There are two methods which have been used for the volumetric 
estimation of sulphuric acid by means of benzidine. Miiller treated 
the neutral solution of the sulphate with a solution of benzidine 
hydrochloride of known acidity. 


CioHs (N Ho) 2° 2HC1+ NaeSO4 = 2NaCl + CioHs (N Ho) 2° H2SOxg. 


—~— 





Insoluble 


The precipitate of benzidine sulphate was filtered off and the- 
filtrate titrated with a standard solution of alkali. The loss in 





+ W. Miller, Ber., 35, 1587 (1902); Miller and Diirkes, Z. anal. Chem., 
42, 477 (1903); F. Raschig, Z. angew. Chem., 1903, 617 and 818; von Knorre, 
Chem. Ind., 28, 2; and Friedheim and Nydegger, Z. angew. Chem., 1907, 9. 


DETERMINATION OF SULPHURIC ACID. 415 


acidity corresponded to the amount of sulphuric acid present. 
Raschig, on the other hand, recommends treating the neutral 
or acid solution of the sulphate with benzidine hydrochloride 
solution, filtering off the precipitated benzidine sulphate, washing 
it, and then suspending it in water and titrating the sulphuric acid 
with tenth-normal sodium hydroxide. | 

The latter method constitutes a direct determination and 
has the advantage that it does not require a neutralization 
of the solution before the treatment with benzidine hydro- 
chloride. ~ 

In the precipitation of sulphuric acid by means of benzidine, 
there are two sources of error. In the first place the precipitate 
is not perfectly insoluble, so that an appreciable amount of 
sulphuric acid is not precipitated. In the second place, the 
precipitate tends to adsorb some benzidine hydrochloride. These 
two sources of error influence the results of an analysis in opposite 
directions and either wholly or partly compensate one another. 
Friedheim and Nydegger have studied the method from this 
point of view and have apparently succeeded in working out the 
conditions whereby the sum of the errors becomes practically 
zero. 

Procedure.—To prepare the solution of benzidine hydrochloride, 
6.7 gms. of the free base, or the corresponding amount of the 
hydrochloride,* is rubbed up in a mortar with 20 c.c. of water. 
The paste is rinsed into a liter flask, 20 ¢.c. of hydrochloric acid 
(sp. gr. 1.12) are added, and the solution diluted up to the mark. 
(1 c.c. of this solution corresponds theoretically to 0.00357 gms. 
H2SO,.) The solution has a brown color and may be filtered if 
necessary. After some time brown flakes are likely to separate, 
but these do no harm. 

The solution of the sulphate is diluted with water until its 
volume corresponds to not less than 50 c¢.c. for each 0.1 gm. of 
sulphurie acid present. An equal volume of the reagent is added . 
while stirring vigorously. A filter is prepared by placing a Witt 
perforated porcelain filter plate in a funnel and covering it with 
two moistened filter papers, one of exactly the same size as the 





* The commercial salt contains varying amounts of hydrochloric acid. 
This can be determined by titration with alkali. 


716 VOLUMETRIC ANALYSIS. 


plate and the upper one a little larger. After ten minutes, the 
precipitate is filtered off upon this filter, using gentle suction. The 
last portions of the precipitate are transferred to the filter with 
the aid of small portions of the clear filtrate, and then the beaker 
and precipitate are washed with 20 c.c. of cold water, added in 
several portions. The precipitate and filter, but not the plate, 
are then transferred to an Erlenmeyer flask, 50 c.c. of water are 
added and the contents of the stoppered beaker shaken until a 
homogeneous paste is obtained. The rubber stopper is then 
removed from the flask, rinsed off with water, a drop of phenol- 
phthalein added, the water heated to about 50° and titrated with 
tenth-normal sodium hydroxide. When the end point is nearly 
reached, the liquid is boiled for five minutes, and the titration 
then finished. ie 

Remark.—According to Friedheim and Nydegger, this method 
gives excellent results in the analysis of all sulphates provided 
no substances are present which attack benzidine, and provided 
the amount of other salts and acids present is not too great. There 
should not be more than 10 mol. of HCl, 15 mol. HNOs, 20 mol. 
HC2H302, 5 mol. alkali salt, or 2 mol. ferric iron present to 1 mol. 
H28SO,4. A satisfactory determination of the sulphur in pyrite may 
be made by dissolving 0.5 of the sample according to the Lunge 
method (see p. 362), evaporating off the nitric acid, taking up the 
residue in a little hydrochloric acid, diluting to 500 c.c. and using 
100 c.c. for the treatment with benzidine hydrochloride. 


9. Determination of Sulphuric Acid: (Hinman).* 


ee < ASO, 98.08". 
1000 c.c. io Na,S,0,= 30 30 = 3.269 gms. H,SO,. 





The solution of the sulphate is treated with an excess of a 
hydrochloric acid solution of barium chromate, by which means 
barium sulphate is precipitated, while an equivalent amount 
of chromic acid is set free. If the solution is then neutralized 

* Am. J. Sci. and Arts, 114, 478 (1877). Cf. Andrews, Am. Chem. J., 2, 
567; Pennock and Morton, J. Am. Chem. Soc., 1903, 2265; Bruhns, Z. anal. 
Chem., 45, 573 (1906); Holliger, Jbid., 19, 84 (1910), and M. Reuter, Chem 
Ztg., 1898, 357. 





DETERMINATION OF SULPHURIC ACID. ye | 


with ammonia or calcium carbonate, the excess of barium chro- 
mate is precipitated and can be filtered off with the barium sul- 
phate. In the filtrate, the amount of free chromic acid is deter- 
mined volumetrically by acidifying with hydrochloric acid, adding 
potassium iodide and titrating the deposited iodine with sodium 
thiosulphate solution. 

The barium chromate used for this determination must be com- 
pletely free from soluble chromate and can .contain no soluble 
barium salt nor barium carbonate; the presence of barium sul- 
phate exerts no influence. 

It is best to prepare the barium chromate by precipitav.ag 
barium chloride with potassium chromate at the boiling tem- 
perature. The precipitate is washed first with boiling water con- 
taining a little acetic acid, then with pure water and dried. The 
solution used for the analysis is obtained by dissolving 2 to 4 gms. 
of the dry salt in 1 liter of normal hydrochloric acid. 

Procedure.—The solution of the sulphate, which at the most 
should contain not over 2 per cent. of SO,, is almost neutralized 
- (in case it reacts acid) with potassium hydroxide solution, pre- 
cipitated at the boiling temperature with an excess of the hydro- 
chloric acid solution of barium chromate and boiled for one minute. 
If the solution originally contained carbonate, the boiling is con- 
tinued for five minutes. The precipitated barium sulphate always 
carries a little barium chromate with it and consequently appears 
yellow in color. 

To the boiling solution, calcium carbonate free from alkali is 
added in small amounts until there is no further evolution of carbon 
dioxide, after which the hot liquid is filtered and the precipitate 
washed with hot water. 

After cooling, an excess of potassium iodide is added and 5 c.e. 
of concentrated hydrochloric acid for each 100 ¢.c. of the solution; 
the liberated iodine is titrated as described on p. 647. 

Remark.—When iron, nickel or zine salts are contained in the 
solution, the acid present cannot be neutralized with calcium carbon- 
ate because these salts when boiled with calcium carbonate and a 
soluble chromate form insoluble basic chromates, so that too little 
chromic acid will be found in the filtrate corresponding to too little 
‘sulphuric acid. In such a case the neutralization is effected with 


718 VOLUMETRIC ANALYSIS. 


ammonia, an excess being added, the solution boiled until the 
excess is almost entirely expelled and then filtered. 


10. Determination of Phosphoric Acid. Method of Pincus. 


Principle—If a neutral solution, or one slightly acid with 
acetic acid, is treated with uranyl acetate, a greenish-white pre- 


cipitate of uranyl phosphate is formed: 
> 


KH,PO,+U0,(C,H,O,),—=KC,H,O, + HC,H,O,-+ UO,HPQ,. 


If at the same time ammonium salts are pars ammonium 
is contained in the precipitate: 


KH,PO,+U0,(C,H,0,), + NH,C,H,0,= 
— KC,H,0,+2HC,H,O, + UO,NH,PO,. 


‘The end of the precipitation is determined by testing a drop 
of the solution on a porcelain tile with potassium ferrocyanide. 
As soon as all of the phosphoric acid is precipitated and the solu- 
tion contains a trace of uranyl acetate in excess, the ferrocyanide 
solution produces a brown coloration. 

In order to completely precipitate the phosphoric acid, it 1s 
necessary to titrate in a boiling hot solution. As, however, a solu- 
tion of calcium phosphate will become turbid on boiling, owing 
to. the formation of secondary calcium phosphate (CaHPO,), it is 
best to precipitate the greater part of the phosphoric acid in the 
cold, then heat to boiling and complete the titration. 3 
Requirements. 1. Potassium Phosphate Solution.—This is ob- 
tained by dissolving 19.17 gms. (corresponding to 10 gms. P,O,) 
of chemically pure monopotassium phosphate (which caa be ob- 
tained commercially) in 1 liter of water. 

The concentration of the solution is confirmed by evaporating 
50 c.c. to dryness in a large platinum crucible, igniting the residue 
over the full flame of a Bunsen burner and weighing as KPO,; also 
by precipitating another portion as magnesium ammonium phos- 
phate and weighing as magnesium pyrophosphate. 

50 c.c. of the solution correspond to 0.5 gm. P.O, 

and should. yield, «i546 Selenite cies 0.8315 gm. KPO; 
BAG i. 3.46 Ses BES iN ke ee 0.7839 gm. Mg2P20,. 


DETERMINATION OF PHOSPHORIC ACID. 719 


2. Calcium Phosphate Solution.—5.461 ems. of Ca,P,O,, cor- 
responding to 2.5 gms. P,O,, are dissolved in a little nitric acid, 
diluted with water to a volume of 1 liter, and the concentration of 
the solution tested by means of the molybdate method of Woy 
(p. 437). : 

3. Uranyl Aceta'e Solution.—This is made by dissolvinz about 
35 gms. of uranyl acetate in a liter of water. 

4. Ammonium Acetate Solution.—100 gms. of pure ammonium 
acetate and 100 c.c. of acetic acid, sp. gr. 1.04, are diluted with 
water to a volume of 1 liter. 


5. Potassium Ferrocyanide.—The salt is used in the powdered 
form. 


Procedure. 
(a) Standardization of the Uranium Solution. 


50 ¢.c. of the potassium phosphate. or calcium paosphate, 
solution are treated with 10 c.c. of the ammonium acetate solution, 
and to it the uranyl acetate solution is run in from a burette until 
a drop of the solution will show a brown coloration when treated 
“with solid potassium ferrocyanide upon a white porcelain tile. The 
solution is then heated to boiling, when a drop of it will no longer 
react with the ferrocyanide. To the hot solution more of the 
uranium solution is added, until the brown color is obtained once 
more. | | 
If for the precipitation of the phosphoric acid contained in 
50 c.c. of the potassium phosphate solution (0.5 gm. P,O,), T 
c.c. of the uranium solution were required, its concentration is 


i em. P,O, per c.c. 


For the analysis of alkali phosphates, the solution is standard- 
ized against the potassium phosphate solution, while for the 
analysis of an alkaline-earth phosphate the solution of calcium 
phosphate is used. , 


(b) Determination of Phosphoric Acid in Alkali Phosphates. 


The solution to be analyzed should be of about the same con- 
centration as that of the potassium phosphate used for the stand- 
ardization, and titrated as above. Phosphate solutions of different 
concentrations give different results by the titration. 


720 VOLUMETRIC ANALYSIS. 


(c) Determination of Phosphoric Acid in Calevum Phosphate. 


A weighed amount of calcium phosphate is dissolved in dilute 
nitric acid, ammonia is added to the solution until a permanent 
precipitate is produced, which is redissolved in a little acetic acid, 
10 c.c. of the ammonium acetate solution are added, and the 
solution is titrated with the standardized solution of uranyl 
acetate. . 

Remark.—In the presence of iron and aluminium this method 
will not give accurate results because the phosphates of these 
metals are insoluble in acetic acid. In such cases, the turbid 
acetic acid solution is filtered and the phosphoric acid determined 
in the filtrate by the above titration. The precipitate consisting 
of iron and aluminium phosphates is ignited, weighed, and, if 
it amounts to less than 0.01 gm., half its weight is taken as P,O;; 
otherwise the phosphoric acid in the precipitate must be deter- 
mined by the molybdate method. 


11. Determination of Nickel by Potassium Cyanide.* 


This method, which permits the volumetric estimation of 
nickel with speed and accuracy even in the presence of iron, 
manganese, chromium, zinc, vanadium, molybdenum, and 
tungsten, depends upon the fact that nickel ions react with 
potassium cyanide in slightly ammoniacal solution, to form a ~ 
complex anion, Ni(CN) 4” 

Ni(NH3)6Clo +4KCN = Kf Ni(CN) ,] +6NH3 +2KCI. 


If the solution of the nickel salt contains a precipitate of silver 
iodide, produced by adding a known amount of silver nitrate and 
a few drops of potassium iodide solution, the turbidity will not 
disappear until all of the nickel has entered into reaction with the 
potassium cyanide. 

AgI + 2KCN =K[Ag(CN)o]+KI. 





* Cf. Campbell and Andrews, J. Am. Chem. Soc., 17, 126 (1895); Moore, 
Chem. News, 72, 92 (1895); Goutal, Z. angew. Chem., 1898, 177; Brearley and 
Jarvis, Chem. News, 78, 177 and 190 (1898); Johnson, J. Am. Chem. Soc., 29, 
1201 (1907); Campbell and Arthur zbid., 30, 1116 (1908); and Gro:smann, 
Chem. Ztg., 32, 1223 (1908). 


DETERMINATION OF NICKEL BY POTASSIUM CYANIDE. 721 


The titration is finished by adding just enough more silver nitrate 
to cause the precipitate of silver iodide to reappear. 


K[Ag(CN) 2] + AgNO3=2AgON + KNOs. 


Requirements.—The potassium cyanide solution should be about 
equivalent to a tenth-normal silver solution, and is prepared by 
dissolving 13.5 gms. of pure potassium cyanide and 5 gms. of 
caustic potash in water and diluting to a volume of one liter. 
The addition of the alkali serves to make the solution more stable. 

The silver nitrate solution is made exactly tenth-normal and 
is prepared by dissolving 8.495 gms. AgNO3 in water and diluting 
to exactly 500 ¢.c. 1 c.c. of this solution is equivalent to 0.01392 
gms. KCN, or to 0.002934 gms. Ni. It is used for standardizing 
the potassium cyanide solution, and in the analysis itself. 

The potassium iodide solution contains 2 gms. KI in 100 c.e. 

Standardization of the Potassium. Cyanide Solution.—About 
30 c.c. of the potassium cyanide solution are diluted to 100 c.c., 
5 c.c. of the potassium iodide solution added, and the solution 
titrated with silver nitrate solution until a faint permanent 
opalescence is obtained which is cleared up by a small drop of 
potassium cyanide solution. 

Analysis.—The solution, containing not more than 0.1 gm. of 
nickel and having a volume of about 100 c.c., is treated with 
ammonia until slightly alkaline * and then with 5 c.c. of the 
potassium iodide solution and 0.5 c.c. of the 0.1N silver nitrate 
solution, the latter being accurately measured from a burette. 
While stirring constantly with a glass rod, the standard potassium 
cyanide solution is added until the precipitated silver iodide 
dissolves completely. Then the silver solution is added until a 
faint, permanent opalescence is obtained which is cleared up by 
a small drop of the potassium cyanide. 

Assuming that the silver solution was exactly tenth-normal, 
that one c.c. of potassium cyanide=f, e.c. of the silver nitrate 
solution, and that 7’ ¢.c. of potassium cyanide and ¢ c.c. of silver 





*If the addition of ammonia does not give a clear solution, in few cubic 
centimeters of ammonium chloride solution should be added. 


722 VOLUMETRIC ANALYSIS. 


nitrate were used in titrating a gms. of a substance, then the 
percentage of nickel is found by the following equation: 


(Tt, — t) X0.2934 
a 





=per cent. Ni. 


Instead of working with two solutions, F. Sutton * states that 
equally reliable results can be Savane’ by using a potassium 
cyanide solution to which a little’ silver nitrate has been added. 
Thus, to the above solution of potassium cyanide there may 
be added about 0.50 gm. of silver nitrate which is first dissolved 
in water by itself. If this solution is used for titrating a nickel 
solution to which potassium iodide solution has been added, a 
precipitate of silver iodide is formed at once which increases 
with the further addition of the potassium cyanide-silver nitrate 
solution until all the nickel is converted into potassium nickelo- 
cyanide, but the precipitate eventually disappears upon the 
further addition of the solution. When this modification of the 
potassium cyanide method is used, however, it is necessary to 
standardize the potassium cyanide solution against a solution 
containing a known quantity of nickel. 

Remarks.—The method can be carried out in the presence of 
most of the other elements of the ammonium sulphide group.t 
If copper is present 1n amounts not exceeding 0.4 per cent., the 
copper will replace almost exactly three-quarters of its weight 
of nickel. In case chromium is present, the dark color due to 
presence of chromic salts may be obviated by adding to the 
original sulphuric acid solution a 2 per cent. solution of potassium 
permanganate until a slight permanent precipitate of manganese 
dioxide is obtained, whereby the chromium is oxidized to chromic 
acid. .The solution is filtered, concentrated in a 400 c.c. beaker 
to about 60 c.c., then treated with sodium pyrophosphate, as 
described above. The method is not applicable in the presence 
of cobalt, the presence of which is betrayed by the solution as- 
suming a dark color upon the addition of potassium cyanide, 
but when the amount of the latter does not exceed one-tenth the 





 * Volumetric Analysis, 8th edition, p. 252. 
+ The addition of alkali pylouhoaphind is-usually necessary, as in the 
following method for determinine nickel in steel. 


DETERMINATION OF NICKEL IN NICKEL STEEL. 723 


amount of nickel present, the titration can be carried out succesfully 
and the results represent the amount of nickel and cobalt present. 

The temperature of the solution should not be much above 
20°, for in hot solutions the results are not concordant. The 
quantity of ammonia present should not be too great, because 
the tendency is for ammonia to impede the reaction if more than 
a slight excess is present. Potassium cyanide containing sulphide 
cannot be used; the reagent should be the purest obtainable. 
The results are accurate. The method has been modified so that 
it can be used to advantage for the 


Determination of Nickel in Nickel Steel. 


One gram of steel is dissolved in a casserole with 10 to 15 ¢.c. 
of nitric acid (sp. gr. 1.2), adding a little hydrochloric’ acid if 
necessary. After the steel has dissolved, 6 or 8 c.c. of sulphuric 
acid (1:1) are added, and the solution is evaporated until fumes 
of sulphuric anhydride begin to come off. The residue is cooled, 
30 to 40 c.c. of water are added, and the contents of the casserole 
boiled until the ferric sulphate has all dissolved. The solution is 
then transferred to a 400-c.c. beaker, filtering if necessary, and 
13 gms. of sodium pyrophosphate * dissolved in 60 c.c. of water at 
about 60° are added. The pyrophosphate solution must not be 
boiled, as this causes the formation of normal phosphate. The 
addition of the sodium pyrophosphate causes the formation of a 
heavy white precipitate of ferric pyrophosphate. The liquid is 
cooled to room temperature, whereupon dilute ammonia (1:1) is 
added drop by drop, while stirring constantly, until the greater 
part of the precipitate has dissolved and the solution has assumed 
a greenish tinge. At this point, it should react alkaline toward 
litmus but should not smell,of free ammonia. Now on gently 
heating the solution, while stirring, the remainder of the pyro- 
phosphate will dissolve, giving a perfectly clear light green 
solution. If the ammonia is added too fast, or the solution is 
not carefully stirred, a brownish color is fiicels to result, but this 





* Instead of using sodium pyrophosphate to prevent the. interference 
of iron and. other metals, many chemists use citric or tartaric acid. . In 
this case the solution is dark colored and the end-point a little harder: to 
detect. 


724 . VOLUMETRIC ANALYSIS. 


can usually be overcome by carefully adding a few drops of dilute 
sulphuric acid. The clear solution is cooled to room temperature 
and 0.5 c.c. of the standard silver nitrate solution is added together 
with 5 c.c. of the potassium iodide. The solution is then titrated 
with potassium cyanide, which is added until the precipitate of 
silver iodide has disappeared. The titration is finished by adding 
just enough more of the silver nitrate to cause the formation of a 
slight turbidity again. ° 


12. Determination of Copper by the Potassium Cyanide Method. * 


Principle.-—If an ammoniacal solution of a cupric salt is 
treated with potassium cyanide, the intense blue color gradually 
disappears. The reaction is essentially as follows: 


2Cu(NH3) 4804-H20-+7KCN =K3NH,Cu2(CN)5 +NH.CNO 
+2K.804+6NH;3+H,0. 


The temperature of the solution, the ammonia concentration, 
and the quantity of ammonium salts present effect the reaction 
so that a given quantity of copper does not always react with 
the same quantity of potassium cyanide. The potassium cyanide 
solution, therefore, must be standardized under exactly the same 
conditions as under which the analysis is carried out. 
Standardization of the Potassium Cyanide Solution.—Twenty 
grams of pure potassium cyanide are dissolved in a liter of water 
and the solution titrated against pure copper. About 0.2 gm. 
of pure copper wire or foil is weighed out into a 200-c.c. Erlen- 
meyer flask and dissolved in 5 c.c. of concentrated nitric acid. 
After solution is complete, 25 c.c. of water and 5 ¢.c. of bromine 
water are added and the solution boiled to expel the excess of 
bromine. Then 50 c.c. of water and 10 c.c. of strong ammonia 
(sp. gr. 0.90) are added, the solution cooled to room temperature 
by placing the flask in cold water, and the potassium cyanide 
added slowly from a burette, while constantly rotating the 





* Cf. Steinbeck, Z. anal. Chem. 8, 8 (1869). Dulin, J. Am. Chem. Soc., 17, 
346. A. H. Low, Technical Methods of Ore Analysis. 


DETERMINATION OF COPPER. 725 


contents of the flask. When the solution has become a pale 
blue, water is added to make the total volume 150 c.c. and the 
addition of the potassium cyanide continued until the solution 
is just decolorized. The weighed amount of copper divided by 
; the number of cubic centimeters of potassium cyanide required 
gives the titer of the solution. 

Low’s Method for Analyzing Copper Ores.—About 0.5 gm. of a 
rich ore, or from two to four times as much of a low-grade ore, 
is weighed into a 200-c.c. Erlenmeyer flask and treated with 
6-10 c.c. of concentrated nitric acid and boiled until nearly all 
the red fumes are expelled. If necessary, 5 c.c. of concentrated 
hydrochloric acid are also added to decompose the ore and the 
boiling continued for a short time. After cooling somewhat, 
7 c.c. of concentrated sulphuric acid are added and the solution 
evaporated until dense fumes of sulphuric acid are evolved. 
Then, after allowing to cool somewhat, 25 c.c. of cold water are 
added and a drop of concentrated hydrochloric acid to pre- 
cipitate any silver as chloride. The liquid is boiled to dissolve 
the copper and ferric sulphates and then the precipitated lead 
sulphate and silicious residue is filtered off and washed with hot 
water. The filtrate is received in a beaker of about 6 cm. diam- ° 
eter and care is taken not to let the filtrate exceed 75 c.c. 

The copper is precipitated from the filtrate by introducing a 
piece of aluminium foil, about 14cm. long and 2.5 cm. wide, 
which is bent into a triangle. The beaker is covered with a 
watch-glass and its contents boiled about ten minutes, whereby 
nearly all the copper is deposited as spongy metal. The beaker 
is now removed from the flame and the sides washed down with 
cold water. To precipitate the last traces of copper and to 
prevent the oxidation of the fine deposit, 15 c.c. of strong hydrogen | 
sulphide water are added, after which the liquid is decanted through 
a 9cm. filter. The copper is washed off the aluminium by 
means of weak hydrogen sulphide water into the flask in which 
the ore was dissolved and the liquid decanted through the filter; 
the beaker containing the aluminium foil and some copper is 
set aside temporarily. The operation of filtering should take 
place without interruption and the filter kept well filled with 
liquid to prevent the oxidation of any precipitate upon it, which 


726 VOLUMETRIC ANALYSIS. 


would cause it to dissolve and give a turbid filtrate. After 
washing the deposit four times, using in each case 20 c.c. of weak 
hydrogen sulphide water, the liquid is allowed to drain from 
the funnel, and then the beaker containing the filtrate is replaced 
by the flask containing the deposited copper. The a’uminium 
foil, to which some copper usually adheres, is now covered with 
10 ¢.c. of nitric acid (1:1), heated | nearly to boiling, and the hot 
acid poured through the filter. "The flask is replaced by the 
beaker containing the foil, and the contents of the flask heated 
until all the copper is dissolved and the greater part of the red 
fumes expelled. The flask is again placed under the funnel, the 
aluminium foil in the beaker covered with 5 c.c. of strong bromine 
water, which is poured through the filter. The aluminium foil 
and the filter are then washed with hot water. The solution is 
boiled to expel the excess of bromine, cooled to room tem- 
perature, treated with 10 c.c. of strong ammonia, and titrated 
with potassium cyanide exactly as in the standardization. | 


13. Determination of Lead by the Molybdate Method.* 


Principle.—The lead is precipitated as molybdate from an 
acid solution and the termination of the reaction is recognized 
by testing a drop of the solution with a drop of tannin solution, 
which gives a yellow coloration when an excess of ammonium 
molybdate is present. 

Requirements.—1. A solution of ammonium molybdate pre- 
pared by dissolving about 4.25 gms. of ammonium molybdate 1 in 
water, and diluting to one liter. 

2. A freshly prepared tannin solution containing 0. I eu, of 
tannin in 20 c.c. of water. 

Standardization of the Ammonium Molybdate Solution.—About 
0.2 gm. of pure lead foil is weighed into a 200-c.c. Erlenmeyer 
flask, dissolved in a mixture of 2 c.c. concentrated nitric acid and 
4 ¢.c. of water and the solution evaporated nearly, if not quite, 
to dryness. The residue is taken up in 30 ¢.c. water, 5 ¢.c. of 
concentrated sulphuric acid added, the liquid shaken, tke lead 





_ * Low, Technical,Methods of Ore Analysis, 9) 9. 0 J) 


is pie 


DETERMINATION OF LEAD. 927 


sulphate allowed to settle-completely, filtered and washed with 
dilute sulphuric acid (1:10). The filter, together with precipi- 
tate, is thrown into an Erlenmeyer flask, 10 c.c. of concentrated 
acid are added, and the liquid boiled until the filter is completely 
disintegrated. Then, after adding 15 c.c. more of the concen- 
trated hydrochloric acid, and 25 c.c.'of cold water, 25 c.c. of 
concentrated ammonia (sp. gr. 0.90) are carefully poured into 
the flask, whereby the greater part of the acid is neutralized. 
A piece of blue litmus paper is thrown into the solution, ammonia 
is added to slightly alkaline reaction, and then glacial. acetic 
acid until the litmus paper turns red. The solution is diluted 
to about 200 c.c. with hot water and about two-thirds of the 
solution transferred to a beaker. The ammonium molybdate 
solution is added to the latter from a burette until a drop of the 
solution, brought in contact. with a drop of the tannin indicator 
upon a white porcelain tile, gives a brown or yellow color. Some 
more of the lead solution is added to the beaker and the opera- 
tion is repeated until finally but a few cubic centimeters of the 
lead solution remain in the flask. The contents of the beaker 
are now poured into the flask, then back again to the beaker, 
and the titration finished by adding the molybdate solution two 
drops ata time. Iftc.c. of molybdate are used in titrating a gms. 
of lead the titer of the solution is 


‘1 ¢.c. ammonium molybdate =" em. lead. 


Procedure.—0:5 gm. of the ore is weighed into a 200-c.c. 
Erlenmeyer flask, 10 c.c. of concentrated hydrochloric acid and 
20 c.c. of water added and the liquid boiled until all the hydrogen 
sulphide is expelled. If the ore should not dissolve completely 
by this treatment, a little concentrated nitric acid is added and 
the heating continued until the ore is completely decomposed. 
As soon as this has taken place, the solution is allowed to cool, 
7 c.c. of concentrated sulphuric acid added, and the liquid 
evaporated over a free flame until dense vapors of sulphuric 
acid are evolved. After allowing to cool, 20 c.c. of water are 
added and the liquid boiled for fifteen minutes in order to dissolve 
all the anhydrous ferric sulphate. 


728 VOLUMETRIC ANALYSIS. 


After cooling, the precipitated lead sulphate and silicious 
residue is filtered off and washed with cold dilute sulphuric acid 
(1:10). \. The lead sulphate is often contaminated with calcium or 
- barium sulphate, and before the titration it must be purified. To 
this end, the precipitate is rinsed by a stream of cold water into the’ 
original flask, 5 gms. of pure ammonium chloride are added and 
about 1 c.c. of concentrated hydrochloric acid. By boiling, all of 
the lead and calcium sulphates areWissolved but the gangue, which 
is easily distinguished from either of the above salts, remains 
behind. The solution is neutralized with ammonia and treated 
with an excess of ammonium sulphide. The precipitate is 
allowed to settle and is then filtered and washed with hot water 
until the filtrate no longer gives a test for calcium when tested 
with ammonium oxalate. As the lead sulphide may be con- 
taminated with some iron sulphide, it is again rinsed into the 
original flask, by means of as little hot water as possible, treated 
with 5 c.c. of dilute sulphuric acid, shaken until the precipitate 
is well broken up, treated with 25 c.c. of strong hydrogen sulphide 
water, filtered through the same filter as was last used, and 
washed with cold water. By this time it is safe to assume that 
the lead sulphide is free from all calcium and iron. The filter 
and precipitate are once more returned to the original flask, dis- 
solved by boiling with 5 ¢.c. of concentrated hydrochloric acid, 
boiled, and then, when the hydrogen sulphide is practically all 
expelled, treated with 2 or 3 drops of concentrated nitric acid 
to remove the last traces of hydrogen sulphide. Now 25 c.c. of 
cold water are added, and the solution treated exactly as in the 
_ standardization of the ammonium molybdate. 

Remark.—The smelter chemists in the western part of the 
United States use a much more rapid method, which gives good 
results in the hands of an experienced operator, provided the lead 
content of the ore is greater than fifteen milligrams. 

Procedure.*— The ore is dissolved in hydrochloric acid or 
hydrochloric and nitric acids, and the solution is filtered 
while hot without diluting any more than to prevent the 
acid attacking the paper. The residue is washed rapidly 
with a hot solution of ammonium chloride until the washings 





* This method was obtained through the courtesy of Mr. Franklin G. Hills 
of the American Smelting and Refining Co. 


DETERMINATION OF LEAD. 7284 


show no blackening when tested with ammonia and a drop of 
ammonium sulphide. The filtrate is made just alkaline’ with 
ammonia and a slight excess of ammonium sulphide added. The 
liquid is heated to boiling and the precipitated sulphides are 
filtered off and washed with hot water. (The alkaline earths may 
be determined in the filtrate if desired.) The sulphides are dis- 
solved in hot, dilute nitric acid and the resulting solution is caught 
in the same beaker in which the sulphides were precipitated. 
About 7 c.c. of concentrated sulphuric acid are added and the 
liquid evaporated until dense vapors of sulphuric acid are evolved. 
After allowing to cool, 20 c.c. of water are added and the liquid 
boiled to dissolve the anhydrous ferric sulphate. The precipitated 
lead sulphate is filtered off, washed free from acid, dissolved in a 
slight excess of ammonium acetate solution* and diluted with 
water. After heating to boiling, the hot solution is titrated with 
ammonium molybdate. 

The above procedure serves when alkaline earths are present; 
but when these are known to be absent, the original solution of 
the ore is at once evaporated with sulphuric acid and the resulting 
lead sulphate can be dissolved in ammonium acetate solution and 
titrated without any purification._ 





* If too much ammonium acetate solution is used, a transitory end point 
is obtained in the subsequent titration. It is necessary to use a hot solution, 
_ which does not contain too much of the salt. See page 176. 








ae ke lds 
GAS ANALYSIS. 





THE chemical analysis of gas mixtures is accomplished usually 
by measuring and rarely by weighing the individual constituents, 
so that it is customary to express the results in per cent. by volume. 
But inasmuch as the volume of a gas is influenced to an extraor- 
dinary extent by the temperature and pressure, it is necessary 
to reduce each measurement to standard conditions of tempera- 
ture and pressure, and further to take care that these remain 
constant during the whole of the analysis. A volume of gas V 
measured over water at ¢° C. and B mm. barometric pressure,* 
is reduced to the volume which it would assume at 0° C. and 
760 mm. pressure in a dry condition by means of the formula 

V.— V(B-w) 
° 760(1+ at)" 

In this formula, V, represents the reduced volume,+ V the 
volume of the gas at ¢° C. and B mm. pressure, w the tension 
of aqueous vapor, and a@ the expansion coefficient of the gas 
(= 0.003665). 





As, however, a= 3 the above formula may be written as 
follows: | 
y _V(B—w)273 
0 760(273 +2)" 





* Here is understood the barometer reading reduced to 0° C. The 
reduction is accomplished by means of the formula: 
ie 1+ ft RB 
l+at 
in which By represents the reduced reading, B the actual reading at t°, a 
the expansion coefficient of mercury (=0.000181), @ the linear coefficient 
of expansion of glass (=0.0000085). For most purposes, however, the 
reduction to 0° C. can be made with sufficient accuracy by making the 
following deductions from the actual readings: 
gies atin ais Re se rien Mae PARR foe Oe ae elie ss heey Vie fd 3 mm. 
WOO Se Os aici | tee 1s awh SR ange bee Salk girs ape Aest8 


+ Or volume under standard conditions. _ 729 





2) 


730 GAS ANALYSIS. 


Instead of reducing the observed volume to the standard 
conditions by computation, it can be effected mechanically by 
compression (see p. 388). 


The Collection and Confinement of Gas Samples. 


Since all gases diffuse rapidly into one another even when 
separated by porous solid bodies or liquids, it is evident that the 
collection of the sample and its préservation offers certain diffi- 
culties. If a gas is confined in a bell jar over water and thus kept 
out of contact with the air, it will be found that different results 
will be obtained in the analysis of the gas from day to day. The 
air gradually penetrates through the water into the bell jar and . 
in the same way the gas within the jar gradually diffuses into 
the atmosphere. This process will continue until finally the 
composition of the gas both within and without the jar is the 
same. The rapidity of the diffusion depends upon the extent 
to which the gases are absorbed by the liquid which separates 
them. Those liquids which absorb the gases readily, allow them 
to pass through it rapidly, and consequently cannot be used for 
keeping the gases apart. Of all liquids, mercury is best suited. 
for the purpose, because it absorbs only minimum amounts of 
the different gases. 

Gases which combine chemically with mercury, such as chlo- 
rine, bromine vapors, hydrogen sulphide, etc., cannot, of course, 
be collected over mercury; it is best to collect them in dry glass 
tubes and to seal the latter by fusing together the open ends in 
case the gas cannot be analyzed immediately. Through glass 
there is no diffusion, so that gases may be kept unchanged in sealed 
tubes for years. 

If the gas is to be analyzed within a few days after the time 
of collection, it can be kept in pipette-shaped tubes. The ends 
are closed by thick pieces of rubber tubing into each of which is 
inserted a piece of glass stirring-rod with rounded ends; where the 
rubber tubing comes in contact with the glass it should be fastened 
tightly with wires. It is not permissible to keep gases in such 
tubes for a considerable length of time. for rubber, particularly 
when it has become hard, permits the diffusion of gases to some 
extent. 


COLLECTION AND CONFINEMENT OF GAS SAMPLES. 73. 


For less accurate analyses, the gases may be collected over 
water which has been previously saturated with the gas to be 
analyzed, and the analysis must be made immediately afterwards. 

From what has been said, it is evident that care must be taken 
in collecting and keeping the gas to be analyzed. We will now 
consider briefly a few practical examples, 


(a) Collection of Gases in Accessible Places. 


1. The neck of a 200-c.c. flask is drawn out somewhat and 

a glass tube is inserted and about 800 c.c. of the gas to be 

analyzed are drawn through the flask by means of suction 

(Fig. 98). The neck of the flask is closed by means of a rubber 
cap and the glass is fused together. 


(b) Collection of Gases from Inaccessible Places. 


The rubber tubing G, Fig. 99, is connected on one side with 
the aspirator A of about 30 liters capacity and on the other with 











bhA 


Fig, 98. 





the source of the gas, and water is allowed to flow quickly from 
the former. After 5 or 6 liters have run out, the air is usually 


732 GAS ANALYSIS. 


eompletely expelled from the rubber tubing and replaced by the 
gas to be analyzed, so that it is now ready for collecting the sample. 
For this purpose the stop-cock H is turned 90° to the right, so that 
th: vessel R, which is to receive the gas, is in communication 
with the outer air, and the air is expelled from it by raising the 
mercury reservoir N. The stop-cock is then turned back to 
the position shown in the figure and RF is filled with the gas by 
lowering N. As the tubing betweéh the 7 tube and the stop- 
cock contained impure gas, # is again filled with mercury and 
the gas expelled into the air. After the process has been repeated 
three times, the receiver is filled for the last time with the gas, 
H is closed, N is lowered so that the pressure in the tube is less 
than that of the atmosphere, and the ends of #& are fused together 
first at a then at b. During this sealing of the tube, it should 
be removed from the ring-stand so that the tube can be revolved 
a little while being heated in the flame. 

In sealing the tube, the ends are drawn out into a capillary as 
shown in #’, Fig. 99. | 

If it is necessary to obtain the gas from places at a very high 
temperature, e.g., frc.c. blast-furnaces, producers, 
etc., glass tubes would melt, and if ordinary 
iron tubes were not melted they would de- ’__A¢ 
compose the gas. In this case it is best to a= 3 
use the water-jacketed iron tube devised by 
St. Claire Deville and shown in Fig. 100. Cold 
water is run into the outer condenser at a 
and allowed to run out at b, and the gas is col- 
lected as described above through the tube c. 
It is important that. the water should run 4 
through the tube fast enough to keep the inner ‘ies ite 
tube cold, otherwise the gas will be decom- ae 
posed. By this means there is no difficulty in collecting gas 
samples from different heights of the glowing layers of coal in 
blast-furnaces or producers, 























Collection of Gases Arising from Mineral Springs. 


The receiver R is connected with the funnel 7’ by means of the 
rubber tubing g (Fig. 101). All these parts of the apparatus are 


COLLECTION AND CONFINEMENT OF GAS SAMPLES. 733 


filled with spring-water and the gas is allowed to ascend up through 
the funnel as shown in the illustration. 
In order that the gas ray pass from 
the funnel into the receiver, RP is raised 
so that only the tubing p remains in 
the water while the funnel is lowered. 
as deep as possible, causing pressure 
enough to drive the gas over. Tho 
tubing is then closed just above a by 
means of a screw-cock, a beaker filied 
with spring-water is placed under p, 
\S \ the apparatus removed from the spring, 
8) ==) and both ends of R are fused together 
Fic. 101. with the blowpipe. If the gas is to 
be analyzed within two or three days, 
the receiver may be closed by pieces of short rubber tubing each 
containing a short piece of glass rod with rounded ends. All of 
such connections must be fastened by means of wires where the 
glass comes in contact with the rubber. According to the above 
method the gas arising from the thermal springs of Baden, Switzer- 
land, was collected and analyzed.* The results obtained showed 
that it makes but little difference which method is used for closing 
the receiver, provided the analysis is made within a short time.t 
100 e.c. of the gas contained: | 








*“Chemische Untersuchung der Schwefeltherme von Baden (Kanton 
Aargau),” by F. P. Treadwell, 1896. 

Tt The following analyses illustrate the importance of analyzing the gas 
soon after it has been collected. Both samples were taken at the same time 
and one was analyzed promptly but the other only after two years and a 
half had elapsed. The gas receiver was closed by fresh rubber tubing con- 
taining a piece of stirring rod with rounded edges, and precaution was taken. 
to wire the connections tightly. The rubber remained soft and flexible. 








I. II. 
CO:= 8.52% 0.43% 
O2= 10.77 14.82 

CO= 0.17 
CH.= 0.15 : 
N2= 80.39 84.75 








100.00% 100.00% 


734 | GAS ANALYSIS. 





I II 
Nifrogen? 60s OS OA eae 69.13 69.15 
Carbon dioxide. ...........- 30.81 30.90 
Hydrogen sulphide........... 0.05 0.05 
OXVRO 565 oh eo weed eens 0.00 0.00 
99.99 100.10 


Sample I was collected and the ends of the receiver fused to- 
gether, while with sample II the ends were closed by means of rub- 
ber tubing and glass rods, and analyzed five days later. 


Collection of Gases Absorbed in Spring-water. 


Of the many different methods which have been proposed for 

the analyses of the absorbed gases in spring-water, the author has 

| found the following 

to give the best re- 
sults. 

The flask A, Fig. 
102, is filled with 
spring-water up to its 
upper edge and the 
rubber stopper con- 
taining the tube L, 
which is fused to- 
gether at the bottom 
but has an opening 
on the side at l, is im- 
mediately placed in 
the neck and pressed 
down to the mark. 
The tube L is raised 
so that the opening 1 
is within the stop- - 
per, thus making an 
air-tight connection. 
The bulb K is now 
connected with L, 
filled half full with distilled water and connected with the 
capillary tubing C, although the latter is not yet connected with 




















Fie. 102. 


COLLECTION AND CONFINEMENT OF GAS SAMPLES. 735 


the measuring-tube B, as shown in the illustration. The levelling- 
tube N is next raised until mercury begins to flow out of the 
right-angled capillary tube, when the stop-cock H is closed. After 
this the water in the bulb K (which is held in an inclined position) 
is boiled for three minutes, meanwhile warming the capillary tub- 
ing connected with the measuring-tube. Unless this last precau- 
tion is taken, the capillary tubing is likely to break, particularly 
in winter. After the water in K has boiled vigorously for three 
minutes, the flame is removed, C is quickly connected with the 
measuring-tube B and the rubber connection is securely fastened 
with wire. By boiling the water in K, a complete vi.cuum is 
produced in the bulb, so that the gas can be at once collected 
from the spring-water. For this purpose the tube ZL is pressed 
down through the rubber stopper until the opening 7 comes 
just below its lower edge, the levelling-tube N is lowered, and 
the stop-cock H is opened. At once there is a lively evolution 
of gas from the water in A and this is subsequently maintained by 
warming the water. As soon as the eudiometer is full the stop- 
cock is closed and the volume of the gas read after bringing the mer- 
cury to the same level in N that it is in B. At the same time the 
temperature of the water in the condenser M is taken by the ther- 
mometer 7’ and the barometer is read. ‘The gas is then driven over 
into the Orsat tube O containing potassium hydroxide solution (1:2) 
and allowed to remain there for the time being. Meanwhile the boil- 
ing of the water in A; measurement of the gas in B, etc., are con- 
tinued until finally no more gas is evolved from the spring-water. 
All of the gas is driven over into the Orsat tube after its volume 
has been noted and by means of the caustic potash, ‘the carbonic 
acid is quantitatively absorbed. The unabsorbed gas is again 
driven over into B and its volume read. By correctly regulat- 
ing the velocity of the current of water flowing through the con- 
denser, it is easily possible to maintain a constant temperature 
throughout the whole of the experiment. The residual gas remain- | 
ing after the absorption of the carbon dioxide consists usually 
of nitrogen, oxygen, and in some cases methane. It is trans- 
ferred to the apparatus of Hempel, and analyzed according to 
methods which will be described further on. 

According to this method, the determination of nitrogen, 


736 GAS ANALYSIS. 


oxygen, and methane gives exact results, but the apparent amount 
of carbon dioxide is sometimes too much and sometimes too little. 
If the water contains large amounts of bicarbonate in solution, 
the carbonic acid found will represent more than was originally 
present in the free state, for such substances are partly decom- 
posed by boiling their aqueous solution. On the other hand, if 
only a little bicarbonate is present, the result will be too low, for 
it is not possible to remove all of the free carbonic acid from a 
solution by boiling it in a vacuum. 

Consequently, in all cases the free carbonic acid must be deter- 
mined by computation. For this purpose, in a fresh sample of 
the water, the total carbonic acid is determined according to p, 393, 
and then if the composition of the solid constituents present is 
known, the volume of the free carbonic acid can be calculated. 

Example. —1000 gms. of Tarasper- Lucius water contain 
7.8767 gms. of total carbonic acid. Of this amount, a part of it 
is present in the water as carbonate (“ combined ”’ carbonic acid), 
an equal amount as “ half-combined ” carbonic acid, and the re- 
mainder is free carbonic acid. If from the total amount of 
carbonic acid the “ combined” and ‘“ half-combined ”’ acid is 
deducted (or what is the same thing, double the amount of the 
‘“‘ combined ”’ carbonic acid), the difference represents the amount 
of free carbonic acid present. 


Calculation of the ‘‘ Combined ”’ Carbonic Acid. 


This is obtained by multiplying the difference between the 
number of, cations and anions (expressed in univalent ions) by 
the molecular weight of carbonic-acid (CO3) ions and dividing by 
two,* because the sum of the cations in every salt solution is 
equal to that of the anions present when both are expressed in 
univalent ions. 

The ‘ univalent ions” are obtained by dividing the amount 
in grams of each element (or radical) present by its atomic (or 
molecular) weight and multiplying by the valence. 





* For the CO; ion is bivalent. 


Sum of the cations = 0.22354 


(b) Calculation of the Anions. 


Chlorine (Ci)e2 ee 2.40000: 35.46=0.06768 
Bromine (Br)........... 0.02890: 79.92=0.00035 
podine: (hy rec ees oS ad 0.00086 : 126.92 =0.00001 
Sulphuric acid (SOQ,)..... 1.72098: 96.07=0.01701 
Boric acid (BO2)......... 0.57600: 43.0 =0.01340 
Phosphoric acid (PO,).... 0.00008: 95.0 =0.00000 
Silicie acid (SiO,)........ 0.01421: 76.3 =0.00019 


expressed in univalent ions. 


Sum of the anions 


x 1=0.06768 
x 1=0.00035 
x 1=0.00001 
X2=0.03582 
x1=0.01340 
x 3=0.00000 
xX 2=0.00038 


=0.11764 


“Sum of the cations =0.22354 


Sum of the anions 


=0.11764 


COs; anions remaining = 0.10590 


COLLECTION AND CONFINEMENT OF GAS SAMPLES. 737 
(a) Calculation of the Cations. 
1000 gms. Lucius water contain: 
Grams. Corapining Valence. gail 
OMIM poorer t weer he, 3.90610 : 23.00=0.16983 «1=0.16983 
POtanentn oS ec ees 0.16603 : 39.10=0.00425 «x1=0.00425 
PORN oe a Ad 0.00914: 7.00=0.00131 x1=0.00131 
PerOU os ka ee 0.01298 : 18.04=0.00072 x1=0.00072 
CMC oS creaee wits. e 0.62691 : 40.09=0.01564 x2=0.03128 
PIRPORPII cates secs, wrace 0.00879 : 87.62=0.00010 «2=0.00020 
Magnesium............. 0.19040: 24.32=0.00783 «x2=0.01566 
SE at CSREES a oe 0.00603 : 55.85=0.00011 «K2=0.00022 
Manganese. .800e 6 ice bs 0.00021 : 54.93=0.000004 x 2=0.00001 
APAMININM 5:0). sss os sca 0.00064 : 27.1 =0.00002 K3=0.00006 


As COQ, is a bivalent ion, half of this amount represents the actual amount 


of CO, ions present: 


0.10590 





‘This Corresponds tO. Co 65 ies oss Cai 
Or the “combined” carbonic acid...... 


= 0.05295. 


ew ee 0.05295 X60=3.177 gms. CO; 


=2.330 ‘$ 


CO, 


738 G4S ANALYSIS. 


Calculation of the Free Carbonic Acid. 


Total amount of carbonic acid (COz) présent........... 7.877 gms. per liter 
Amount of “combined” carbonic acid................. 25330... 4614 Ue 


Amount of free+ “half-combined” carbonic acid .5.547 ‘* ‘* 
Amount of “half-combined” carbonic acid............. 2.330“ “ * 


Amount of free carbonic acid...%.......... dpe O62) «oo ee 
This weight of carbon dioxide occupies 1637 c.c. under standard con- 
ditions. 


By boiling 828.3 gms. of the water, 1868.9 c.c. of CO, were 
obtained at 8.4° C. and 651 mm. pressure, containing only traces 
of nitrogen. ‘This corresponds to 1851.4 ¢.c. at 0° C. and 760 mm, 
pressure, per liter, which is more than the calculated amount, 
because the carbonic acid gas consisted partly of free and partly 
of “‘ half-combined”’ carbonic acid. 

In cases where the amount of bicarbonate present is very 
small, the total amount of carbonic acid obtained by boiling the 
water is always too small. Thus in the case of the thermal water 
of Baden, by boiling there was obtained: 


Nitrogen 5274.0 04 tis ses sas ooh 14.43 c.c. per liter 
Carbon dioxide. ....... seve eh vB bias Vd ag Ft a, 





126. 55 iT 4 ft) cc 


while from the analysis, the free carbonic acid was computed to 
be 180.52 ¢.c. The absorbed gas in the thermal water of Baden 
is, therefore, 


NitfOGeD. 3 32004 a5 sews vewe wes 14.43 c.c. per liter 
Carbon dioxide $535 da sel 04% ROG 52 oS es 





104. Ob ad oe 


Remark.—With the above method of collecting the gas, it is 
difficult to prevent some water getting into the measuring-tube B, 
by means of which a small amount of the gas will be reabsorbed. 
This difficulty is avoided, however, if the flask shown in Fig. 103 is 
used to contain the water. 


. ’ 


COLLECTION AND CONFINEMENT OF GAS SAMPLES, 739 


This flask is provided with a short tube blown into its neck 
near the top and connected by means of thick-walled rubber 
: tubing w.th the mercury reservoir 

R. In order to determine the eon- 
tents of the flask, a seratch is made 
on the small tube about 4 cm. 
from the neck of the flask, the 
mercury is driven over just te 
this mark, and the rubber tubing 
tightly closed by means of a screw- 
cock. The reservoir is then emptied 
of mercury, and the flask is weighed 
together with the stopper, glass 
tube L, rubber tubing, and what 
mercury remains above Q. The 
flask is then filled with water, the 
.stopper pressed down to the mark 
Fic. 103. in the neck of the flask, and the tube 
L is raised until the lower opening 
1 comes within the stopper. After drying the tube L with blotting- 
paper, the flask and its contents are weighed. Its capacity is 
then etched upon it. 

For the determination of the gases absorbed in a liquid, the 
flask A is filled with it in the same way as in the determination. 
of its capacity, the bulb-tube K, half filled with distilled water 
is connected with L, and the latter is connected with a capillary 
tube as shown in Fig. 102. The air is removed from K and the 
capillary tubing by boiling the water in the former, as described 
on p. 735, and the capillary is then connected with the measuring- 
tube B, Fig. 102. The heavyrubber tubing is now connected with 
the reservoir as shown in Fig. 103, and the latter is placed in a 
beaker of hot water. The tube L is introduced into the neck 
-of the flask until the opening / can just be seen, and the gas is 
expelled in the same way as described on p. 735, except that in 
this case the liquid is not allowed to rise so high in AK. After 
three-quarters of an hour the gas will be completely expelled 
from the liquid. The last portions of the gas are driven over into 
B by lowering the levelling tube N (Fig. 102), raising the mercury 














740 GAS ANALYSIS. 


reservoir R (Fig. 103), and carefully opening the screw-cock Q. 
A warm stream of mercury will then flow into the flask, expelling 
the gas into the measuring-tube. As soon as the liquid in A has 
been driven over as far as the stop-cock H, this is immediately 
closed. Otherwise the procedure is the same as was described 
on p. 735. 

In order to test the accuracy of this method, the author made 
a few determinations of the oxygen absorbed in the lake-water at 
Zurich, and the results were compared with those obtained by 
E. Martz in this laboratory by means of the method of L. Winkler 
(see p. 760), 


OXYGEN IN 1 LITER OF ZURICH LAKE-WATER, 

















Modified Pettersson Method. Method of L. Winkler. 
I II I II 
7.66 c.c. 7.74 c.c. 7.67 c.c. 7.175 6.e, 





Collection of Gases Absorbed by Defibrinated Blood. 


The author has used for this purpose the apparatus shown 
in Fig. 104. The experiment is carried out as follows: 

First, the rubber tubing which connects N’ and A is filled 
with mercury by raising the levelling tube N’, and the pinch- 
cock h” is closed. Tyhen the gas burette C is likewise filled with 
mercury by raising D and the stop-cock h is turned to the position 
shown in the drawing. By raising the leveling tube N, the 
bulb K and the vessel A are filled with mercury, which is allowed 
to flow into the funnel M up to the line a and then the stop- 
cock h is closed. The blood to be examined is poured into M, 
N’ is lowered, and, by carefully opening h, the mercury is allowed 
to fall from M until it just reached the stop-cock h, which is 
then closed. M is now filled with blood to the mark b, h is 
opened, and the blood is sucked down until its upper level is 
exactly at the line a, when h is closed once more. The levelling 
tube N is lowered until a good vacuum is produced in A and 
the mercury level falls to near the bottom of K. When this.is 


COLLECTION OF GASES ABSORBED BY DEFIBRINATED BLOOD. 741 





i 
(fined 


























Fie. 104. 


742 GAS ANALYSIS. 


accomplished, the gas escapes from the blood so rapidly that 
all of A is filled with foam; after a few minutes, however, the 
effervescence subsides and all the blood collects in K. Then, 
the leveling tube N is raised until the blood reaches the cock h’ 
and the latter is then closed. In this way the greater part 
of the gas absorbed is separated from the blood. The leveling 
tube N’ is raised, h’’ opened so that the gas in A is under pressure, 
and by properly opening h and the burette stop-cock, the gas 
is driven over into the measuring burette C; when this is accom- 
plished, h is closed, N’ lowered until the mercury reaches the 
cock h’’ which is then closed and h’ opened. The blood again 
effervesces, but not so vigorously as before. The bulb K is now 
surrounded by water at 55°, which causes further effervescence 
from the blood. As soon as the foam subsides, the gas is again 
driven over into C and the process of evacuating.is continued 
until the blood ceases to effervesce. The volume of the gas in C 
is finally read, the temperature and pressure noted, and the 
analysis carried out as described on p. 775 or 786. 


re ee = - 


The Pnontensaes of Gases in Sealed, Tubes to the Apparatus 
Used for the Analysis. 


We will assume the gas to be contained in R, Fig. 105. Over 
one of the short tubes connected with the three-way stop-cock H is 
placed a piece of thick-walled rubber tubing which contains a short 
piece of heavy glass tubing r. The stop-cock is then revolved so 
that the rubber tubing is above it and the latter is filled with mer- 
cury. H is then turned 180° toward the left so that the left and 
upper tubes communicate with one another. As soon as the 
mercury begins to run out, the stop-cock is closed. One end of R 
is then introduced into the rubber tubing containing the mercury 
so far that its drawn-out point reaches within r, and the rubber 
tubing is securely fastened by wiring.* In a similar way, the other — 
end of & is connected with the rubber tubing filled with mercury 
of the levelling-tube N, and after this the stop-cock H is connected 
with the measuring apparatus W by means of the capillary tubing 





* Annealed iron wire is used. Copper or brass wire would be likely to 
become amalgamated with mercury. 


CALIBRATING GAS MEASURING VESSELS. 743 


E. By raising the levelling bulb K, the air is expelled from W 
and the capillary #, and the mercury is allowed to rise in the fun- 


























nel T. The stop-cock H is turned so that communication is estab- 
lished between & and W, and the ends of the former are opened 
by pressing the capillaries against r and r’. Then, by raising N 
and lowering K, the gas is readily driven over into W. 


Calibrating Gas Measuring Vessels. 


When vessels are purchased to be used in measuring gases, 
the correctness of the calibrations should always be tested; the 
testing can be done with water or with mercury. The calibration 
‘with water is carried out in exactly the same way as was described 
for vessels to be used in measuring liquids (cf. pp. 522-530). 


‘ 


744 GAS ANALYSIS. 


The calibration by means of mercury will be illustrated by an 
example. We shall assume that it is desired to calibrate the 
apparatus shown in Fig. 106. The vessel must be thoroughly 
cleaned and then placed in a vertical 
position as shown in Fig. 106 II. The 
lower capillary a is connected by means 
of thick-walled rubber tubing with a 
leveling wessel containing mercury, and 
the mercury is made to rise slowly in 
the vessel to a little above the upper 
mark. The stop-cock is then closed, the 
leveling tube together with the rubber 
tubing is removed, and the mercury al- 
lowed to flow out slowly until the highest 
point in the meniscus is exactly tangent 
to the horizontal plane through a’a’. To 
avoid a parallax error, the reading is 
taken by means of a _ telescope placed 
2 or 3mm. away from the glass. The 
whole contents of the vessel, including 
the space in the stop-cock, is next al- 
lowed to run into a tared flask, which 
is then weighed to the nearest centigram. 
After determining the temperature of the 
mercury, its volume can be found by 
means of the table (top of page 745) pre- 
pared by Schlésser.* 

If the weight of the mercury at 20.3° 
amounted to 2025.26 gms., then its volume corresponds to 
2025.26 
13.5483 
cus and the volume is desired up to the plane aa’, it is evident 
that the volume of mercury weighed did not include the space 
a’a-aa’ and, moreover, since the instrument is to be used in 
the reversed position, the error is really twice as much, as is 
evident from the inspection of Fig. 1061. This is called the 





p 








ee [PUT] La cre ee IY LPT 








= 149.41. Since, however, mercury forms a convex menis- 





* Schlosser and Grimm, Z. Chem. App.-Kunde, 2, 201 (1907). 


CALIBRATING. GAS MEASURING VESSELS. 745 


WEIGHT OF 1 C.c. OF MERCURY IN AIR AT TEMPERATURES BETWEEN 15° anpb 20°. 
Normal temperature 15°. 





























Temperature Weight Temperature Weight Temperature Weight 
of Mercury. of Mercury. of Mercury. 

Deg. C. Gms. Deg. C. Gms. Deg. C. Gms. 
15 13.5593 20 13.5489 25 13.5385 
15.5 13.5583 20.5 13.5479 25.5 13.5374 
16 13.5573 21 13.5468 26 13.5364 
16.5 “1 13.5562 7 oo Mi 13.5458 26.5 13.5353 
17 13.5552 22 13.5447 27 13.5343 
17.5 13.5541 22.5 13.5437 27.5 13.5332 
18 13.5531 23 13 . 5426 28 13.5322 
18.5 13.5520 23.5 13.5416 28.5 13.5312 
19 13.5510 24 13.5405 29 13.5301 
19.5 13.5499 24.5 13.5395 29.5 13.5291 

30 | 13.5280 








double meniscus correction. Its value is dependent upon the bore 
of the tube, as is shown by the following table: 


TABLE OF MENISCUS CORRECTIONS.* 











4 Double Meniscus, Double Meniscus, Simple Meniscus Cor- 
Diameter of Correction for Hg in | Correction for H2O in | rection (H20—Hg) in 
Tube in mm. mgs. mgs.=cubic millimeters cubic millimeters. 

3 76 Aas 3 
‘ 108 20 6 
5 174 31 9 
6 314 44 10 
7 550 61 10 
8 790 81 11 
9 1038 106: )<;) 15- 
10 1288 134 20 
11 . 1540 167 27 
12 1796 _ 204 36) 
13 2058 245 46 
14 2326 289 59 
15 2596 ? 336 72 
16 2872 387 88 
17 3152 441 104 
18 , 3436 499 128: 
19 3726. 560 _ 148 
20 4016 624 164 
21 4314 691 187 
22 4614 757 208 
23° 4920 821 229 
24. 5230 881 247 
25 5544 938 264 
26 5864 991 279 
97 6185 1042 293 
28 6515 1090 308 
29 6845 1135 315 
Peet 7182 1179 324 











* W. Schlésser. Private Communication. 


746 GAS ANALYSIS. 


If the diameter of the vessel in question is 20 mm., then the 
correction, according to the vane, would be 4.016 gms. and the 
true volume will be 


2025.26+4.016 2029.276 


13.5473 *:13.5483 149-78. 





The volume of this instrument, tMferefore, is 0.22 cm. less than 
the intended 150 c.c. The volume of the narrower parts of the 
tube can be found in a similar manner. 

The diameter of the tube is best determined by filling with 
mercury up to a mark, then allowing it to run out until a lower 
mark is reached, weighing the escaped mercury, and measuring 
the distance between the two marks with a millimeter rule. 
If the weight of the mercury is p, the distance between the marks 
h, the temperature of the mercury 20.3°, then the 








diameter = 2y hXn a 5483" 


In many cases it is sufficiently accurate to compute the diameter 
from the circumference of the tube and then subtract twice the 
thickness of the glass. 

If it is desired to determine the total volume of a tube provided 
_ with stop-corks at both ends, the apparatus is weighed empty 
and then filled with mercury. In this case, it is obvious that no- 
meniscus correction is necessary. | 

For a measuring vessel calibrated with water, when in a 
reversed. position, the meniscus correction is obtained from the 
table on page 745. If an instrument calibrated with water is 
to be used subsequently with mercury, the water meniscus in 
calibrating the reversed tube occupies a similar position to that 
of the mercury meniscus when the instrument is in use (see Fig. 
107) but the mercury meniscus is not so deep as that of the water. 
The volume of the gas is therefore found as much too large as the 
difference between the simple meniscus corrections (H2O —Hg). 
Thus if the volume of a gas measuring instrument of 70 mm. 
diameter is found by weighing with water to be 10.167, according 


PURIFICATION OF MERCURY. 747 


to the table on page 745, then if the instrument is to be used with 
mercury, the gas volume will be 10.167 —0.020 =10.147 c.c. 


Purification of Mercury. Lothar Mayer’s Method. * 


The mercury used for gas-analytical operations must be 
purified. The principal impurities are copper, cadmium, zinc 
and sometimes silver and gold. The base metals are removed 










Ue 


ut 





i 










i 


i 


SSS 





ml 






i 
w 


j 








| 
if 






us 





ih 


i 








sien 











Fie. 107. Fie. 108. 


most readily by allowing the mercury to run in a fine stream 
through about a meter of 8 per cent. nitric acid. This is done 
in the apparatus shown in Fig. 108. The bottom of the tube B 
is first filled with impure mercury and the nitric acid is added. 
The mercury is then poured through the funnel A, the stem of 
which is drawn out to a capillary and bent to an angle of 60°. 
This causes the mercury to take a zig-zag course as it flows slowly 
through the nitric acid. The dry mercury that first passes over 
into the flask Cis impure and must be poured into the funnel and 
allowed to flow through the acid. In this way a fairly pure mercury 

*Z. anal. Chem., 2, 241 (1863). C. J. Moore (Chem. Ztg., 1910, 735) 


has used a similar apparatus for purifying large quantities of mercury, but 
filters through buckskin before allowing it to fall through the acid. 





748 GAS ANALYSIS. 


is obtained which can be used as it is for most purposes. If the 
mercury is to be used for calibrating apparatus, it must be distilled. 

For this purpose, Hulett’s apparatus, shown in Fig. 109, may | 
be used. The mercury.is placed in the long-necked flask K which 
is connected with the receiver V. The flask is covered with a 
hood of asbestos paper and heated on the sand bath. Through 
the arm a, the receiver is connected with a suction pump and at 


b 




















Fie. 109. 


b a slow current of nitrogen (or carbon dioxide) which has been 
dried by passing over calcium chloride, is introduced into the 
flask through the long glass tube that ends in a capillary. The 
- distillation is regulated so that the mercury condenses in the glass 
arm c, where it leaves the hood of-asbestos paper. About 150-200 
c.c. of mercury can be distilled in an hour with this apparatus. 
Frequently, especially when the nitrogen used contains a little 
oxygen, the distilled mercury is covered with a thin coating of 
oxide. This may be removed by filtration. To filter the mercury, 
the point of a paper filter is perforated several times with a 
needle, the filter placed in a funnel and the mercury poured 


SUBDIVISIONS OF GAS ANALYSIS. 749 


through. The pure metal runs through the holes in the paper 
while the impurity remains behind. 


Subdivisions of Gas Analysis. 


According to the manner of determining the amount of gas, 
we distinguish between: 

1. Absorption Methods. 

2. Combustion Methods. 

3. Volumetric Methods, 


In the case of an Uinje tion method. the- minture of gases is 
treated with a series of absorbents. The difference in the volumes 
of the gas before and after it has been acted upon by each absorbent 
represents the amount of gas absorbed. The absorption of the 
gas may take place in the measuring-tube itself, or, what is better, 
in separate absorption vessels. 

- In this way, the amount of carbon dioxide, heavy hydrocar- | 
bons (ethylene, benzene, acetylene, etc.), oxygen, and carbon mon- 
oxide may be determined in illuminating-gas, producer gas, water- 
gas, or Dowson gas. 

After the constituents capable of absorption have been re- 
moved, a gas residue is left consisting of hydrogen, methane, 
and nitrogen; the two former constituents are determined by 
combustion, while the latter is always determined by subtracting 
the total amount of other gases found from 100 per cent. 

For a combustion analysis the unabsorbed constituents of the 
gas mixture are mixed with air, or oxygen, in more than sufficient 
amount to ensure complete combustion, and burnt in a suitable 
apparatus; the amount of combustible gas is determined ' by 
measuring the contraction, the amount of carbon dioxide formed, 
and the excess of oxygen. | 

Finally, if the gas evolved by means of a chemical reaction is 
measured and from the volume of, the latter the weight of the body 
producing it is calculated, we have made use of what is called 
a gas-volumetric method. (Cf. Determination of Carbonic and 
Nitric Acids, pp. 384 and 456), 


750 GAS ANALYSIS. 


DETERMINATION OF THE GASES. 
Carbon Dioxide, CO. Mol. Wt. 44. 


Density =1.5290 * (Air=1). Weight of 1 liter =1.9767 gms, 

Molar volume =22.26 1. 

Critical temperature = +31.5° C. 

Carbon dioxide is absorbed to @ considerable extent by water; 
‘1 vol. water absorbs: 


At OPO. ud aca ec ee ee 
8 BO Oe ee ee ae ne 
tt O80 Osu alpen ah ecaean aaa eek IRRS Sic ae 


or in general 


£8 t =1.7967 —0.07761 Xt+0.0016424 x 2, 


Absorbent.—Potassium H ydroxide Solution 1:2. 

1 c.c. of caustic potash of the above strength will absorb at 
least 40 c.c. of COg. Sodium hydroxide solution is not used on 
account of the difficult solubility of sodium bicarbonate. 

Small amounts of COg may be absorbed by means of a definite 
amount of standardized Ba(OH)2 solution, and the excess of the 


latter titrated with a HCl, using phenolphthalein as indicator. 
(See p. 593). 





* This number is the mean from the observations of Lord Rayleigh 
(1897) =1.52909, Leduc (1898) =1.52874, and Christie (1905) =1.52930. 

+ £ is ealled the absorption coefficient of the gas. This signifies the 
volume of gas, measured at 0° and 760 mm. pressure, which 1 c.c. of a liquid 
at ¢° will absorb when the pressure upon the surface of the liquid is 760 mm. 
If h c.c. of liquid, at ¢ and B mm. pressure, absorb V; c.c. of the gas, then 
the absorption coefficient can be computed by the equation: 


Vi 
Pr hited)" 


THE HEAVY HYDROCARBONS. 751 


The Heavy Hydrocarbons. 


Ethylene (Ethene), CoH4; Benzene, CgsHg; Acetylene (Ethine), 
CoH. 


Ethylene, C2.H4. Mol. Wt. 28.03. 


Density =0.9739* (Air=1). Weight of 1 liter =1.2590 gms. 
Molar volume =22.27. Critical Temperature = +9° C. 


Preparation of Ethylene—One of the most satisfactory methods 
consists in treating an alcoholic solution of ethylene bromide 


with zine dust ft: % 


CoH 4Breo + Zn = ZnBre fe CoH. 


A round-bottomed flask of about 200 c.c. capacity, and having 
a short wide neck, is chosen for the experiment. In the neck is 
inserted a rubber stopper with three holes, carrying respectively 
a safety tube provided with mercury seal, a gas delivery tube, 
and a dropping funnel. 

A sufficient amount of zine dust, moistened with alcohol, is 
placed in the. flask and gently heated at the start by placing the 
flask in a bath of warm water at about 50° C. A mixture of 1 
part ethylene bromide and 20 parts absolute alcohol is placed in 
the dropping funnel, and allowed to flow slowly upon the zinc 
dust. The escaping gas is passed first through olive oil, to remove 
a little ethylene bromide which is carried over mechanically, then 
through caustic potash solution, and finally through water; it is 
collected over mercury, perhaps in the Drehschmidt pipette 
(Fig. 105, p. 743). The gas thus obtained is almost pure, par- 
ticularly when the mixture of ethylene bromide and alcohol has 
stood for some time over anhydrous sodium carbonate to remove 
traces of hydrobromic acid. 

A sample of the gas prepared by W. Misteli was found to con- 





*M. Bretschger (Inaug. Dissert. Ziirich, 1911) found the density of ethylene 
to be 0.9724, but M. Stahrfoss and P. A. Guye (Arch. soc. phys. et nat, 28 
1909) found the value 0.9758. In the text the mean of these two values 


is used. 
+ Gladstone and Tribe, Berichte, 7, 364 (1874). 


752 VOLUMETRIC ANALYSIS. 


tain 98.84 per cent. of ethylene, 1.00 per cent. of hydrogen and 
0.16 per cent. of nitrogen. 


Absorption Coefficient for Water. 


1 volume of water absorbs at 


Pas esas bs Rath ti wash 0.256 c.c. C,H, 
1PM du ieee a a Rao 0.161 “ « 
DI Be CMMI Ren Cudahy cyte hm Gueaid 0.149 “« « 


or in general, 
8 =0.25629 =0.00913631¢+-0.00018810822. 
Alcohol absorbs more ethylene; the general formula is 
8=3.594984 —0.077162-t+0.0006812- ¢?, 


Absorbents.—1. Fuming sulphuric acid * (with 20 to 25 per 
cent. free SO3), 1 c.c. absorbs 8 c.c. of CoH4. 2. Bromine water.t 

Ammoniacal cuprous chloride solution will also absorb ethy- 
lene. 

By means of bromine, the ethylene is absorbed with the for- 
mation of ethylene bromide, C2H4Bre. If the absorption is 
effected with a titrated bromine water, the amount absorbed 
can be determined by titrating the excess of bromine. This 
excellent method, proposed by Haber,t is at present the best 
known for the determination of ethylene in the presence of 
benzene. (See p. 818.) 


Benzene, CgHs. Mol. Wt. 78.04. 


78.04 gms. of benzene vapor occupy a volume of 22.391 liters 
under standard conditions. 

Benvene is readily soluble in alcohol, ether, carbon bisulphide, 
caoutchouc, ethylene bromide, bromine, and fuming. sulphuric 
acid. : is 





_ * Ethionic acid, C2H,S20,, is formed. eg we 
{ Treadwell and Stokes, Berichte, 21 (1888), p. 3131. 
{ Haber and Oechelhauser, Berichte, 29,.p. ‘2700, . 


BENZENE. 753 


_Absorbents.—Fuming sulphuric acid* and bromine water con- 
taining an excess of bromine. 

Inasmuch as benzene is neither brominated nor oxidized by 
bromine at ordinary temperatures, it was difficult to under- 
stand why bromine water should absorb it quantitatively. In 
fact, Berthelott and Cl. Winklert disputed it, but the results 
of Treadwell and Stokes$ have recently been confirmed by 
Haber. He suggested that the absorption of benzene by bromine 
was of a purely physical nature, and M. Korbuly|| has shown 
that such is the case. Just as bromine can be removed from 
aqueous solution by shaking with benzene, so benzene can be 
removed by shaking with bromine, or even ethylene bromide and 
other like solvents. 

By means of highly concentrated nitric acid (specific gravity 
1.52) benzene is also absorbed; this solvent cannot be used in the 
analysis of gases containing carbon monoxide, for the latter is 
quantitatively oxidized to carbon dioxide by nitric acid of this 
strength, and is therefore removed with the benzene { when the 
acid vapors are neutralized by caustic potash solution. | 


Behavior of Benzene to Water. 


Benzene vapors are absorbed to a considerable extent by water 
and all aqueous salt solutions, a circumstance which must be 
considered when an exact gas analysis is to be made. 

In order to determine how much benzene is absorbed by water, 
M. Korbuly performed the following experiments: 

Different amounts of air containing 3.16 per cent. of benzene 
vapor were shaken in a Drehschmidt’s pipette with the same 
amount of water (5 c.c.) until no more benzene was absorbed. He 
obtained the following results: 7 





* Benzene sulphonic acid is formed, C,H,SO3. 
t Compt. rend., 82, p. 1255. 

{ Zeitsehr. f. anal. Chem., 1889, p. 281. 

§ Treadwell and Stokes, loc. cit. 

|| Inaug. Dissertation, Zurich, 1902. 

{| Treadwell and Stokes, loc. cit. 


754 GAS ANALYSIS. 








Per Cent. Benzene | Amountof Benzene Absorbed 
Experiment. Gas Taken in c.c. Present by at the End of Three 
Volume. Minutes. 
1 58 .92 3.16 1.28 c.c.= 2.17% 
2 61.14 3.16 0.80 ‘ =1,.31% 
3 58.32 3.16 0.52::** =0.89% 
4 59 .86 3.16 0.44 §§ =0.73% 
5 60.78 3.16 2a =0.46% 
6 59.88 #3.16 0.08 ‘4 =0.01% 
7 60.20 3.16 0.02 ‘f =0.00% 














Potassium hydroxide behaves similarly. 

In the analysis of a mixture of carbon dioxide and benzene, 
it is customary to first remove the carbon dioxide by means of 
potassium hydroxide solution and then the benzene with fuming 
sulphuric acid or bromine. It is evident, then, that both of the 
results obtained will be inaccurate if a fresh solution of potassium 
hydroxide is used for the absorption of the carbon dioxide, for 
this will absorb not only the whole of the carbon dioxide, but in 
‘many cases nearly all of the benzene. Accurate results may be 
obtained by using a solution of potassium hydroxide which has 
been saturated with benzene vapors. 


Acetylene, CoH». Mol. Wt. 26.02. 


Density =0.9134 * (Air=1). Weight of one liter =1.1808 gms, 
Molar volume =22.03 1. Critical temperature = +37° C, 
Boiling Point = —80.6° C. 


Acetylene is quite soluble in water; 1 volume of water at the 
ordinary temperature absorbs an equal volume of this gas. In 
amyl alcohol, chloroform, benzene, glacial acetic acid, and acetone 
it is much more soluble; thus 1 volume of acetone absorbs 31 
volumes of acetylene.t 





*M. Bretschger (Inaug. Dissert. Zurich, 1911) found the density of 
acetylene =0.9157, whereas M. Stahrfoss and P. A, Guye (Arch. sci. phys. et 
nat., 28, 1909) found it=0.9120. The mean of these two values is 0.91335. 

+ Hempel, Gasanalytische Methoden (1900), p. 206. 


PREPARATION OF PURE ACETYLENE. 755 


Preparation of Pure Acetylene. 
(a) Method of M. Bretschger.* 


The crude acetylene, prepared from calcium carbide, is passed — 
through an acid solution of copper sulphate, then through aqueous 
chromic acid, caustic potash, and finally over slaked lime; it is 
then subjected to a fractional distillation. The gas is passed 
through a small bulb cooled by liquid air which causes the acetylene 
to solidify. By gentle warming, the acetylene is then evaporated 
and is caused to pass through calcium chloride tubes, 


(b) Method of M. Stahrfoss and P. A. Guye.t 


The impure acetylene, prepared from calcium carbide, is passed 
through a solution of potassium permanganate, then through 
caustic potash solution and finally over phosphorus pentoxide. 
It is frozen by means of liquid air and then fractionated. 

The method of preparing acetylene by decomposing copper 
-acetylide cannot be recommended, because the gas is then strongly 
contaminated with ethylene (C2H4) and vinyl chloride (C2H3Cl). 
Thus M. Bretschger ¢ found from 5 to 10 per cent. of ethylene 
in such gas. 

Absorbents: Fuming sulphuric acid.§ By bromine water 
acetylene is absorbed extremely slowly in the cold, a fact which 
permits the titration of ethylene in the presence of ian 
(see page 821). 

_By means of ammoniacal cuprous chloride, acetylene is 
absorbed and forms red copper acetylide (CusCeH2)O. This 
reaction is so characteristic that it is used for the 


Qualitative Detection of Acetylene 


in gas mixtures. This test is best performed by the method of 
L. Ilosvay von Nagy Ilosva.|| 





“ * Loc. cit. | 
{ Private Communication from Professor Guye. 
t Loe. cit. 
§ C,H,SO, is formed. 
|| Berichte, $32 (1899), p. 2698. 


756 GAS ANALYSIS. 


Preparation of the Reagent.—One gram of copper nitrate 
(chloride or sulphate) is placed in a 50-c.c. measuring-flask and 
dissolved in a little water. To the solution 4 c.c. of concentrated 
- ammonia (20-21 per cent. NH3) and 3 gms. of hydroxylamine 
hydrochloride are added, and the liquid is shaken until it becomes 
colorless, when it is immediately diluted with water up to the 
mark. r 

The Qualitative Test——A few cubic centimeters of the reagent 
are placed in a 500-c.c. glass-stoppered cylinder and the gas to be 
tested for acetylene (illuminating-gas) is passed over it until the 
color of the reagent becomes pink. The cylinder is then stoppered 
and its contents thoroughly shaken. If acetylene is present, a 
beautiful red precipitate is immediately formed. Another method 
of making the test is to pass the gas through a small bulbs 
containing glass-wool moistened with the reagent. 

Remark.—lf the reagent is placed under petroleum it can be 
kept for about one week, but if copper wire is added to the solu- 
tion, it can be kept for a much longer time, as L. Pollak has shown. 
Such a solution gave a distinct reaction after it had been kept 
for one year, but the precipitate obtained, instead of being a bright 
red, was more the color of sealing-wax. The solution is much 
less permanent when it is prepared from the chloride or sulphate, 
even when copper is added to it. Without the copper, the chloride 
would give no reaction after being a week old, and with the addition 
of copper it was spoiled at the end of two weeks. The sulphate 
behaved about the same. | 


Separation of the Heavy Hydrocarbons from One Another. 


It has been attempted repeatedly to separate ethylene from 
benzene, but usually in vain. The separation as proposed by 
Berthelot, of absorbing the ethylene with bromine water and 
afterwards removing the benzene by means of concentrated nitric 
acid, is erroneous in every respect.* The method of Harbeci: 
and Lungef is correct in principle but very tedious, and the 
original modification of Pfeiffer t always gives too high results. 





* Treadwell and Stokes, loc. cit. 

{ Zeit. f. anal. Chem., X VI (1898), p. 26. 

t J. f. Gasbeleuchtung und Wasserversorgung, 1899, p. 697, and Berichte, 
29, p. 2700. 


OXYGEN. 757 


Recently Pfeiffer * has improved his method so that it gives 
the same results as that of Harbeck and Lunge. 

Haber and Oechelhiuser,t on the other hand, have devised 
a method which is accurate and to be recommended. 


Principle.—In one portion of the gas, the sum of the ethylene 
and kenzene is determined by absorption with bromine water or 
fuming sulphuric acid, while in a second portion the gases are 
absorbed in titrated bromine water, and the excess of the latter 
is determined iodimetrically. From the amount of bromine re- 
quired the ethylene is calculated: 


N 


06: i0 I=1.114 ¢.c. CoH, at 0° C. and 760 mm. pressure. 


As this analysis is performed in the Bunte burette, it will not 
be described in detail until we have become acquainted with this 
important form of apparatus. (See p. 798.) 


| Oxygen, O=i6. Mol. Wt. 32. 
Density =1.1053 (Air=1). Weight of 1 liter=1.4289 gms. 
Molar volume=22.39 1. Critical temperature = — 119° C. 


Oxygen is only slightly soluble in water; according to the 
experiments of L. W. Winkler,t one liter of water at 60 mm. 
pressure will absorb the following quantities of air: 


ABSORPTION COEFFICIENTS OF ATMOSPHERIC AIR. 
(1000 ¢.c. absorbed) 


Temperature. Oxygen. Nitrogen. Air. 
IR te acc ona ui 10.24 c.c. 18.57 c.c. 28.81 c.c. 
Brose Gye et gits 8.98 ‘ 16.45. °° 25.43°.** 
i” Lethe o caer ere ri” 7 tags 14:67 *¢ ay .64 
BGro Sorh ue Deuces yO) | RA, 13.29. ** 20.45. ** 
Wy ee Soars ae we 6.50: ** 1239 18.69 ‘‘ 
WO Ose ea Pees 5.93: ** je) aa 17. 24.4" 
WO Napa de oe baz <"* 10.59 ‘‘ 16.06 ‘‘ 
SO eT eed at 1, 2s Beh POA 15.03 ‘* 
RR EO ore y eae oo 4-83." 9. 35.** 45ie** 
OBS SPOS pats 4.58:*‘ oan. 13.51 
DP oprah eee Ba eG 4.38 ‘ 8.59 ‘ 12.07" 
BO se iaticae ahah wk ae ts Sok. 2.53°°* 





* See Chem. Ztg., 1904 (884). 

+ J. £. Gasbeleuchtung und Wasserversorgung, 1896, p. 804, and Berichte, 
29, p..2700. 

{Berichte, 34, 1410 (1901). 


758 GAS ANALYSIS. 


From these data, the absorption coefficient of pure oxygen for 
water at 0 to 55° can be computed. . 


ABSORPTION COEFFICIENTS OF OXYGEN FOR WATER, 


Temperature. B Temperature. B 
Oo yi ary > eee | a Sa ey eRe 0.02608 
Rees ig gat 0.04286 Be ee 0.02440 
LO: ker ee 0.03802 WR os eno. 0.02306 
1 eee 0.03415 WAR ie 0.02187 
abe Ghee yes 0.03102 BOM Pe ses os 0.02090 
BO Sins wale Mere 0.02831 BOA GAR 0.02012 


Oxygen can be determined by combustion or by absorption. 


The Determination of Oxygen by Combustion. 


The determination of oxygen by combustion may be effected 
by exploding it with hydrogen (Bunsen) or by conducting a 
mixture of the two gases through a glowing platinum capillary 
(Drehschmidt), exactly as in the determination of carbon mon- 
oxide (cf. p. 765). In both cases the combustion takes place in 
accordance with the equation : 


O + He = HO 


1 vol. 2 vols. 0 vol. 


Three volumes of gas, therefore, disappear for each volume of 
oxygen present. If the contraction resulting from the com- 
bustion of a mixture of oxygen and an excess of hydrogen is 
designated by V,., then the amount of oxygen present =3Vc. 


The Determination of Oxygen by Absorption. 


The absorbents of oxygen are: 


1. Alkaline Pyrogallol Solution (Liebig). 


One volume of a 22 per cent. aqueous solution of pyrogallol is 
mixed with five or six times as much potassium hydroxide solution 
(3:2). 1c.c. of this solution absorbs 12 ¢.c. of oxygen. 


OXYGEN. 759 


At a temperature of 15° C., or higher, the absorption takes 
place quickly; the oxygen in 100 e.c. of air will be absorbed in 
three minutes or less. 

At lower temperatures the absorption takes place less readily 
and at 0° C. the above quantity of oxygen cannot be absorbed 
completely in half an hour. 

A pyrogallol solution of the above concentration will not evolve 
carbon monoxide during the absorption. 


1 


2. Phosphorus (Lindemann). 


| The absorption of oxygen by means of phosphorus takes 

place by simply allowing the gas containing the oxygen to remain 
over moist phosphorus. The formation of white clouds indicates 
the presence of oxygen, and their disappearance shows that 
the absorption is complete. A temperature of 15 to 20° C. is 
best suited for the absorption. 

The oxygen is completely absorbed at the end of three min- 
utes from 100 c.c. of air at this temperature. At lower temper- 
atures the absorption requires more time and at 0° more than an © 
hour is necessary. | 

If the gas contains more than 60 per.cent. of oxygen, moist 
phosphorus will absorb none of it at the ordinary atmospheric 
pressures. In this case the gas must be diluted with nitrogen or 
hydrogen until a mixture is obtained containing less than 60 
per cent. oxygen, or the gas must be allowed to act upon the moist 
phosphorus under diminished pressure. In the latter case, how- 
ever, the phosphorus easily becomes heated enough to melt it. 

Further, oxygen is not absorbed by moist phosphorus if the 
gas contains traces of heavy hydrocarbons, ethereal oils, alcohol, 
or ammonia. According to Hempel* 0.04 per cent. of ethylene, 
and according to Haber { 0.17 per cent., suffices to prevent com- 
pletely the absorption of oxygen. 





* Gasanalytische Methoden. 
+ Experimental-Untersuchung iiber Zersetzungen und Verbrennungen 
von Kohlenwasserstoffen, Habilitationschrift, Munich, 1896. 


760 GAS ANALYSIS. 


3. Chromous Chloride. 


Consult the paper by Otto von der Phordten, ‘Aohat Chem. 
Phys. 228, 112. 
4. Copper. 


The gas is either conducted over glowing copper, or it is intro- 
duced into a Hempel pipette containing rolls of copper gauze and 
an ammoniacal solution of ammonium carbonate. 


5. Sodium Hydrosulphite,* NagS2O4 (Franzen ft). 


An alkaline solution of sodium hydrosulphite, which can now 
be obtained commercially at a low price, is an excellent absorbent 
for oxygen. The reagent may be prepared for use in the Hempel 
pipette by dissolving 50 gms. of the salt in 250 ¢.c. water and 40 
c.c. caustic potash solution (500 KOH:700H2O). For absorption 
in the Bunte burette, the above solution is too concentrated; in 
this case 10 gms. hydrosulphite in 50 c.c. water plus 50 c.c. of 10 
_ per cent. caustic soda, may be used. 

The absorption takes place in accordance with the eq ibaa a 


2NagS204 + 2H2O + Ovo = 4NaHSOsz. 


Sodium hydrosulphite has the advantage over all other 
absorbents that the absorption is always complete at the end of 
five minutes. 


Determination of Absorbed Oxygen in Water. Method of 
L. W. Winkler.{ 


1000 c.c. > 9 Nan8.05 pohution miceeawile onl: 8 gm. or 559.8 ¢.c. oxygen at 0° 


and 760 m.m pressure. 


Principle—Ilf water containing dissolved oxygen be heated in 
a closed vessel with manganese hydroxide, the latter is oxidized 
to manganous acid according to the following equation: 


Mn(OH)2+0 =H2MnQOsz. 


* This is really sodium hyposulphite, although sodium thiosulphate, NaS,0,, 
is commonly called “ hyposulphite.”’ 

t Berichte, 39, 2069 (1896). 

{ Ibid, 21 (1888), p. 2843. 





DETERMINATION OF ABSORBED OXYGEN IN WATER 761 


The amount of oxygen taken up is determined iodimetrically by 
adding hydrochloric acid and potassium iodide to the manga- 
nous acid and titrating the liberated iodine, 


H,MnO,+4HCl=MnCl,+2H,0+ Cl, and 2KI+Cl,=2KCl+ 1. 


Hence 1 gm.-at. [=8 gms. = 5597.8 c.c. oxygen at 0° C. and 
760 mm. pressure. 

Reagents Required—1..An approximately 4N-MnClz solution 
obtained by dissolving 400 gms.of MnCle+4H20 in water and dilut- 
ing to 1000 c.c. The manganese chloride must be free from iron. 

2. Sodium Hydroxide Solution Containing Potassium Iodide-—On 
~ account of the nitrite usually present in commercial sodium hy- 
droxide, the alkali solution is prepared from sodium carbonate and 
calcium hydroxide. The clear liquid is siphoned off and con- 
centrated in a silver dish until its specific gravity is 1.35. In 100c.c. 
of this solution, 10 gms. of potassium iodide are dissolved. 

A portion of the alkaline potassium iodide solution on being 
acidified with hydrochloric acid should not immediately turn 
starch paste blue, and, furthermore, large amounts of carbonate 
must not be present. 


3. " Sodium Thiosulphate Solution. 


Procedure.—A_ glass-stoppered flask of about 250-c.c. capacity 
is taken and its exact capacity is determined by weighing it first 
empty and then filled with water at 17.5° C. If the water to be 
analyzed is saturated with air, it is simply poured into the flask, 
otherwise the water is conducted through it for ten minutes. 
Then, by means of a pipette reaching to the bottom of the flask, 
1 c.c. of the alkaline potassium iodide solution is introduced and 
immediately afterwards 1 c.c. of the manganese chloride solution. 
The flask is closed, shaken, and allowed to stand until the previp- 
itate has settled. Then, by means of the long-stemmed pipetie, 
about 3 ¢.c. of concentrated hydrochloric acid are introduced and 
the contents of the flask once more shaken. The precipitate dis- 
solves readily with liberation of iodide and the latter is titrated 
with sodium thiosulphate in the usual way. 

Remark.—The results obtained by this method agree closely 
with those obtained by boiling the water as described on p. 739. 


762 GAS ANALYSIS. 


Carbon Monoxide, CO. Mol. Wt. 28. 
Density =0.96702 (Air=1). Weight of 1 liter 1.2502 gms. 
Molar Volume= 22.397 liters. Critical temperature = — 136° C. 


Preparation.—Some concentrated sulphuric acid is heated in a 
fractionating flask to a temperature of 140° to 160° C. upon an oil 
bath, and formic acid (sp. gr. 1.2) is allowed to drop into it: 


HCOOH = 420 +C0O. 


In order to free the escaping gas from water and acid vapors, it is - 
conducted first through a Liebig condenser, which leads to an ~ 
empty flask to receive the condensed water, and from thence into 
a concentrated caustic potash solution. 

This method * yields about 60 liters of carbon monoxide in 
half an hour, using about 500 ¢c.c. of concentrated sulphuric acid. 
The method of Wade and Panting,f according to which very pure 
carbon monoxide can be prepared by allowing concentrated 
sulphuric acid to drop upon potassium cyanide, is not, according 
to Allner, a suitable process for preparing large quantities of the 
gas; because considerable potassium cyanide becomes enveloped 
in pyrosulphuric acid during the reaction, so that there is con- 
siderable danger involved in working with the residues. 

By the action of hot concentrated sulphuric acid upon oxalic 
acid, it is very easy to prepare a mixture of equal volumes carbon 
monoxide and carbon dioxide; on account of the large amount 
of the latter, however, this method is less satisfactory than that 
of Allner. 

The gas is only very slightly soluble in water; 


ABSORPTION COEFFICIENTS OF CARBON MONOXIDE FOR WATER.{ 


Temperature. B Temperature. B 
chs cece ss 0.03537 a... os Ree 0.01998 
Bier yee 0.03149 DW ee ees 0.01877 

TO epi csr; 0.02816 rt gigas tery 2 SEB 0.01775 
Be Chea ea 0.02543 rs eee EN, Gee 0.01690 
A airs Bee bias 0.02319 BOP ite Leese 0.01615 
BOF ics, ete S's 0.02142 Ge ieatiacces: 0.01548 





* W. Allner, Inorg. Dissert. Karlsruhe, 1905. 
t J. Chem. Soc., 73, 255. 
t L. W. Winkler, Berichte, 34, 1414 (1901). 


F: CARBON MONOXIDE. 763 


In alcohol the gas is about ten times more soluble than it 
is in water. 

Its determination is effected either by absorption or by com- 
bustion. 

Absorbents.—Ammoniacal Cuprous Chloride. 200 gms. of com- 
mercial cuprous chloride are shaken in a closed flask with a solu- 
tion of ammonium chloride (250 gms. in 750 ¢.c. water), and to 
every three volumes of this mixture 1 vol. of ammonia, specific 
gravity 0.91, is added. In order that the solution may remain 
active, a spiral of copper wire is introduced into the flask long 
enough to reach from the bottom up to the stopper. 

1 c.c. of this solution will absorb 16 ¢.c. of carbon monoxide. 

Formerly it was the almost universal custom to absorb this 
gas by means of a hydrochloric acid solution of cuprous chloride, 
but to-day this is not done on account of the following reasons. 
The absorption of carbon monoxide by means of cuprous chloride 
takes place according to the.following equation: 


CuosCle + 2CORCueCle.2CO.* 


The compound CugCl2.2CO is extremely unstable and can only 
be formed when there is a certain pressure exerted by the carbon 
monoxide, so that when the acid solution is used the absorption will 
never be quantitative. Further, if a gas free from carbon monoxide 
(nitrogen or hydrogen) is shaken with such a solution after it has 
been used several times, a part of the CuoCle.2CO in solution will 
be decomposed according to the above equation in the direction 
of right to left, until the partial pressure of the carbon monoxide 
set free is sufficient to restore equilibrium. Consequently the 
volume of the gas will appear greater after it has been treated 
with the cuprous chloride solution than it was originally. 

When an ammoniacal cuprous chloride solution is employed, 
the absorption of the carbon monoxide is almost quantitative, but 
after such a solution has been used repeatedly it will readily give 
up some of the gas, although not so readily as is the case of the 
‘solution of cuprous chloride in hydrochloric acid or calcium chlo- 





* The compound has been isolated in the solid state, according to W. A. 
Jones (Am. Chem. J., 22, 287) its formula is CuCl,-2CO-4H,0, but according 
to the experiments of C. v. Girsewald in the author’s laboratory, the formula 
is Cu,Cl,+2CO-2H,0. 


764 GAS ANALYSIS. 


ride.* It is advisable, therefore, to adopt the suggestion of Dreh- 
schmidt, and first absorb the greater part of the gas by means of 
an old solution of cuprous chloride, afterwards removing the last 
traces by means of a freshly-prepared solution, or one which has 
beep used but a few times. 

Besides carbon monoxide, the ammoniacal cuprous chloride 
solution will absorb acetylene, ethylene, etc., so that these gases 
must be removed previously by means of fuming sulphuric acid or 
bromine water. 

By long shaking with concentrated nitric acid (specific gravity 
1.5), carbon monoxide is completely oxidized to carbon dioxide, 
and the latter can be removed by treatment with potassium 
hydroxide solution.} 


Determination of Carbon Monoxide by Combustion with 
Air or Oxygen. 


The following reaction shows how carbon monoxide may be 
determined by combustion: 


CO'+ 0 = CO, 


2vols. 1 vol. 2 vols. 


From the reaction we can make the following deductions: 

1. The difference in the volume of the gas mixture before and 
after the combustion is for 2 vols. CO; 3—2=1 and for 1 vol. 
CO=4. This difference is designated as the contraction. The 
contraction caused by the combustion of carbon monoxide is, there- 
fore, equal to half the original volume oj CO. 

2. The volume of the carbon dioxide formed 1s equal to the volume 
of the carbon monoxide originally present. [f, then, the carbon 
dioxide is determined by absorption with caustic potash, the 
volume of the carbon monoxide is at once obtained, provided no 
other combustible gas containing carbon is present at the same 
time. 





* Cuprous chloride is soluble in a concentrated solution of calcium chlo- 
ride. 1 ¢.c. of this solution absorbs 12 to 15 c.c. of CO. 
+ Treadwell and Stokes, Berichte, 21, p. 3131. 


CARBON MONOXIDE. 765 


3. For the combustion of 2 vols. of CO, 1 vol. of oxygen is neces- 
sary, and consequently the amount of oxygen consumed is equal to 
half the volume of the carbon monoxide. 


Methods of Effecting the Combustion. 


The combustion of the carbon monoxide can be carried out in 
several different ways: | 

1. By explosion. © 

2. By conducting the gas over glowing palladium or platinum. 

3. By conducting the gas over copper oxide. 

1. Combustion by Explosion.—The gas is mixed with a 
sufficient amount of air in a measuring vessel, such as is 
shown in Fig. 105, and the latter is connected by means of 
the capillary E with the Hempel’s explosion pipette shown 
in Fig. 110. The gas is completely driven over into the latter 
so that the capillary is entirely 
filled with mercury, the stop-cocks 
of the capillary and of the explo- 
sion pipette are both closed, and 
an electric spark is made to pass be- 
tween the two platinum points which 
are fused into the glass walls of the 
pipette; this immediately causes 
an explosion to take place. After- 
wards the gas is once more driven 
back into the measuring burette, 
and its volume again determined. The difference in volume 
before and after the explosion represents the contraction. 

This most excellent method can in some cases lead to erroneous 
results. In practice. it is almost always a question of determining 
the amount of combustible gas in a mixture containing nitrogen 
obtained after treatment with the different absorbents. If the 
amount of combustible gas present is too small in proportion to the 
amount of non-combustible gas, there will be no combustion what- 
soever; while on the other hand, if this relation is too large, a part 
of the nitrogen will be burnt to nitric acid (hydrogen is usually 








Fie. 110. 


766 GAS ANALYS S. , ee 


present). According to Bunsen, the combustion is complete when 
30 parts of combustible gas are present for each 100 parts of non- 
combustible gas. Consequently, if the explosion method is to be 
used for the analysis, the approximate composition of the gas must 
be known. 

2. Combustion by Conducting the Gas over Glowing Palladium.— 
This is the most certain of all methods for effecting the com- 
bustion, because it is entirely indépendent of the proportion 
of combustible gas present,* and there is no danger of any of the 
nitrogen being oxidized. The combustion is best effected, as 
proposed by Drehschmidt, by passing the gas through a thick- 
walled platinum capillary tube containing three palladium wires, 
The platinum capillary (Fig. 105, V) is placed between the gas 
burette and the Drehschmidt pipette S (Fig. 105), and it is heated 
by means of the non-luminous flame of a Teclu burner. The 
gas is repeatedly passed through the glowing capillary until there 
is no further diminution in volume, showing the combustion to 
be complete. There is no dangér to be feared from explosions 
even when pure detonating gas is passed through the platinum 
tube, and by this method CO, H, and CH, are completely oxidized. 
In the analysis of gases containing only small amounts of the 
above gases (e.g. exhaust gases from gas-motors) the so-called 
fractional combustion is employed. By this means either hydrogen. 
and carbon monoxide are oxidized while methane is not, or car- 
bon monoxide is alone burned. 

Fractional Combustion.—If, according to Haber,t an abso- 
lutely dry gas mixture, consisting of considerable nitrogen 
and oxygen with little carbon monoxide, hydrogen, and 
methane, is slowly conducted (at the rate of about 700- 
800 c.c. per hour) through a glass U tube 3 mm. in diam- 
eter which contains a palladium wire 55 cm. long, folded into 
three lengths of about 18 cm., then if the temperature of boiling 
sulphur is maintained, the hydrogen and carbon monoxide will 
be completely burned, while methane will escape from the tube 





* It is only necessary to make sure that a large excess of oxygen is pres 
ent (cf. Hempel, Zeitschr. f. anorg. Chem., XX XI (1902), p. 447. 
t Loc, cit. 


CARBON MO\ OXIDE. 797 


in an unchanged condition. By connecting the U tube with a 
weighed calcium chloride tube and then with two weighed soda- 
lime tubes (see p. 380) the increase in the weight of the former 
will show the amount of water formed from the hydrogen, and 
the gain in weight shown by the soda-lime tubes corresponds to 
the amount of carbon dioxide formed from the carbon monoxide. 
If, after passing through the soda-lime tubes, the gas is passed 
through a combustion-tube filled with platinized asbestos, or 
copper oxide, which is heated to a dark-red heat, the methane 
is quantitatively burned to water and carbon dioxide; the former 
is absorbed in a calcium chloride tube and the latter in two soda~ 
lime tubes, all three tubes being weighed before the gas is passed 
through them. In this way a check is obtained upon the accuracy 
of the determination, for the proportion of carbon to hydrogen 
found should be 1:4. 

The combustion of carbon monoxide alone from a mixture 
of this gas with hydrogen, methane, and air can be effected satis- 
factorily as follows: 

After the gas has been freed from CO,, unsaturated hydrocarbons, 
and aqueous vapor, it is conducted through a U tube * containing 
60-70 gms. of pure iodine pentoxide ¢ heated to 160° C.; by this 
means the carbon monoxide is alone oxidized with liberation of 
iodine according to the equation 


L,0,4+5C0=5C0,+1L.t 


Tf the gas is now conducted through two Péligot tubes containing 
potassium iodide solution, the iodine will be absorbed and can 


be titrated at the end of the experiment with x sodium thiosulphate 
solution. : 


1 ¢.c. > Na.S,0, solution corresponds to 5.6 c.c. CO, measured 


under standard conditions. 

* The U tube is heated in a small paraffin bath. 

+ Iodine pentoxide is prepared by heating iodic acid in a current of dry 
air at 1£0° until the water is completely removed. 

+ Nicloux, Compt. rend., 126, p. 746, and Kinnicutt, Journ. of the Am. 
Chem. Soc., XXII, p. 14. 





fe :4S ANALYSIS. 


If, after the carbon dioxide and water have once more been 
removed from the gas, it is passed through a combustion-tube 
half filled with copper oxide and half with platinized asbestos, 
both heated to dark redness, the hydrogen and methane will 
be completely burned to water and carbon dioxide, which 
can be absorbed and weighed as before. From the amounts of 
each, the hydrogen and methane present in the gas can be calcu- 
lated. 


Qualitative Detection of Traces of Carbon Monoxide in the Air. 


If blood be diluted with water until the solution shows only a 
slight red color, it will give a characteristic absorption spectrum; 
two dark absorption bands appear between the D and £ lines. 
If to this dilute blood solution a few drops of a concentrated, 
freshly-prepared ammonium sulphide are added, the dark bands 
disappear, and instead a single broad band will appear at a place 
between the positions of the previous bands. Blood containing 
carbon monoxide behaves quite differently. When the latter gas is 
present, the blood takes on a rose color and the solution gives 
almost the same absorption spectrum as pure blood (the bands 
shift slightly toward the violet) but in this case the two bands 
do not disappear on the addition of ammonium sulphide. 

To detect traces of carbon monoxide in the air, Vogel directs” 
that a 100-c.c. bottle, filled with water, be emptied in the room 
containing the gas, .and that 2 to 3 ¢.c. of blood, highly diluted 
with water, and showing only a very faint red color (although 
still giving the blood spectrum in a column as thick as a test-tube) 
be poured into the bottle and shaken for some minutes. To the 
solution a few drops of ammonium sulphide solution are added 
and the liquid is examined by means of the spectroscope. If 
the two bands are now visible, carbon monoxide is present. Ac- 
cording to Vogel as little as 0.25 per cent. of CO can be detected 
in this way. : 

Hempel has improved this method to a marked degree. He 
found that it was not possible to completely remove small 
amounts of carbon monoxide by shaking with the dilute solution 


CARBON MONOXIDE. 769 


of blood, and furthermore concentrated blood solutions could not 
be used because they foam so much. By using a living animal, its 
lungs furnish a better means of absorption, for the gas then 
comes in contact with undiluted blood. A mouse is placed 
between two funnels which are joined together by means of a 
broad band of thin rubber and the gas to be tested is passed 
through the funnels at a speed of ten liters per hour. At the 
end of two or three hours the mouse is killed by immersing 
the funnels in water and a few drops of its blood are taken from 
the region near the heart. In this way Hempel was able to detect 
with certainty as little as 0.032 per cent. CO. With such small 
amounts of CO the live mouse showed no symptons of poisoning; 
this was first apparent when 0.06 per cent. of the gas was 
present. In the latter case after half an hour the mouse_ breathed 
with difficulty and lay exhausted on its side. 

Potain and Drouin detect small amounts of carbon monoxide 
by passing the gas through a dilute solution of palladous chloride, 
when metallic palladium is precipitated: : 


PdClp,+ CO + H20 = 2HCl+CO2+ Pd. 


The solution is decolorized, or turns a pale gray, when large 
amounts of CO are present, but appears a light yellow in color 
when only traces are present. 

In order to estimate better the decrease in ‘color, Potain and 
Drouin filter off the deposited palladium sod compare the color 
of the filtrate. 

For the detection of small amounts of carbon monoxide, C. 
Winkler recommends a method which, as the author has found, 
will often lead to error. . According to Winkler, the gas to be tested 
is conducted through a solution of cuprous chloride in a saturated 
solution of sodium chloride, afterwards diluting with four to five 
times as much water, causing the precipitation of snow-white 
cuprous chloride. If this turbid solution is treated with a drop 
of sodium palladous chloride, a black precipitate of metallic 
palladium is obtained. Unfortunately, however, the palladium 
is often precipitated even in the absence of a trace of carbon 


re Be GAS ANALYSIS. 


monoxide; for cuprous chloride itself will readily reduce salts of 
palladium. 

It is true, on the other hand, that at a definite concentration 
the reduction of the palladous chloride is only effected by means 
of carbon monoxide, but it is difficult to always obtain the right 
conditions, and herein lies the inaccuracy of the method. If the 
solution be too concentrated with respect to sodium chloride, even 
large amounts of carbon monoxide will fail to precipitate a trace 
of palladium, because in that case the solution contains not 
only copper but also palladium in the form of complex sodium 
salts: 


[Cu,Cl,]Na, and [PdCl,jNa,. 


The sodium palladous chloride is not reduced by carbon mon- 
oxide and there is even less likelihood of the two sodium salts acting 
upon one another. If the solution be diluted with water, both 
salts break down according to the equations 


[Cu,Cl,JNa, 2 2NaCl+Cu,CL, 
fPdCl,]Na,z~ 2NaCl+ PdCl,, 


and only when the palladium is in the ionic condition is it capable 
of entering into the reaction. The fact that the reduction of the 
palladous chloride is effected by means of CO at a concentration 
at which Cu,Cl, is incapable of causing any reduction is easy to 
understand, for the gas, CO, comes in contact more readily with a 
sufficient number of palladium ions than does the difficultly soluble 
cuprous chloride. 


Hydrogen, H. Mol. Wt. 2.016. 


Density =0.06960 * (Air=1). Weight of 1 liter =0.089978 em. 
Molar volume =22.405 1. — Critical temperature = —238° C, 


Hydrogen is practically insoluble in water. 





* Lord Rayleigh, Proc. Roy. Soc., 538, 1134 (1893). 


HYDROGEN. ; 771 


ABSORPTION COEFFICIENTS OF HYDROGEN FOR WATER.* 


Temperature. B Temperature. B 
Eero tes 0.02148 Kf Sapo wee... 0.01699 
gaa Veppeabe GE 0.02044 A eee ASTe ERI ».. 0.01666 
1 6 soba iain iba 0.01955 > eles! ARNO aS 0.01644 
LO vet, 0.01883 SE .. 0.01624 
A i seh tre te 0.01819 Aes peas 0.01608 


Be ee 0.01754 Ba i ies - 0.01604 


The usual way for determining this gas by absorption is by 
means of metallic palladium,t but in the majority of cases it is 
determined by combustion with oxygen and observing the con- 
traction: 

Heats ree ©) 


2 vols. 1 vol. 0 vol. 


It is evident that by the combustion of two volumes of hydro- 
gen, three volumes of gas will disappear (the water formed oe 
a negligible volume). The contraction, therefore, is equal to } the 
volume of the hydrogen consumed. If the contraction is denoted 
by Ve and the volume of the hydrogen by Vy, then 


Vo air 3 Vu, 
Vua=4Vo. 


and consequently 


In many cases the weight of the water formed is determined 
by absorbing the latter in weighed calcium chloride tubes, and 
from the gain in weight the volume of hydrogen is computed as 
follows: 

18.02 :22,405=p:z, 
_ 22, 405 


= T3809 xX p= 1243.6 Xp c.c. hydrogen understandard conditions. 





* L. W. Winkler, Berichte, 24, 99 (1891). 

+ The absorption can also be accomplished very satisfactorily by means 
of a one per cent. solution of palladous chloride. Campbell and Hart, Am, 
Chem. J., 18, 294.—{Translator.] 


772 GAS ANALYSIS. 
Combustion of Hydrogen, according to Cl. Winkler. 


The following method is employed frequently in technicai 
analyses for the separation of hydrogen from methane. 

A mixture of hydrogen and air is conducted over gently-ignited 
palladium-asbestos, by which means the hydrogen is quantita- 
tively burned to water and the methane is not affected. Fig. 111 











Fig. 111. 


represents the apparatus required. A is the eudiometer and is 
connected by means of the canillary /, in which is found a short 
fibre of palladium-asbestos, with a Hempel pipette filled with water. 

The capillary, /, is heated by means of the small flame J’, at 
the place where the palladium-asbestos rests, to a temperature of 
about 300 to 400°, but not hot enough to soften the glass. After 
the gas, which is mixed with air,* has been passed back and forth 





* If oxygen is used instead of air, some of the methane is sure to be 
oxidized. Cf. O. Brunck, Zeit. f. angew. Chem., 1903, p. 195. 


HYDROGEN, 773 


through the capillary three times, the combustion is complete. If 
the above-specified temperature is not exceeded, no trace of meti- 
ane will be burned and the hydrogen determination will be accu; 
rate. It is, however, difficult. to regulate this temperature closely 
énough to prevent the combustion of some methane unless, as 
recommended’ by Haber, the tube is heated by means of sulphur 
vapor; the results are usually from 0.5 to 1 per cent. too high. 
Preparation of Palladiwm-asbestos.—Three gms. of sodium 
palladous chloride are dissolved in as little water as possible, 
3 c.c. of a cold saturated solution of sodium formate are added 
and enough sodium carbonate solution to make the solution 
alkaline. Then about 1 gm. of soft, long-fibred asbestos is 
added, which sucks up the whole of the liquid, and the 
mixture is dried on the water bath; by this means finely- 
divided palladium is deposited uniformly through the asbestos: 


Na,PdCi,+ HCOONa=3NaCl+ HCi+.CO,+ Pd. 


The hydrochloric acid formed by the above reaction is neutral- 
ized by the sodium carbonate. In acid solutions formic acid 
hardly reduces palladous chloride at all. 

After the asbestos has thoroughly dried, the mass is softened 
with hot water, placed in a funnel and washed with hot water 
until the soluble salt is completely removed. It is then dried 
once more and preserved in a well-stoppered bottle. 

_ The palladium-asbestos fibre is introduced into the capillary 
tube as follows: The fibre is rolled between the fingers to a little 
round wad, the latter is placed in the opening of the unbent capil- 
lary tubing and by gentle tapping upon the table it is made to pass 
along to the centre of the tube. The latter is then bent as shown 
in the figure. 

Remark.—inasmuch as the palladium-asbestos is likely to 
become shoved into the capillary, it is perhaps more satisfactory 
to use instead a palladium wire which is wound into a spiral.* 





* Private communication from Dr. Leutold of Hamburg. 


74 GAS ANALYSIS. 


Methane, CH,. Mol. Wt. 16.03. 


Density =0.55297. (Air=1.) Weight of 1 liter =0.71488 gms, 
Molar volume =22.43 1. Critical temperature = —82° C. 
Preparation.—Methane is conveniently prepared by a process 

analogous to that used in making ethylene * (cf. p. 751). A 
mixture of equal parts methyl iodide and alcohol (sp. gr. 0.805) is 
allowed to act upon a zinc-copper couple which has been washed 
with alcohol. 

2CH3I +2Zn +2HOH =ZnI2+ Zn(OH)2+2CH4. 


The zine-copper couple is obtained by pouring a 2 per cent. 
copper sulphate solution four times over granulated zine, then 
washing with water, and finally with alcohol. 

By allowing the mixture of methyl iodide and alcohol to drop 
upon the copper-coated zinc, a steady stream of methane is 
obtained at the ordinary temperature. The gas is purified by 
shaking it with fuming sulphuric acid, and then with caustic 
potash solution. It then contains nearly 99 per cent of CH, and 
about 1 per cent. of nitrogen. 

Methane, also called marsh-gas or fire damp, is only slightly 
soluble in water. 


ABSORPTION COEFFICIENTS OF METHANE FOR WATER.{ 


Temperature. Temperature. 
O° oa cheng 0.05563 Yb ase OE RA ee 0.02762 
Be) etcamhn eo 0.04805 Bote powers 0.02546 
Mie eae eae 0.04177 WAN: SN phe 0.02369 
aap eer ate: Sie 0.03690 BATS ccs ses Ss 0.02238 
PY costars been 0.03308 Beavis ates 0.02134 
BO sed Ca ore 0.03006 DA eS Sa Re 0.02038 


In alcohol, the gas is about ten times as soluble as it is in water. 
Inasmuch as no satisfactory absorbent for methane is known, 

it is always determined by combustion. | 
From the equation representing the combustion, 


CH,+20,=CO,+ 2H,0, 


2 vols. +4 vols. 2 vols. 0O vol. 


we can make the following deductions: 

1. Contraction——The contraction caused by the combustion 
of methane is equal to twice its original volume. 

2. Carbon Dioxide.—By the combustion of methane an equal 
volume of carbon dioxide is produced. 

3. Oxygen Consumed.—For the combustion of one volume of 
methane two volumes of oxygen are necessary. 

* Gladstone and Tribe, J. Chem. Soc., 45, 154. 


+L. W. Winkler, Berichte, 34, 1419 (1901). 
? 





ILLUMINATING AND PRODUCER GASES. 775 


ANALYSIS OF ILLUMINATING AND PRODUCER GASES. 


The analysis of all such gases is best performed either by the 
method of Hempel * or that of Drehschmidt.t 


Hempel’s Method. 


Hempel’s apparatus is shown in Fig.105,p.743. It consists of a 
eudiometer, W, divided into 4 ¢.c. and connected by means 
of rubber tubing with the levelling-bulb K. The eudiometer is 
also connected with the compensation-tube D and the latter is 
connected with a manometer C; both the tubes W and D are sur- 
rounded by a cylinder containing water. 

Calibration of the Apparatus.—First of all the manometer-tube is 
filled with mercury by raising the levelling-bulb K with the stop- 
cock p in the position shown in Fig. 105, so that there is an open 
connection between W and c; the mercury is allowed to pass over 
into C until the mark mm is reached. The volume of the manom- 
eter-tube from the mark m to the point a (Fig. 105) is now deter- 
mined as follows: 

By carefully lowering the bulb K the mercury is drawn over 
into C exactly to the point a when the stop-cock p is closed. A 
little air is allowed to enter into the eudiometer through the right- 
hand capillary tube above p (the tube # should be withdrawn as 
in Fig. 112), the levelling-bulb K is placed upon a solid support at 
about the same height as the mercury in W, and with the stop- 
cock p still open the position of the mercury in W is read. The 
stop-cock is closed, K is raised a little and p is turned to the 
position shown in Fig. 105, By raising K still higher, the air 
is driven over into the manometer-tube C until the mercury has 
exactly reached the mark m, when the stop-cock A (Fig. 105) is 
closed. The exact position of the mercury is then adjusted by 
turning the stop-cock p one way or the other, and the position 
of the mercury in W is once more read. The difference between the 





* Gasanalytische Methoden (1900), p. 48 ff. 
+ Berichte, 21, p. 3242 (1888). 


77° GAS ANALYSIS. 


two readings represents the volume of the tube between the marks 
m and a, an amount which must be added to all subsequent 
readings. , 

A drop of water is now introduced at c, by means of a fins 























=- x 


Fig. 112. 


pipette, into the compensation-tube D and the end of the tube ¢ 
is either fused together, or closed with a cork stopper and made 
air-tight with sealing wax. 

Procedure for the Analysis.—If the analysis is to be carried 
out on the spot, a large sample of the gas is collected in a Dreh- 
schmidt pipette (Fig. 105 S). To accomplish this the capillary 
tube E’ is connected by means of rubber tubing with the source 


ILLUMINATING AND PRODUCER GASES. 7a 


of the gas, and the stop-cock M is turned so that the tube 
E’ is in connection with the bulb of the pipette, the levelling-bulb 
being in a low position and the stop-cock s left open. The pipette 
is entirely filled with the gas, then the stop-cock M is turned 
so that it communicates with the outer air, and the gas is com- 
pletely expelled from the pipette. The gas is in this way drawn 
in and out of the pipette at least three times in order to make sure 
that all foreign gas (air) is removed from the rubber tubing. The 
sample of gas is then taken and the two stop-cocks M and s are 
closed. 

In order to bring the gas to be tested from the Drehschmidt 
pipette into the eudiometer, the two instruments are connected 
by means of the capillary EH’ (imagine the capillary E in Fig. 105 
to be replaced by £’) and the rubber connections are firmly 
wired to the glass. The stop-cock M is turned to the posi- 
tion shown in Fig. 105, the levelling-bulb K is raised (after 
previously causing the mercury in the manometer-tube to reach 
to the point a) and the burette is entirely filled with mercury 
until the latter begins to flow from out of the tip of the key at 
M, when the cocks A and p are closed. The cock M is then 
turned so that the pipette S and the burette W are in con- 
nection, K’ is raised, s opened, K lowered, and both p and A are 
opened. 

After about 40 c.c. of the gas have passed over into the 
eudiometer, the cocks A and M are closed, the key of the stop- 
cock M (which must be entirely filled with mercury) is dipped 
into a beaker containing mercury, and the gas in the capillary is 
sucked into W by lowering K and opening A and p. As soon as 
the capillary E is entirely filled with mercury, A, p and finally M@ 
are closed. 

The volume of the gas in W is now determined as follows: 
A is opened and K raised so that the mercury in the bulb is a 
little higher than it is in W. After this p is opened and 
the gas is driven over towards C until the mercury in both 
arms of the manometer-tube is at about the same _ height, 
when A is immediately closed. The last fine adjustment of the 
mercury ievels within the tubes is made by closing or opening 


778 GAS ANALYSIS. 


the serew-cock Q;* the volume is now read, and to the reading 
the correction corresponding to the volume between the marks 
M and a is added. 

From this point begins the analysis. 


1. Determination of Carbon Dioxide. 


With the stop-cock p.closed,#the cock M is turned as 
shown in Fig. 105 the Drehschmidt pipette is removed and 
replaced by a second, clean pipette completely filled -with 
mercury. On connecting the stop-cock M with the rubber 
connector of the capillary H’, it should be in the position 
shown in the drawing. By this means the mercury in the 
rubber tubing can flow out through the key. After wiring 
the rubber tightly to the glass, from 3 to 5 c.c. of caustic 
potash solution (1:2) are introduced through the key into the 
pipette M and the alkali in the capillary is washed out with | 
about 2 ¢.c. of distilled water and then with a little mercury; 
after this the gas itself is driven over into the pipette. When 
the mercury has filled the whole capillary, both to the right and. 
left of M, then A, p, and M are closed. The bulb K’ is raised so — 
that extra pressure is placed upon the gas in the pipette and s is 
closed. The pipette is now gently shaken for three minutes with- 
out disconnecting it from the eudiometer, after which the gas is 
returned to W as follows: M, p, and A are opened, K is lowered, K’ 
raised, and s opened. When almost all of the gas has been driven 
out of the pipette, M, p, A, and Q are closed, the levelling-bulb 
is placed on the table below, and K’ is placed upon the support 
(missing from Fig. 105, but shown in Fig. 112) upon which the pi- 
pette itself rests. M,p, A, and s are now opened and @ screwed up 
a little so that the gas is very slowly sucked into the burette. 
As soon as the caustic potash solution has reached M the latter 
is closed. The gas remaining in the capillary to the left of - 
M is now removed by sucking mercury through the key of M 
into W. Finally the volume of the unabsorbed gas is read in the 





* The reading is best made with the help of a small telescope, the ocular 
of which is provided with cross-hairs. For this purpose the telescope con- 
nected with a Bunsen spectroscope is suitable. 


ILLUMINATING AND PRODUCER GASES. 779 


same way as before. The difference between the two readings 
represents the amount of CO,,. , 


2. Determination of the Heavy Hydrocarbons. 


The pipette containing the caustic potash solution is removed 
and replaced by another containing fuming sulphuric acid.* The 
gas is driven over into the latter, shaken with the acid for three 
minutes,t and the pipette emptied in precisely the same way as 
before. The gas is now returned to the pipette containing the 
caustic potash in order to remove the acid vapors, and finally trans- 
ferred to the burette W and its volume read. The difference 
before and after the treatment with fuming sulphuric acid repre- 
sents the sum of the heavy hydrocarbons (C,H,, C,H,, C,H,, etc.). 
It is not usually customary to attempt to separate the venzene 
trom the ethylene. 
3. Determination of Oxygen. 


This part of the analysis is carried out in exactly the same way 
as the determination of the CO,, except that in this case the absorp- 
tion pipette contains an alkaline solution of pyrogallol (ef. pp. 
758-9). | 

- 4. Determination of Carbon Monoxide. 


The determination of carbon monoxide may be effected either 
by absorption with ammoniacal cuprous chloride or by simul- 
taneous combustion with hydrogen and methane. 

For the absorption method, the procedure is the same as in 
the case of the determination of the heavy hydrocarbons, i.e., the 
absorption is effected in a pipette containing only ammoniacal 
cuprous ch'oride (no mercury). The gas is shaken for three 
minutes with a solution of cuprous chloride which has already 
been used frequently, and then the same length of time with a fresh, 





*In this pipette the bulb-tube K’ is fused on to the absorption-bulb, 
so that it is a little higher than the latter, in the same way as in the Hempel 
pipette (Fig. 115). Mercury is acted upon by fuming sulphuric acid. 

+ From the experience of the Massachusetts Gas Inspectors it would seem 
as if more time were necessary for the complete absorption of the heavy 
hydrocarbons—perhaps thirty minutes instead of three.—{Translator.] ° 


780 GAS ANALYSIS. 


or little used, solution (cf. pp. 763-4). Before reading the volume 
of the unabsorbed gas it must be freed from ammonia vapors, 
which is accomplished by shaking with hydrochloric acid (1:2) 
in a Drehschmidt pipette. } 


5. Determination of Hydrogen and Methane. 


After the removal of the carboh monoxide, the gas may con- 
sist of hydrogen, methane, and nitrogen. An excess of oxygen 
is added to this mixture (with illuminating-gas twice its volume 
is added, while with Dowson, water, and producer gas only a 
little more than half as much oxygen ‘s necessary). The eudiom- 
eter W is connected with a Drehschmidt pipette entirely filled 
with pure mercury * by means of a Drehschmidt platinum eapil- 
lary (Fig. 105, V), and the latter is heated to bright redness 
with the non-luminous flame of a Teclu burner, taking care 
that the inner flame mantle does not come in contact with the 
platinum. The gas mixture is conducted three times in a slow 
stream through the hot platinum tube, but taking care that no 
mercury enters the latter. The volume of the unconsumed gas 
is then measured without removing the platinum capillary, and 
the carbon dioxide is determined by introducing some caustic 
potash into the pipette and then shaking the gas with it; after 
three minutes’ shaking, the unabsorbed gas is returned to the eudi- 
ometer, closing the stop-cock M as soon as the caustic potash 
solution reaches it. 


Calculation of Hydrogen and Methane. 


Assume V ec.c. of gas to be taken for the analysis. The 
residue remaining after the absorption of the CO,, CpH,,, O, and 
CO was mixed with oxygen and burned. The contraction pro- 
duced was V¢ and the CO, formed amounted to Vx. 

We saw on p. 774 that the volume of the methane is equal 





* There must be no trace of caustic potash in the pipette, because in 
that case CO, would be absorbed and an inaccurate result would be obtained. 
To make sure that all the alkali is removed, the pipette is washed first with . 
water, then with hydrochloric acid, and finally with water once more. 


ILLUMINATING AND PRODUCER GASES. 781 


to the volume of the CO, formed, Vx, and in per cent.; 


¥V:Vxe=100:2 
Ve 
t= 7; 100=per cent. CH,. 


Since by the combustion of one volume of CH, two volumes 
of gas disappear, it is evident that by the combustion of Vx c.c. 
of CH, the contraction will amount to 2V x. | 
If the latter value be subtracted from the total contraction Vc, 
the difference represents the contraction caused by the combus- 
tion of the hydrogen present (V¢ —2V x) and two-thirds of the lat- 
ter represents the amount of hydrogen, 


MRom2V a) 
oe 





H, 
and in per cent.: 
| V:2(V_ —2Vx):=100:2 


pn 200 Ve—2V x) 
ne 3V 





=per cent. H. 


Determination of Carbon Monoxide, Methane, and Hydrogen 
by Combustion. 


After the absorption of the CO,, C,H», and O, the residual 
gas consists of CO, CH,,H, and N. To it a measured volume of 
oxygen * is added, the mixture burned, and both the contraction, 
Vc, and the carbon dioxide formed, Vx, are estimated. After this 
the unused oxygen is determined by absorption with alkaline 
pyrogallol solution. If the excess of oxygen is subtracted from 
the amount originally added, the difference will give the amount 
of oxygen necessary for the combustion, Vo. 





* The purity of the oxygen must be tested before the analysis, because 
the commercial product almost always contains nitrogen. For the analysis a 
measured volume of nitrogen is added to a definite amount of oxygen. as 
otherwise the amount of the residual gas might be too small to fill the 
manometer-tube between the marks a and m (Fig. 105). The nitrogen 
is prepared by allowing air to stand over phosphorus in a Hempel pipette. 
(Cf. p. 759). 


782 : GAS ANALYSIS. 


If the amount of CO is denoted by zx, the CH, by y, and finally 
the hydrogen by z, we have the following three independent equa. 
tions: 


1. Vo=h0+2y4+3z, 
2. Ve=<xt+y, 
3. Vo=4a+2y+ 32; 
. wo - 
and from these equations we find that 


=$Vx«t+4Vc—Vo=CO,* 
y¥=Vo—4(VetVc)=CH, 
2=Vco—Vo=H. 





* According to A. Wohl (Berichte, 1904, 433) the results are not quite 
accurate when obtained in this way because the molecular volume does not 
always equal the theoretical value of 22.41 liters. Nernst, in his book on 
Theoretical Chemistry, gives the following molecular volumes: 


For 1 gm.-mol. of the gas, or referred to oxygen. 
H, =22.43 1. H,=1.0017 
O,=22.391, O,=1.0000 
CO =22.39 1. CO=1.0000 

CH,=22.441. CH,=1.0020 
CO, =22.26 1. CO, =0.9939 


Taking these values into consideration, A. Wohl obtains for x CO, y CH,, 
and z H,, the following formulas: 


t=0.3329Ve— Vo +1.3394V K, 
y = —0.3336Ve + 1.0020Vo —0.3340V1; 
z=1.0005V-— 1.0017 Vo —0.0060V x. 


F. Haber (Thermodynamik techn. Gasreaktionen, p. 289) sees no reason 
for modifying the Bunsen formulas in this way, for when a combustion 
analysis is carried out by explosion, the volume of gas after the explosion 
is so poor in carbon dioxide that the partial pressure of the latter does not vary 
much from that of an ideal gas, and, therefore, follows Avogadro’s Rule. 

It is quite another matter in the case of mixtures rich in carbon dioxide, 
as often occur in gas-volumetric analyses. In that case the weight of carbon 
dioxide (or of carbonate) is computed from the volume of the gas and accurate 
values are obtained by using the observed molecular volume of 22.26 for 
this gas (see p. 386). 

The necessity of using the observed molecular volume instead of the 
theoretical value has been shown by Treadwell and Christie (Z. angew. Chem., 
1905, 1930) for chlorine. With other vapors (NH;, HCl, SO,, N,O) the 
observed molecular volume should be used unquestionably. 


ILLUMINATING AND PRODUCER GASES. 783 


In order to illustrate the accuracy of the method, the results 
obtained in the analysis of the gas from a Dowson gas generator 
with the help of the Deville tube (Fig. 100, cf. p. 732) will be given. 
Two samples of the gas were taken, one 35 cm. and the other 
45 cm. above the grate. The height of the coal layer in the 
producer amounted to 45 cm. 


DOWSON GAS. 
Sample I (35 cm. above the grate). 











I. II. Mean. 

co, ae $8.54 8.48 8.51 

C,H... 0.30 0.30 0.30 
O = 0.36 : 0.27 0.31 

CO... = 20.79 20.81 20.80 

CH, = 1.32 1.26 1.29 

H ae 21.84 22.20 22.05 
N = 46.85 46.61 46.74 

100.00 * 100.00 100.00 


The above analysis was performed by Korbuly in the author’s 
laboratory, and the carbon monoxide was determined by absorp- 
tion in ammoniacal cuprous chloride, but in the following 
analysis this gas was determined, as described above, by simul- 
taneous combustion with hydrogen and methane 


DOWSON GAS. 
Sample II (45 cm. above the grate). 
I 











: II. Mean. 

CO, = 8.58 8.55 8.56 
CrHn= 0.48 0.48 0.48 
O es. O17 : 0.26 0.21 
CO- .=.20779 20.59 20.69 
CH, = 0.43 0.43 0.43 
H = 19.31. 19.22 19.26 
N = 50.24 50.47 50.37 
100.00 100.00 100.00 





* These analyses add up to exactly 100 per cent. simply because the 
nitrogen is determined by difference.—{Translator.] 


784 GAS ANALYSIS. 


Obviously, the above results are perfectly satisfactory; it. 
is worth mentioning, however, that according to the former method 
{absorption of the CO and combustion of the residue) the value 
obtained for the methane is almost invariably somewkat higher, 
and that for hydrogen a trifle lower than according to the second 
method. ‘To illustrate this, the results of a third analysis * will 
be given, which was also made by, Korbuly in the sample of gas 
taken 35 cm. above the grate . 


Sample I (Dowson Gas, 35 cm. above the grate). 








CO determined CO determined 

by absorption. by combustion, 
CO, =  S$i8t4 8.43 
CrHn= 0:30 0.33 
O és Qi 3l 0. 295 
GO. = 20.30 20.91 
CH, “= 1.20 0.79 
H ox 22.05. 23.38 
N = 46.74 45.89 
100.00 100.00 


Of the two methods, the author decidedly prefers the latter. 


Analysis according to H. Drehschmidt.t 


The apparatus of Drehschmidt, like that of Hempel, con- 
sists of the gas-burette B and the compensation-tube C, both 
of which are contained in a‘ cylinder filled with water (Fig. 
113). 

Through the stop-cocksa and 6, B and C are connected by 
means of capillary glass tubing in which a drop of a colored 
solution (indigo and sulphuric acid) is placed; in order to deter- 
mine the position of the latter, the capillary is provided with 





* The gas came from the same tube as in the case of the other analyses. 
‘The gas was removed from the tube, as described on pp. 731-2. 
+ This is the analysis given on p. 783. 
t Berichte, 21 (1888), p. 3242. 


iLLUMINATING AND PRODUCER GASES. 785 


a millimeter graduation. The three-way cock a can be turned 
so that C connects with the outer air or with the capillary, 
or so that the capillary is in connection with the air; it has an 

















Fig. 113. 


opening through the top of the key. The cock 6 has a right- 
angled boring like H, Fig. 105. The burette is divided into milli- 
meters and must be calibrated with mercury before using. The 
apparatus is used in the same way as described under the Hempel 
method, p. 775. 


786 GAS ANALYSIS. 


TECHNICAL GAS ANALYSIS. 
Method of Hempel. 


The apparatus necessary is depicted in Fig. 114. It consists of 
a long measuring-tube ending at thé top in a thick-walled capillary 
tube and connected at. the bottom by means of rubber tubing 
about a meter long with the levelling tube. 
_ The gas is confined over water which has been saturated with 
the gas to be examined, and the absorption is effected in Hempel’s 
absorption pipettes such as are shown in Figs. 115, 116, 117, and 
118.* Fig. 100 represents a simple pipette for liquid absorbents, 
while Fig. 101 shows a compound absorption pipette. The latter is 
used for solutions which undergo change on exposure to the air, e.g., 
an alkaline solution of pyrogallol, or an ammoniacal cuprous chloride 
solution. The liquid in the two right-hand bulbs serves to protect 
the solutions on the left. Fig. 117 shows the pipette used for 
fuming sulphuric acid. The small bulb is filled by the glass- 
blower with glass beads, which serve to give to the sulphuric acid 
the largest possible surface, so that the absorption is effeeted much 
more readily. Fig. 118 is a pipette used for solid absorbents, such 
as phosphorus, ete. In order to fill it with phosphorus, the pipette 
is placed upside down, the cylindrical part is filled with distilled 
water, and small: sticks of colorless phosphorus are introduced. 
Afterfilling the pipette, the rubber stopper is inserted, theapparatus 
is placed right side up, water is poured into the bulb, and any 
air-bubbles in the cylindrical part of the pipette are removed by 
blowing through the bulb until the water flows out from the top : 
of the left-hand capillary, which is then closed by means of rubber 
tubing and a pinch-cock. 


Analysis of Illuminating-gas. 


First of all the confining liquid is prepared by conducting the 
gas through distilled water in a wash-bottle for several minutes 
with constant shaking. 





* These wooden pipette stands are no longer much used; iron ones are 
preferred.—_[Translator. ] 


ANALYSIS OF ILLUMINATING GAS. 787 ! 


, 


The gas-burette is filled entirely full with this liquid and 
then the upper rubber tubing is closed with a pinch-cock. In 
order to fill the burette with gas, the receiver is connected with 


TUG 
PLOTS LIT 





SSS 


Fig. 114. 


the burette by means of a piece of rubber tubing through which 
the gas has been flowing for two or three minutes, the levelling-tube 
is lowered, the pinch-cock opened and a little more than 100 c.c. 
of the gas are allowed to flow into the burette. The upper cock 
is now closed, the levelling-tube raised until the lower meniscus 
of the confining liquid is exactly at the 100-c.c. mark, when the 
rubber between the levelling-tube and the burette is closed near 
the burette with a pinch-cock. The apparatus is allowed to 
stand until the water no longer rises in the burette; this requires 


788 GAS ANALYSIS. 


two or three minutes. When the water is stationary, the lower 
pinch-cock is carefully opened (for there is extra pressure in the 
burette), which causes the water-level to sink. When the 100-c.c. 
mark is again reached, this cock is closed, the upper pinch-cock 
is opened an instant in order to allow the excess of gas to escape 
and then immediately closed. Then, to make sure that the burette 
contains exactly 100 c.c. of the gasjthe lower pinch-cock is opened 
and after bringing the water in the levelling-tube to the same height 
as in the burette, the reading is taken; the lowest point of the 
meniscus should coincide exactly with the 100-c.c. mark of the 
burette. Finally the lower pinch-cock is closed. 


1. Determination of Carbon Dioxide. 


The burette is connected with a pipette containing caustic 
potash solution by means of a capillary filled with water, as shown 
in Fig.114,the levelling-tube is raised, first the lower pinch-cock 
and then the upper one * is opened and the gas is driven over into 
the pipette. The confining liquid should now fill the entire capil- 
lary. The upper pinch-cock is closed, the pipette taken up and 
shaken for three minutes,} and the gas is returned to the burette, 
‘taking care that none of the alkali enters with it. 

The liquid in the levelling-tube is brought to the same level 
as that in the burette; the lower pinch-cock is closed and after 
the water has completely drained from the sides of the tube, the 
volume of the unabsorbed gas is read. 


2. Determination of the Heavy Hydrocarbons, CnHon. 


The burette is connected by means of a dry, empty capillary 
with sulphuric acid pipette (Fig. 117) and the gas is passed back 
and forth four times, taking care that no water enters the pipette ~ 
and that the sulphuric acid does not reach the rubber connection. 

Before the experiment the position of the sulphuric acid is 





* In the figure this pinch-cock is lacking. 

+ The absorption takes place more rapidly with one of Hempel’s new 
pipettes, which is similar to the one shown in Fig. 117, except that the right- 
hand bulb is replaced by a movable levelling-bulb, as in Fig. 105. The latter 
is filled with mercury, upon which the liquid absorbent floats. For the 
absorption of COs, it is only necessary to pass the gas back and forth once. 


DETERMINATION OF OXYGEN. 789 


marked upon the milk-glass plate back of the pipette and at the 
end of the experiment the acid must come to the same mark. The 
gas in the burette is now contaminated with acid vapors which 








Fig. 118. 





are removed by passing it into the potash pipette, afterwards 
returning it to the burette. 


3. Determination of Oxygen. 

This can be effected by shaking the gas in the compound pipette 
with alkaline pyrogallol solution, but far preferably by means of 
phosphorus. In the latter case, the gas is driven over into the 
phosphorus pipette and allowed to remain there until the white 
vapors disappear; this usually requires but three or four minutes 
(cf. p. 759). If no white vapors can be detected, this shows con- 
clusively that the absorption of the heavy hydrocarbons was 
incomplete (cf. p. 759). In such a case, the gas must be again 
treated with sulphuric acid and afterwards with phosphorus. 
If no white fumes are then formed, no oxygen is present, a case 
which practically never occurs, for in the determination of 
the hydrocarbons a little air containing oxygen always reaches 
the gas from the small capillary. 


x Determination of Carbon Monoxide. 


The gas is shaken three minutes with an old solution of 
ammoniacal cuprous chloride and then the same length of time 
with a fresh solution. (See pages 763, 779.) 


790 GAS ANALYSIS. 


5. Determination of Hydrogen and Methane. 


After the absorption of the carbon monoxide the residual gas is 
placed in the hydrochloric acid pipette, while the burette is 
washed out with hydrochloric acid in order to remove traces of 
alkali, and then filled with distilled water. 

About 15 to 16 ¢.c. of the gas in the hydrochloric acid pipette 
are transferred to the burette, and after reading its volume it is 
driven over into an exp‘osion pipette containing mercury (Fig.110).. 
100 c.c. of air (containing 20.9 c.c. of oxygen) are accurately meas- 
ured off in the burette and added to the contents of the explosion 
pipette. The latter is then closed by means of a pinch-cock, 
the contents of the pipette are mixed by shaking, the levelling- 
tube is lowered so that the gas is placed under reduced pressure, 
and the giass stop-cock of the pipette is closed. The platinum 
wires which are fused in the upper part of the bulb are now con- 
nected with the poles of a small induction coil so that sparks pass 
between the platinum points within the pipette. The explosion 
at once occurs with a flash without ever breaking the pipette. 
The gas is returned to the burette. It would seem natural to 
read the volume of the gas and then determine the amount of | 
carbon dioxide formed, the latter being a measure of the amount 
of methane burned. This is not advisable, however, because 
the gas in the burette is confined over water which absorbs ap- 
preciable quantities of carbon dioxide.* Consequently without 
reading the volume of the gas, it is transferred to the potash pipette, 
the carbon dioxide removed, and the volume of the gas then read; 
this gives the contraction V,. Finally, the amount of unused 
oxygen is determined by means of absorption with phosphorus. 
If the excess of oxygen is subtracted from the total amount added 
(20.9 ¢.c.), the amount of oxygen required for the combustion 
is determined (V,), so that we have two equations from which the 
amount of hydrogen and methane can be computed. 





* Subsequent experiments have shown that the error caused by absorption 
of CO, by the water in so slight, during the short time of waiting, that 
it is better to determine the CO, with caustic potash after the explosion, 
as Hempel also recommended. The results thus obtained are usually more 
concordant than those by the method described in the text. (See p. 792.) | 


ANALYSIS OF ZURICH ILLUMINATING-GAS. 791 


If we represent by x the volume of the hydrogen, and by y the 
volume of the methane we have 


Pe V.=$r+3y, 
a Vo=4r+2y, 


and from these equations we find 


x=4V.—2Vo, 
1 Baas Vo—4V ce 


The values thus obtained are referred to the total gas residue 
and in this way the amount of hydrogen and methane pan 
in the illuminating gas is determined. 

Great accuracy is naturally not to be expected by such an 
analysis, but the procedure is very satisfactory for an approxi- 
mate estimation. In order to illustrate this point, the results of 
analyses made by two different students in the author’s labora- 
tory at the same time will be given. 


_ Analysis of Zurich Illuminating-gas by Hempel’s 
Technical Method. 


IL. 


I. 
ROM TUMOR Cita cc ie eens 100 c.c. 100 c.c. 
—1.8% CO, —1.8% CO, 
After removal of CO,...... 98.2 98.2 
—3.6% CnHon —3 .6%CnaHon 
re os ** CaHon ... 94.6 94.6 <i 
—0.6% O —0.6% O 
Ue RO: Deo 94.0 94.0 
—8.6% CO —8.8% CO 
er ois PS IUD od chee 85.4 85.2 


For the H and CH, deter- 
mination were taken of 


GBB ia ek as ORS 16.0 15.6 
WBS Sc tes des ewes ae 116.0 115.6 
—30.0=V-~. —29.8=Ve, 
After the explosion,....... 86.0 85.8 
: —5.2 excess oxygen —5.6 excess 
oxygen 
‘© removal of excessofO 80.8 80.2 


V.=20.9—5.2=15.7. V.=20.9—5.6=15.3. 


792 GAS ANALYSIS 


If the values of Vc and Vo are inserted in the above equations, we 
have: 








Hydrogen r=8.6 | z=9.1 
Methane y=5.7 y=5.4 
and in per cent.: 
z=45.9% H x=49.7% H 
y= 30.42% CH, y= 29.5% CH, 
. 
SUMMARY OF THE TWO ANALYSES. 
I Il. Difference. 
Co, 1.8 1.8 0.0 
CrHan 3.6 3.6 0.0 
O 0.6 0.6 0.0 
CO 8.6 8.8 0.2 
H 45.9 49.7 3.8 
CH, 30.4 29.5 0.9 
N 9.1 6.0 3.1 




















From the results obtained, it is obvious that in each case the 
values obtained by absorption agree closely; on the other hand, 
the two determinations of hydrogen differ by almost 4 per cent. — 
while that of methane shows a divergence of nearly 1 per cent. 

It is possible to obtain a much closer agreement than the 
above in the determination of hydrogen and methane, but the 
analysis is inaccurate on account of the fact that only one-fifth 
of the residual gas is taken for the explosion; thus every error 
is multiplied by five. 

As was mentioned in the foot-note on page 790, the gas residue 
may be analyzed as under (a) with the exception that the CO 
obtained by combustion is measured, Then, if the volume of the 
hydrogen =a and that of the methane =y, the 


contraction =V,=3r+2y. 
CO2(=CHa) =Vi=y 


from which the hydrogen (x) can be computed as follows: 


2 =§(V,—2V;x). 


ANALYSIS OF ZURICH ILLUMINATING-GAS, 793 


As an example of this kind of an analysis, two of the author’s 


students analyzed independently a sample of illuminating gas from 
Montbéliard. 


I II 
Taken 100 c.c. : 100 c.c. 
C0: 97.4 c.c.=2.6% CO, 97 .2 c.c.=2.8% CO, 
—CrHin 92.7 c.c.=4.7% CrH»n 92.4 c.c.=4.8% CrHin. 
oF s 2 92.4 ¢.c.=0.3% O, 92.0 c.c.=0.4% O, 
—CO 83.1 c.c.=9.3% CO 83.0 c.c.=9.0% CO, 
Of the residual gas there 
was taken for H and 
CH, determination + 
air 15.4 c.c. 15.6 c.c. 
115.4 c.c. 115.6 c.c. 
After the explosion 90.6 c.c.=24.8=V- 90.2 ¢.c.=25.4 Ve 
—CO, _ 84.5¢.0.=6.1=Ve=CH, 83.8 ¢.c.=6.4=VE = 


5 4 
80.3 c.c.=4.2=excess oxy- 79.8 c.c.=4.0=excess 
gen of O 


—O, 


If the values of V, and Vx are inserted in the above equa- 
tions: 


Hydrogen....... 2=8.4=45.3 per cent H: x=8.4 44.7 per cent H; 
Methane........ y=6.1=32.9. “ CH, y=6.4. 34.0 ms CH, 


SUMMARY OF THE TWO ANALYSES 


I. te Difference. 
CO,= 2.6 2.8 0.2 
CrHon= 4.7 4.8 0.1 
= 0.3 0.4 0.1 
CO,= 9.3 9.0 0.3 
H,= 45.3 44.7 0.6 
CH,= 32.9 34.0 it 
H,= 4.9 4.3 0.6 


—_——_— 


794 GAS ANALYSIS 
Much better results are obtained by the 


(6) Method of Winkler-Dennis. 


In this methdd, the entire gas residue is transferred to a 
Hempel pipette containing mercury and connected with a 
leveling bulb (Fig. 119). Through the rubber stopper at the 
bottom two steel needles are inserted (knitting needles), the 
longer of which is enveloped throughout its whole length by a 
glass tube, and the upper end is connected, at about three-quarters 





Fia. 119. 


the height of the cylindrical part of the pipette, with a thin 
platinum spiral. 

The pipette is now connected with a Hempel burette containing 
100 ¢.c. of oxygen * over water, a low pressure is produced in the 





* The oyxgen used for experiments in gas analysis should preferably be 
prepared in the laboratory by heating potassium chlorate in a small retort, 
which is prepared by blowing a bulb (of about 20 c.c. capacity) at the end 
of a narrow piece of glass tubing; after introducing about 5 gms. of potassium 
chlorate, the tubing is bent to a right angle close to the bulb. The end of 
the tube is connected with a short piece of rubber tubing and the bulb heated 
over a free flame. As soon as oxygen begins to come off freely (lighting 
a glowing splinter) the rubber tubing from the retort is connected with a 
Drehschmidt absorption pipette, which contains a little caustic potash 
solution and is filled with mercury (ef. Fig. 105, p. 743). The oxygen is not 


ANALYSIS OF ZURICH ILLUMINATING-GAS. 795 


oxygen burette by lowering the leveling tube and then closing 
the rubber tubing with a screw-cock, after which the leveling 
tube is placed in a high position. The bottom ends of the two 
needles of the pipette are now connected with the wires of a 
small storage battery of such a strength that the platinum spiral is _ 
heated to dull redness. By lowering the leveling bulb, a slightly: 
lower pressure is produced in the pipette, and by opening the. 




















Fig. 120. 


two upper screw-cocks between the pipette and the oxygen burette, 
and gradually opening the lower screw-cock on the burette, a 
very slow stream of oxygen is conducted into the pipette. Since 
a large excess of the gas residue is present at the start, the com- 
bustion takes place quietly; explosions never occur. During the 





introduced at once into the pipette, but is allowed to pass through the cock 
M into the air. After about a minute, one can assume that the air from 
the retort and rubber tubing bas been entirely replaced by oyxgen. The 
leveling bulb K of the piptte is lowered, the cock s opened, and the cock M 
turned 90° so that the pipette fills with oxygen. When the filling is accom- 
plished, M is closed and the retort removed. By shaking the pipette, any 
carbon dioxide formed by the burning of dust, etc., is absorbed. 


790 GAS ANALYSIS. 


combustion the platinum spiral begins to glow more brightly; 
to prevent its melting, a resistance * must be placed in the circuit 
by means of which the strength of current, and thus the glowing 
of the platinum, may be regulated as desired. 

As soon as all the oxygen is in the pipette, the spiral is allowed 
to glow two or three minutes longer, the electric current is then 
stopped, and the gas allowed to remain in the pipette for fifteen 
minutes so that it will assume the room temperature. It is then 
transferred to a Hempel burette and its volume measured; the 
carbon dioxide is determined in the usual manner. 

To illustrate the accuracy of this technical method, the follow- 
ing three analyses were“carried out independently by three of the 
author’s students. 


ANALYSIS OF ZURICH ILLUMINATING-GAS ON JULY 14, 1909, 


I II III 
co, 2.0% 2.2% 1.9% 
CaHyn 4.4 4.4 4.6 
O, 0.7 0.5 0.6 
CO 9.2 9.2 9.3 
CH, 27.6 28.2 27.9 
H, 49.8 49.4 49.3 
N, 6.3 6.1 6.4 





100.0% 100.0% 100.0% 


Remark.—By this method it is possible to burn pure acetylene 
without any explosion. The oxygen, however, must not be 
conducted, as above, into the acetylene because in that case the 
combustion of the acetylene will be incomplete and considerable 
carbon will deposit. If the oxygen is first placed in the combustion 
pipette, the platinum wire brought to glowing, and then the 
acetylene introduced, the combustion takes place nicely without. 
deposition of any carbon. 

The Winkler-Dennis pipette is open to the objection that the 
rubber stopper eventually leaks; for this reason the author 
prefers the form of apparatus devised by his assistant, M. Bretsch- 
ger, as shown in Fig. 120. 





* The resistance mentioned on page 178 is suitable to use here. 


ORSAT’S APPARATUS. 797 


Instead of burning the gas residue according to the Winkler- 
Dennis method, it may be conducted over glowing cupric oxide.* 


Orsat’s Apparatus. 


For the analysis of flue gases, Orsat has constructed the appa- 
ratus shown in Fig, 121. It consists of the 100 c.c. measuring-tube 








Fie. 121. 


B surrounded by a cylinder containing water, and connected on 
the one hand with three Orsat tubes by means of the cocks J, 
II, and III, and the other hand with the outer air through the 
stop-cock h. The Orsat tube I/I contains caustic potash, IJ alka- 
line pyrogallol solution, and J ammoniacal cuprous chloride 
solution. 3 





* Jager, J. Gasbeleuchtung, 1898, 764. G. v. Knorre, Chem. Ztg., 1909, 717. 


708 GAS ANALYSIS. 


Manipulation.—By raising the leveling-bottle N and open- 
ing the stop-cock h, the measuring-tube B is filled with water. 
As soon as the water is above the mark in the widened part of 
the measuring-tube, the rubber tubing between the levelling-bottle 
and the measuring-tube is closed by means of a pinch-cock, a is 
connected with the source of the gas, and the gas is sucked into 
the measuring-tube by lowering the levelling-bottle and opening 
the pinch-cock. The U tube on the outside of the apparatus is 
filled with glass-wool and serves as a filter; any smoke being 
removed from the gas to be examined. The sample thus col- 
lected is naturally contaminated with the air from the rubber 
tubing, the U tube, and the capillary, which must be removed. The 
cock h serves for this purpose and is provided with a T boring. 
The cock is turned so that the burette communicates with the 
outer air through a small tube (not shown in the illustration) and ~ 
the gas is expelled by raising the bottle N. This process of 
filling and emptying is repeated three times, and the fourth filling 
of the tube 4 is taken for the analysis. The gas in the burette is 
brought to the 0 mark, and it is placed under atmospheric pres- 
sure by quickly opening and then closing h. After this the gas 
is driven over into the potash-bulb and back again to the meas- 
uring-tube several times, until there is no further absorption, after 
which the volume of the gas is again read. In the same way 
the gas is successively passed into the pyrogallol and the cuprous 
chloride tubes, thus obtaining the amount of CO,, O, and CO in 
the gas. 

Bunte’s Apparatus. 


This apparatus, shown in Fig. 122, differs from those previously 
described, inasmuch as the absorption takes place in the measuring 
vessel itself, whereas in the other cases the absorption takes place 
in the pipettes. 

The Bunte burette has a capacity of about 110 to 115 c.e. 
between a and b; a isa three-way cock, while b is bored only once. 
_ Manipulation.—The burette is connected with the levelling- 
bottle N, as shown in the illustration, a and b are opened, and the 
water is allowed to run up to the mark in the funnel above a. 
The key of the stop-cock a is connected with the source of 
the gas, N is lowered, a turned to the proper. position, and 


BUNTE’S APPARATUS. 799 


the gas is sucked into the burette. After about 101 to 
103 c.c. of the gas have entered the burette, a and b are 
closed, N is raised, and by opening b the gas in the burette is 
compressed until the confining liquid has exactly reached the 
zero mark. The cock a is now cautiously opened, when some of 
the gas in the burette will escape through the water in the funnel. 
The gas in the burette is now under a pressure equal to that of the 
atmosphere plus the pressure from the column of water in the 
funnel, and all subsequent measurements are taken under the 
same conditions. | 

Absorptions.—In order to introduce the different absorbents 
into the burette, its lower end is connected by means of the rub- 
ber tubing h with the bottle F containing a little water, the water 
having been blown up into the rubber tubing. The cock 6 is 
opened, as is the screw-cock at h, and 
the water in the burette is allowed to 
run out until it exactly reaches the cock 
b, which is then closed. The absorbent 
is placed in a small dish, the lower 
tip of the burette is introduced into the 
liquid, and the cock 6 is opened. Inas- 
much as the gas in the burette is under 
less than atmospheric pressure, the ab- 
sorbent is sucked up into the burette. 
The cock 6b -is now closed, the burette 
grasped above a and below 6 (in order 
not to warm the gas), and its contents 
well shaken, after which the burette is 
again dipped into the absorbent in the 
dish and a little more of the latter 
drawn up into the burette. This process 
is repeated until no more of the ab- 
sorbent is sucked up into the burette. 
It would now be incorrect to read the 
volume of the unabsorbed gas, for it | 
is under quite a different pressure than Fig.-122. 
at the beginning of the analysis; namely, 
the atmospheric pressure less the pressure of the dohivha of 











800 GAS ANALYSIS. 


liquid remaining in the burette with the cock b open. Further- 
more the vapor tension of the liquid in the burette is different 
from that of the water originally present. In order to obtain 
the original conditions, the burette is connected with the 
bottle F, which now only contains enough water to fill the 
rubber tubing and the glass tube, and the absorbent is sucked 
from the burette into the bottle until the upper level of the 
liquid reaches the cock b.* The*end of the burette is then 
dipped into a dish containing water, which rises into the burette 
on opening b. The latter is closed and water is allowed to run 
into the burette from the funnel until the original pressure is 
established, when the volume of the gas is once more read. The 
difference gives at once the per cent. of absorbed gas. 

By means of this excellent method the carbon dioxide can be 
removed by caustic potash, heavy hydrocarbons by bromine 
water, oxygen by alkaline pyrogallol solution, and carbon 
monoxide by cuprous chloride. 


ANALYSIS OF GASES WHICH ARE ABSORBED CONSIDERABLY 
BY WATER. 


Under this heading belong 
N,0, SO,, H,S, Cl, SiF,, HF, NH,, ete. — 


Nitrous Oxide, N2O. Mol. Wt. 44.02. 


Density =1.5297 T (Air=1). Weight of 1 liter =1.9766 gms. 
Molar volume =22.261. Critical temperature = +36° C. 
This gas is best prepared according to the method of Victor 
Meyer,t by allowing sodium nitrite to act upon a concentrated 
solution of a salt of hydroxylamine: 





* The absorbent is now by no means exhausted, so that it is returned 
to the proper bottle, and can be used for several other determinations. 

+ Lord Rayleigh, Proc. Roy. Soc., 74, 181 (1904). 

{ Ann. Chem. Pharm., 157, 141. 


BUNTE’S APPARATUS. 801 


It is best to proceed as follows: 

_ A concentrated, aqueous solution of sodium nitrite is added 
drop by drop from a separatory funnel, with constant cooling,’ 
to a concentrated solution of hydroxylamine hydrochloride, I 
is contained in a small evolution flask; in this way the gas} 
evolved is pure and escapes in a regular stream. It is not advis-' 
able to proceed in the opposite way, namely, to add the hydroxyl-, 
amine solution to a concentrated nitrite solution, for in the latter’ 
case the decomposition is likely to take place with explosive vio- 
lence; it is still less advisable to add one of the reagents in the 
solid form. In a very dilute condition the solutions scarcely act 
upon one another. 

Nitrous oxide is never pure when it is prepared by heating 
ammonium nitrate; it is always contaminated with nitrogen and 
nitric oxide, but the latter may be removed by washing the gas 
with a solution of ferrous sulphate. 

According to L. Pollak the solubility of nitrous oxide between 
0° and 22° C. is expressed by the formula 


8=1.13719 —0.042265 - t+ 0.000610: 2, 


while according to Bunsen its solubilty is greater, being expressed 
by the formula 


8 =1.3052 —0.045362 -t+ 0.0006843 - ?. 


The gas is absorbed to a much greater extent by alcohol than 
by water. According to Pollak, the absorption coefficient for 
alcohol is . 


8 =3.22804—0.04915-t+ 0.00023 - 2, 
wnile according to Bunsen it is somewhat greater: 
£B ~ 4.17805 —0.069816- t+. 0.000609-#. 
The determination of nitrous oxide can be effected with accu- 


racy by combustion, and this may be carried out in two different 
ways: 


802 GAS ANALYSIS. 


1. According to Bunsen, by exploding with hydrogen, or accord- 
ing to Knorre, by means of the Drehschmidt capillary. The con- 
traction produced is equal to the original volume of the nitrous 
oxide: 

NJOr + hy BO EONS 


2 vols. 2 vols. 0 vol. 2 vols. 


2. According to Pollak, by c®mbustion with pure carbon 
monoxide, either by explosion or with the help of the Drehschmidt 
capillary; the volume of the CO, formed, which is measured, is 
equal to the volume of the nitrous oxide: 


NO: +400 = 6 2 


2 vols. 2 vols. 2 vols. 2 vols. 
There is no contraction in this case. 


Nitric Oxide, NO. Mol. Wt. 30.01. 


Density =1.0366,* (Air=1). Weight of 1 liter =1.3402 gms, 
Molar volume =22.391. Critical temperature = —94° C, 


Preparation of Pure Nitric Oxide. 


The best way to prepare pure nitric oxide is the method of A. 
Deventer,t in which a solution of potassium ferrocyanide and 
potassium or sodium nitrite is acidified with acetic acid and 
shaken: | 


2K4Fe (CN) g+2KNOo+4HCoH302 =4KC2H302+2K3Fe (CN) 6 
+2H20+2NO. 


According to Emich { a very pure gas is obtained by shaking 
a nitrate with concentrated sulphuric acia in a nitrometer con- 
taining mercury. 





* Computed from observations of Gray (1905), Guye and Davila (1906). 
+ Berichte, 26, 589 (1893). 
t Monatshefte, 13, 73 (1892). 


NITRIC OXIDE. 803 


ABSORPTION COEFFICIENTS OF NITRIC OXIDE FOR WATER.* 


Tempetature, p __-"Femperature. B 
0° 0.07381 30° 0.04004 
5 0.06461 35 0.03734 
10 0.05709 40 0.03507 
15 0.05147 45 0.03311 
20 0.04706 50 0.03152 
25 0.04323 55 0.03040 


Although nitric oxide is only slightly soluble in water, its 
determination will be discussed at this place because this gas 


‘frequently occurs with nitrous oxide, and must therefore be 


determined at the same time. | 

Nitric oxide may be determined by absorption with a con- 
centrated solution of ferrous sulphate or an acid solution of potas- 
sium permanganate, likewise, according to E. Divers,{ by an alka- 
line solution of sodium sulphite (40 gms. NagSO3+4 gms. KOH 
in 200 c.c. H2O) with the formation of NagN2O02SO3.t It is 
better, however, to carry out a combustion by the method of 
Knorre and Arndt,§ in which the gas is mixed with hydrogen and 
very slowly passed through a Drehschmidt’s platinum capillary 
heated to bright redness. Under these conditions the nitric oxide 
is quantitatively burned according to the equation. 


9NO + 2H, = 2H,0 + N,. 


4 vols. 4 vols. 0 vol. 2 vols. 


The contraction produced by the combustion of one volume 
of nitric oxide is equal, therefore, to $ the original volume of the gas. 

Remark.—lf the gas mixture is passed too quickly through 
a platinum capillary heated to bright redness, or slowly through 
a less strongly heated platinum capillary, an appreciable amount 
of ammonia is formed and the results obtained are inaccurate. 

By explosion with hydrogen it is not possible to burn NO 

*L. W. Winkler, Berichte, 34, 1414 (1901). 

+ Journ. Science Coll. Imp. University; Tokio, Vol. XI (1893), p. 11. 

t Nitric oxide is only partially absorbed by an alkaline solution of pyro- 
gallol, where alkali nitrite, N,O, and N, are formed. (C. Oppenheim, Berichte, 
36, 1744 (1903). 

§ Berichte, 21 (1889), p. 2136. 





804 GAS ANALYSIS. 


when it is pure; when it is mixed with considerable nitrous oxide, 
violent explosions take place, yet the combustion of the NO is 
even then not quantitative. 

The gas may be determined, however, by combustion with 
carbon monoxide in the Drehschmidt capillary. 

According to Henry a mixture of carbon monoxide and nitric 
oxide is not explosive. On the other hand, according to Pollak, 
by conducting a mixture of these: “cases through a Drehschmiae 
platinum capillary heated to bright redness, the combustion is 
quantitative if at the same time the carbon dioxide formed is 
removed by means of caustic potash; * otherwise the oxidation 
is not quantitative. According to the equation 

2NO+2C0O =2C02+ Ne 

4vols. 4vols. Ovol.  2vols. 
the contraction produced is equal to $ the volume of the nitric 
oxide. 

Remark.—lf cone lesatie nitrous oxide is present at the same 
time, the combustion in the Drehschmidt capillary takes place 
quantitatively without the removal of the carbon dioxide. In 
this case the contraction is 4 the volume of the nitric oxide. 


Analysis of a Mixture of Nitrous and Nitric Oxides. 


I. Combustion with Hydrogen. 


- The gas is mixed with an excess of hydrogen and oxidized 
according to Knorre in the Drehschmidt platinum capillary heated 
to bright redness. If the volume of the Ne0=z and that of the 
NO=y, we have: 

N20. NO. 
1. c+y=V 
2. xt+y=V. (contraction) 


from which can be calculated: 


xr=3V—2V. 
y=2(V.—V). 





* The mercury in the Drehschmidt tube is covered with caustic potash 
solution, by which means the CO: is absorbed immediately after its formation. 


DETERMINATION OF NITROUS OXIDE, ETC. 805 


Il. Combustion with Carbon Monoxide, 


_ The gas mixture is treated with an excess: of carbon monoxide 
and burned in the red-hot platinum capillary; the contraction, 
V., and the carbon monoxide, Vz, are both determined: 


_N,O. NO. 
z+ y= V;z 
yy = Ve 


from which we can compute: 


t= Vi.—2V- 
y=2V.. 


Determination of Nitrous Oxide, Nitric Oxide, and Nitrogen in 
the Presence of One Another. 


I. By Combustion with Hydrogen in a Drehschmidt Capillary. 


After noting the contraction formed by the combustion with 
hydrogen, an excess of oxygen is added to the gas residue and the 
mixture is burned in the Drehschmidt capillary; two-thirds of the 
contraction which now takes place is equal to the amount of unused 
hydrogen in the first oxidation. If this quantity is deducted 
from the amount of hydrogen originally added, the difference, Vy, 
represents the amount of hydrogen necessary. 


We have now: 
NLO. NOUN, 
Let y+ ee VV 
2.2 + 3y am Vo 
3.2 + Y = Vy 


806 ma GAS. ANALYSIS. 


II. By Combustion with Carbon Monoxide in the Drehschmidt 


Capillary. 
We have: ; 
N,O. NO. N. 
f+ y + 2=V 
sy =V, (contraction) 
ct y = Ve (CO), 
from which it follows: 
c= Vi — 2V~ 
y= 2V- 
z=V—Vjx. 


Determination of Nitrous Oxide, Nitric Oxide, and Nitrogen in 
the Presence of Carbon Dioxide. 


The accurate determination of nitrous oxide in the presence 
of carbon dioxide offers certain difficulties. It is not possible 
to determine the former by combustion with hydrogen in the 
Drehschmidt capillary, because when the carbon dioxide is present 
it takes part to some extent in the combustion, 


CO,-+H,=H,0+C0, 


and the previous absorption of the carbon dioxide by means of a 
large quantity of caustic potash is equally unsatisfactory, because 
a considerable amount of nitrous oxide will be absorbed by the 
reagent. The only way which can be recommended to effect 
this determination consists in absorbing the carbon dioxide by 
means of the smallest possible quantity of caustic potash, in which 
case the error introduced by the absorption of the nitrous oxide 
is reduced to a minimum; the residual gas is examined as described 
above. 


Nitrogen, Mol. Wt. 28.02. 
Density =0.9673 (Air=1). Weight of 1 liter=1.2505 gms. 
Molar volume 22.41 liters. Critical temperature = — 149° C. 


Pure nitrogen is best prepared by heating a concentrated 
solution of potassium nitrate and ammonium chloride, present 
in amounts proportional to their molecular weights, and then 


NITROGEN. 807 


conducting the escaping gas over glowing copper to reduce traces 
of nitric oxide. 
Nitrogen is but slightly soluble in water. According to L. | 


Winkler, * 


ABSORPTION COEFFICIENTS OF NITROGEN FOR WATER. 


Temperature. : Bes Temperature, B 
o 0.02348 30° 9.01340 
a 0.02081 35 0.01254 
10 0.01857. 40 0.01183 
15 0.01682 45 0.01129 
20 0.01542 50 0.01087 
25 0.01432 55 0.01051 


Nitrogen cannot be determined by any of the ordinary methods 
of gas analysis. It is always estimated by determining all the 
other constituents present in a mixture and subtracting the sum 

- of the percentages found from 100. 

Technical preparations o: nitrogen, prepared from the air, 
always consist of nitrogen and small amounts of rarer elements. 
According to-Cavendish these latter may be obtained by adding 
oxygen and allowing a strong electric spark to pass through 
the mixture. In this way the nitrogen is completely oxidized to 
nitric acid, which can be removed by means of caustic potash 
solution. Then, by absorbing out the oxygen, the rarer gases are 
obtained. A still better process is that of Hempel, in which the 
nitrogen is absorbed by passing the gas over a glowing mixture 
of 1 gm. magnesium, 5 gm. freshly burnt lime, and 0.25 gms. 
sodium. The rare gases are not absorbed by this treatment. 

According to Bunsen, there is no combustion of nitrogen 
when detonating gas explodes in the presence of air, provided nct 
more than 30 volumes of combustible gas are present for each 100 
volumes of non-combustible gas. There is no oxidation of nitrogen 
during a combustion of a gas mixture which is passed through a 
Drehschmidt platinum capillary. 


—"= 


bn 





* Berichte, 24, 3506 (1891). 


808 GAS ANALYSIS. 


Analysis of Gases by Titration of the Absorbed Constituents. 


If a mixture of gases contains several constituents, of which 
two are removed by the same absorbent, and one of these can 
be determined by titration, it is a matter of no difficulty to 
determine the amount of each. The diminution in volume after 
treatment with the absorbent repgesents the amount of the two 
constituents, the titration value represents the amount of one of . 
them, and the difference shows the amount of the other. Such 
problems can be solved in a variety of ways, and only a Saw 
examples will be mentioned. 


Chlorine, Cl. Mol. Wt.=70.92. 


Density =2.488 (Air=1).* Weight of 1 liter =3.2164 gms. 
Molar volume = 22.049 liters. Critical temperature = + 146° C. 


Determination of Carbon Dioxide in Electrolytic Chlorine.+ 


The author has used the apparatus shown in Fig. 123 with the 
best success for this purpose. 

The absolutely dry eudiometer, B, the capacity of which 
between the two stop-cocks is accurately known, and for con- 
venience may be 100 c.c., is filled through the lower cock, 
after the gas has been dried by passing it through a long 
calcium chloride tube.t After five or ten minutes it is safe 
to assume that the air has been completely replaced by the 
gas. The lower three-way cock is now closed and then the 
upper one. The temperature and barometric pressure are 
both noted at this point. 

The tip of the burette is connected by rubber tubing with 
the reservoir N, the three-way cock is turned so that the 
reservoir communicates with the outer air, and then the lower 
es, Leduc, Compt. rend., 116, 968 (1893) and Treadwell and Christie, Z 
angew. Chem., 47, 446 (1905). . 

+ Treadwell and Christie, Z. angew. Chem., 47, 1920 (1905). 

t Ifthe burette and gas are not perfectly dry, some chlorine will be 


absorbed by the water. This will not affect the gas reading, but will be 
harmful in the subsequent titration. 





CHLORINE. 809 


tip of the burette and the stop-cock are thoroughly washed, 
after which the latter is closed. A solution of potassium 
arsenite is prepared by dissolving 4.95 gms. 
As2O3 in dilute potassium hydroxide, adding 
dilute sulphuric aoid until the solution is neu- 
tral to phenolphthalein and then diluting to 
1 liter.* 100 c.c. of this solution are placed in 
N and any air in the rubber tubing is expelled 
by pinching it with the thumb and finger. 
By raising N and opening the stop-cock, a 
little of the arsenite solution is made to flow 
into the burette, which is inclined from side 
to side in such a way that the walls are 
- thoroughly wet with the arsenite solution. 
The chlorine is slowly absorbed, as is evident 
from the fact that the solution slowly rises 
in the burette. As soon as there is no 
further absorption, the lower stop-cock is 
closed and the solution in the burette is 
made to flow back and forth in the burette 
several times, by inverting the burette and 
then turning it back again. After one or 
two minutes all of the chlorine will have 
been absorbed. Then in order to absorb all the carbon dioxide 
present, the tube N is lowered, 10 c.c. of potassium hydroxide 
solution (1:1) are poured in the funnel, and carefully made 
to flow into the burette. The stop-cock is again closed and the 
alkali solution poured back and forth in the burette. 

After the liquid in the burette and in the leveling tube has been 
brought to the same height, the reading is taken. The unabsorbed 
"gas residue on being deducted from the original volume of the gas 
gives the volume of the chlorine plus that of the carbon dioxide. 
For the determination of the chlorine, the contents of the burette 
leveling tube, N, are emptied into a large Erlenmeyer flask and 
the stop-cock is turned to the position shown in the drawing, so 








Fig. 123. 





* An ordinary solution of arsenite prepared with sodium bicarbonate 
cannot be used here. : 


810 GAS ANALYSIS. 


that the liquid in the rubber tubing can flow out. The tubing is 
then removed from the burette and washed out with distilled 
water which is also allowed to run into N. The contents of the 
burette are added and the burette itself is rinsed with distilled 
water. 

The contents of the Erlenmeyer flask are treated with two 
drops of phenolphthalein solution and then with hydrochloric acid 
untilthered color just disappears, 60c.c. of sodium bicarbonate solu- 
tion are added (35 gms. in 1000 c.c. water), a little starch solution, 


and the excess of the arsenious acid is titrated with a iodine solu- 


tion. 

It will be assumed that n c.c. are used in the titration. The 
ratio of the arsenite solution to the iodine is then established 
in the same way as in the above titration. 100 c.c. of arsenite 
solution are placed in an Erlenmeyer flask, 10 c.c. of caustie-potash 
solution (1:2) are added, two drops of phenolphthalein, hydro- 
chloric acid to decolorization, and then 60 c.c. of sodium bicar- 
bonate solution. The solution is then diluted to the same 
volume as that of the original experiment and titrated with 
tenth-normal iodine. Hereby n’ c.c. are required. The difference 
n’—n multiplied by 1.102 * gives the number of cubic centimeters 
of chlorine gas at 0° C. and 760 mm. pressure. In other words, 


V’o=(n’—n) X1.102. 


As, however, the original gas was measured at the temperature 
#° C. and under the pressure B mm., it follows according to page 
666, that y? V’.B-273 

760: (273-+1)’ 

* This number is derived from the observation that the density of chlorine 

is 2.488 at 20°, 70.92 
| 0.001293 X 2.488 

Therefore, 35.46 gms. of chlorine at 0° and 760 mm. occupy a volume 
of 11,020 c.c. In the paper just cited, the value 22,039.2 c.c. is taken for 
the molecular volume of the chlorine and 0.001293 for the density of air. 

In the analyses of gases rich in chlorine, correct values are obtained by 
using the observed molecular volume of chlorine, which is 22.049 liters, and 
according to the experiments of N. Busvold in the author’s laboratory, correct 
results are also obtained with this value in the analysis of gases containing 
little chlorine. sore 








= 22,045. 





CHLORINE. 811 
from which can be computed 


9: 760(273 +1) 


V 
? — 
£ B-273 





If V is the original volume of the gas used and R that of 
the residual gas in the burette, then 


Cle+CO2+ Residue = V 





Residue = R 
Cle+COz=V—R 
—Cle=V, 





COz=V—(R+V’) 
and in per cent. 


_[V—(R+V]-100 
= V 





=per cent. COsz. 


Remark.—In the first edition of this book. the mixture of 
chlorine and carbon dioxide was absorbed by means of 5 per 
cent. caustic soda solution and the solution titrated with arsenious 
acid. . 

This method is not entirely correct, however, because it is 
based upon the assumption that the chlorine is absorbed by the 
alkali in accordance with the equation, 


2NaOH +Cl, =NaCl+NaClO+H,0, 


whereas in reality there is always some chlorate formed and 
escapes the titration.* Offerhaus,t therefore, uses two burettes 





* The error here is practically constant and amounts to 0.77 per cent. 
chlorine. The work can be carried out according to the original method, 
adding, for the sake of accuracy, 0.7 per cent. to the value of chlorine found. 
Cf. O. Steiner, loc. cit., and Treadwell and Christie, loc cit. 

If the weight of chlorine fou. by titration is used in connection with the 
theoretical molecular volume, a considerable error will be introduced for, in 
spite of the formation of chlorate, about 0.9 per cent. too much chlorine will 
be found in electrolytic chlorine. 

_+Cl. Winkler (Industriegase II, 318) and Offerhaus . angew. Chem., 
1908, 1033), also Lunge-Berl, Chem. techn. Untersuchungsmethoden, 6th 
edition, Vol. I, p. 582, and O. Steiner, Z. Elektrochemie, 1904, 327. 


312 | GAS ANALYSIS. 


for the determination, absorbing the chlorine and carbon dioxide 
in one by means of caustic-potash solution, and absorbing the 
chlorine in another sample of gas by means of potassium iodide 
and the titrating with tenth-normal thiosulphate solution. 

This is, however, an unnecessary complication, for besides 
requiring an additional burette it involves the use of more of the 
expensive potassium iodide. It is possible, however, to carry 
out the analysis in one burette By absorbing first the chlorine 
with 10 per cent. potassium-iodide solution, and then intro- 
ducing caustic potash solution from the top of the burette. 
Hereby the carbon dioxide is absorbed and the iodine is trans- 
formed into iodide and ae (the solution becomes nearly 
colorless) : 


Cl,+2KI =2KCI+L, 
3I1,+6KOH =5KI+KI10,+3H.C. 


In order to determine the amount of iodine originally set 
free, the contents of the burette are allowed to flow into a 
potassium iodide solution which is acid with hydrochloric acid: 

SKI + KIO; +6HCl =6KC1+3H,0 +3I,, 
and the liberated iodine is titrated with tenth-normal sodium 
thiosulphate solution. 

The method has no advantage over that described above and 
is not quite as accurate. 

Recently Schloetter has described another method for the 
examination of electrolytic chlorine gas. The chlorine is absorbed 
by means of hydrazine sulphate, whereby two volumes of chlorine 
set free one volume of nitrogen. The carbon dioxide is then 
absorbed by means of caustic soda solution. 

P. Ferchland* determines the chlorine by absorption with 
mercury in the residual gas after the CO, has been absorbed with 
caustic potash. This last method, according to the experiments 
of Busvold,t} gives good results; it is to be reeommended especially 
for the analysis of chlorine gas from the Deacon process. 

Examination of the Unabsorbed Gas Residue.—Usually the 
residual gas is too small in amount (as in the above case) to 





* Z. Elektrochem., 13, 114. 
+ Inaug. Dissert. Zuriel 1909, also P. Philosophof. Chem. Ztg., 1907, 959. 


CHLORINE. 813 


examine quantitatively, so that for this part of the analysis a 
larger sample of the gas is taken. The author has used the 
apparatus shown in Fig. 124 for this purpose with good results 





5 RB 


Fia. 124. 





The thick-walled filter-bottle A has a capacity of about 1.5 liters. 
It contains about 500 c.c. of strong caustic potash solution and 
the absorption-tube with stop-cock H is fastened air-tight within it. 

Manipulation.—First of all, the absorption-tube is entirely 
filled with the caustic potash solution by suction through H, finally 
closing the latter. The patent cock is then turned to the position 
ZI, and by suction through the left side-arm, the glass tube 
is filled with the alkali up to the cock. The latter is then turned 
to the position J, the left side-arm is connected, by means of a 





* This apparatus has been used often by the author in the study of elec- 
trolytic chlorine gas and was described in the first edition of this text-book. 
Since then a similar apparatus has been recommended by Thiele and Deckert, 
Z. angew. Chem., 20, 437 (1907). 


814 GAS ANALYSIS. 


short piece of rubber tubing and a long piece of glass tubing, with 
the source of the gas and several liters of gas are drawn through 
this tube by connecting the right side-arm with an aspirator.’ 
As soon as it is safe to assume that all of the air has been driven 
out from the tubing, the cock is turned to the position //, the 
aspirator is connected at a with the flask A in which a slight vacuum © 
is produced, whereby the gas beging to collect in the absorption- 
tube. Chlorine and carbon dioxide are completely absorbed, 
while the residual gas collects in the upper part of the absorption- 
tube. The gas is allowed to enter the tube until from 50 to 70 
c.c. of the gas residue are obtained; the cock J is then closed, 
the aspirator removed, and the gas driven over into a Hempel’s 
gas-burette and analyzed according to the methods already de- 
scribed. : | | : 
60.9 c.c. of the gas residue from the above-mentioned elec-. 
trolytically prepared chlorine * gave: 
Oxygen =40.7 O, = 66.9 
Carbon monoxide= 2.6 and in per cent. CO= 4.3 
Nitrogen =17.6 N = 28.8 


—_— —_——_—— 


60.9 * 100.0 


At the carbon electrode (the anode) not only chlorine but also 
a small amount of oxygen is liberated. The latter attacks the 
carbon of the electrode, forming carbon monoxide, the greater 
part of which in turn combines with the chlorine, forming phosgene 
gas, COCI,, but the latter is decomposed by water with the forma- 
tion of CO, and HCl: 


COCI,+H,0=CO,+2HCI. 


This accounts for the presence of the CO, and CO in chlorine which 
has been prepared electrolytically. 


Hydrochloric Acid HCl. Mol. Wt. 36.47. 
Density =1.2686 (Air=1).t Weight of one liter= 1.6400 gms. 
Molar volume =22.24 liters. Critical temperature = +52° C. 
Hydrochloric acid is determined in gas mixtures by absorbing 
with standardized alkali. 
 * Consisting of 99.0 per cent. Cl, and 0.6 per cent. CO,. 
+ Ledue, Compt. rend., 125, 571 (1897), found the density of hydrogen to 


be 1.2692 and Busvold, Inaug. Dessert. Zurich, 1910, obtained the value 
1.2680. The above figure is the mean of these two determinations. 





SULPHUR DIOXIDE. 815 


Sulphur Dioxide, Mol. Wt. 64.07. 


Density =2.2639 (Air=1).* Weight of one liter =2.9267 gms. 
Molar volume= 21.89 liters. Critical temperature= +155° C. 


For the determination of sulphur dioxide from pyrite burners, 
F. Reich recommends that the gas should be sucked by means. 
of an aspirator through a measured amount of 2 iodine solution, 
colored blue with starch, until the latter is decolorized. The 
amount of the gas is equal to the quantity of water which has 
flowed from the aspirator+the volume of the absorbed SOx. 

For example, 10 c.c. of iodine solution were decolorized 
after V e.c. of water had flowed from the aspirator; the gas was 
at 2 C. and 760 mm. pressure. Since in the absorption of the 
SO2 by the iodine the following reaction takes place, 


SO02+H20+1,.=2HI+S80sz, 


it is evident that the amount of SO, absorbed, measured dry at 


0° and 760 mm. pressure, will be 10.95 c.c., for 1 e.. iodine 
solution corresponds to 1.095 ¢.c. SOg. - | 3 
It follows, then, that the volume of gas taken for the analysis 
equals 
V-(B—w) -273 


760-73 +0) +10.95 c.c.= Vj, 





and from this the per cent. of SOz in the gas can be calculated: 


V,:10.95 = 100z 


5 
Sige per cent. SOo. 


Other examples of gas analyses in which the absorbed con- 
stituent is estimated by titration are found in the determination 


* Leduc, Compt. rend., 117, 219 (1893). 





816 "GAS ANALYSIS. 


of the hydrogen sulphide in gas mixtures (see below) and in the 
determination of carbonic acid in the atmosphere by-the method 
of Pettenkofer (cf. p. 593). 


Hydrogen Sulphide, H2S; Mol. Wt. =34.09. 
' Density =1.1895 (Air =1).* “Weight of one liter =1.5378 gms. 
Molar volume = 22.16 liters. @ritical temperature = 100° C. 


Determination of Hydrogen Sulphide in Gas Mixtures. 


Hydrogen sulphide, when present in the gases escaping from 
mineral springs, is estimated as follows: 

A large funnel, of from 2 to 3 liters capacity, is lowered into 
the spring and held in place by means of a wooden frame B 
weighted with stones, s (Fig. 125), The rubber tubing d is 


Hl] 
i{! 








HT inti 














V7~ 
ZY 
Y 
Z ” 
—~— 
SS 


—— 
f 
























\N 





SSS 
Z 

Z . 

Z 








| tutu 


SSeS /. WC \\ \ 


Fic. 125. 


removed from the flask a, the stop-cock h is opened, and the 
funnel 7 is filled, by means of suction, with water up to the 
stopper, and then f is closed. As soon as the water in the funnel 
has been replaced by the ascending bubbles of gas, the flask a 
is connected on the one side with the stop-cock tube h and on 
the other side directly with the aspirator A by means of a long 
rubber tubing. Suction is then started by opening H and is 


* Leduc. Compt. rend., 125, 571 (1897). 





HYDROGEN SULPHIDE. 817 


continued with h open until the water in the funnel 7 again 
reaches the stopper, when h is closed. The funnel is allowed 
to fill with gas again, and this is eventually removed through a 
by means of suction. This operation is repeated twice more. 
In this way the neck of the funnel, the glass tube h, the rubber 
tubing d, and the flask a, all have the air originally present in 
them replaced by gas from the spring; a few drops of water 
are carried along mechanically into a. Ten c.c. of hundredth- 
normal iodine solution are then introduced into the ten-bulb 
tube b and 10 c.c. of hundredth-normal thiosulphate solution are 
placed in the tube c. The flask a is now quickly connected with 
b by means of a short piece of rubber tubing /, and c is con- 
nected with the aspirator A by means of a longer piece of tubing. 
Meanwhile the funnel 7 is again filling with gas. A measuring 
cylinder C is placed under the outlet tube of the bottle A, H is 
opened, and then the stop-cock h is cautiously turned. The gas 
is allowed to bubble slowly through 6 until the iodine solution 
becomes light yellow, but is not decolorized. H is now closed 
and after about two minutes h also. The contents of c are poured 
into b, starch is added, and the excess of the thiosulphate is 
titrated with hundredth-normal iodine solution (cf. page 650). 
The number of cubic centimeters, n, of iodine required for the 
titration, represents the amount of iodine which reacted origin- 
ally with the hydrogen sulphide. The position of the water in 
the graduate is noted (V c.c.), the temperature of the room 
‘©, and the barometer reading B; w is the tension of aqueous 
vapor at the temperature 1°. 
In computing the amount of hydrogen sulphide in the gases 
escaping from the spring, it is to be remembered that the volume 
of gas which is taken for the analysis is equal to the amount of 
water which has flowed into c plus the volume of hydrogen 
sulphide which has been absorbed by the iodine in b during the 
experiment. Inasmuch, however, as the amount of the latter 
is small in comparison with the total amount of gas taken for 
analysis, it may be neglected here. Furthermore, it is neces- 
sary to call attention to the fact that the volume of gas escaping 
from the spring is at a different temperature than that of the 
analys.s; all the volumes, therefore, should be reduced to corre- 


818 GAS ANALYSIS. 


spond to the temperature at the spring. The amount of hydrogen 
sulphide present per liter in the spring gases at the temperature 
i? and the barometric pressure B is 


{W273 +1) _ 
308-45 0B ay 





c.c. H2S per liter.* 


Determination of Ethylene, according to Haber. 


The principle of this method was discussed on p. 752. The 
determination is effected in the Bunte burette (cf. p. 799, Fig. 
122), 

First, the contents of the lower portion of the burette from 
the lowest scale division to the cock is determined by weighing 
the water drawn from between these points, after allowing the 
burette to drain. Then about 90 c.c. of the gas to be examined 
are placed in the burette and the thermometer and barometer 
readings are taken. Then, exactly, as described on p. 799, the 
liquid is sucked down to the stop-cock,} a little bromine water 
is poured into a small evaporating-dish, about 10 ¢.c. of the 
liquid are allowed to rise into the burette, and in order to wash 
the bromine water from the tip into the burette, 2 or 3 c.c. of 
water are added. 

The walls of the burette are now thoroughly wet with the ~ 
bromine water by suitably turning and inclining the tube, and 
in this way the ethylene is quickly absorbed. In order to de- 
termine the excess of bromine, a strong solution of potassium 
iodide is allowed to rise into the burette, and the contents of 
the latter are vigorously shaken. The liquid is then run out 
into an Erlenmeyer flask, the burette is carefully washed out 
with water and the deposited iodine is titrated with ~ sodium 
thiosulphate solution. The titre of the bromine water added 
is next determined by pouring a little into a porcelain dish, 





* In this formula the temperature of the spring does not come into con- 
sideration because the gas is at the laboratory temperature when measured. 

+ After about one minute liquid will collect above the stop-cock, owing to 
the drainage of the liquid from the sides of the burette; this is removed before 
adding the bromine. 


DETERMINATION OF ETHYLENE. 819 


pipetting off 10 c.c. of it, allowing this amount to run into a 
solution of potassium iodide and titrating the liberated iodine with 
i0 sodium thiosulphate solution. 

The method of calculating the results will be illustrated best 
by means of a single example. 

Example.—A gas consisting of 90 volumes of air and 10 volumes 
of ethylene was used for the analysis. 

Taken for analysis, 91.2 c.c. of the mixture, 

Temperature, 18.3° C. 

Barometer reading, 725 mm. 

Tension of aqueous vapor at 18.3° C.=15.6 mm, mercury. 


Volume of the ungraduated portion of the burette... 6.10 c.¢. 
Reading of the bromine water in the graduated part.. 10.00 “ 


Bromine water used......... «- Tee ea wcrors < 16.10 c.c. 
Titre of the bromine water: 


10 ¢.c. of the bromine water correspond to 12.0 c.c. = sodium 
thiosulphate solution, so that 16.10 ¢.c. of bromine water are 


equivalent to 19.32 c.c. of = = 


To sodium thiosulphate. 


We have now: 


16.1 ¢.c. bromine water.......... = 19.32 c.c. < solution. 


16.1 c.c. bromine water+ ethylene =12.23 “ “ 


es 


The ethylene corresponds to..... 7.09 “ “ & 


Since the absorption of the ethylene by the bromine water 
takes place according to the equation 


C,H,+ Br, =C,H,Br, 
it follows that 


2Br=21=20.000 c.c. a sodium thiosulphate solution =22,270* 


c.c. ethylene, and since 1 c.c. os sodium thiosulphate corresponds 





* Cf. page 751. 


820 GAS ANALYSIS. 


to 1.100 c.c. CoH4, the 7.09 c.c. of = solution used represent 


7.09 X 1.100 =7.94 c.c. CoH, at 0° C. and 760 mm, pressure, or 9.10 
c.c. C,H, at 18.3° C. and 725 mm., measured moist. 
The gas consists, therefore, of: 


CH = ait and in per cent. Me Sake per cent. 


Air =82.1 Air =90.0 “ “ 
91.2 100.0 per cent. 


This method is especially suited for the determination of ethylene 
present with benzene ;in illuminating-gas. In one sample the 
sum of the two gases is determined by absorption with fuming 
sulphuric acid or bromine water, and in a second sample the 
ethylene is determined as described above. . 

This method is suitable for determining ethylene mixed with 
benzene vapors in illuminating gas. In one sample the sum of 
the two is obtained by absorbing with fuming sulphuric acid 
or bromine and in a second sample the ethylene is determined 
as above. 

Remark.—Instead of using bromine water, which changes its 
strength so rapidly, the author uses a tenth-normal solution of 
potassium bromate; on acidifying an equivalent quantity of 
bromine is obtained. 

The experiment is carried out as follows: Exactly as described 
above, 90 c.c. of the gas are led into the Bunte burette, the water 
withdrawn till the lower cock is reached, and then some potas- 
sium bromate solution is placed in a small porcelain dish and about 
10 c.c. of it is sucked up into the burette and the volume deter- 
mined, Then, after wiping off the lower capillary, an excess of 
concentrated potassium bromide solution and finally an excess 
of dilute hydrochloric acid is introduced. After shaking eight 
minutes, all the ethylene will be brominated. At the end of this 
time, 10 per cent. potassium iodide solution is allowed to enter 
the burette, the contents shaken, and emptied into an Erlenmeyer 
flask. The iodine thus liberated is titrated with tenth-normal 
sodium thiosulphate solution. The calculation is carried out as 
before. Using this modification of Haber’s method, the author’s 


DETERMINATION OF ETHYLENE. 821 


assistant, M. Bretschger, analyzed a mixture containing air and 
known quantities of ethylene with the following results: 


saa er taken. Found. 
1 5.20% 2.23% 
2 5.20 5.16 
3 5.20 5.15 
4 3.90 3.88 
5 3.70 3.70 
6 3.70 3.68 


Determination of Ethylene in the Presence of Acetylene. 


Since acetylene is not attacked in the cold by either bromine 
or iodine, whereas ethylene is readily attacked, the author has 
caused his assistant to carry out a number of experiments in thus 
analyzing gas mixtures. 

In one sample of the mixture, the sum of the iciccincctylenc 
was determined by absorption with fuming sulphuric acid and in 
another sample the ethylene was determined by the above 
bromate-bromide method. The accuracy is attested by the 
following analyses of M. Bretschger: 


Ethylene taken. Ethylene found. Acetylene found. 


4.38% 4.33% 0.61% 
4.17 4.23 0.32 
¢ oe, 9.71 0.35 
9.83 9.84 0.43 
7.85 pb SS 4.65 
7 83 te aS: 4.60 
2.69 2.80 19.18 
2 


. 64 2.74 19.60 


822 GAS ANALYSIS. 


GAs-VOLUMETRIC METHODs. 


If in consequence of a chemical reaction a gas is evolved, from 
the volume of the latter the weight of the original substaace may 
be determined. 

Examples of this sort of an analysis were given under the 
determination of COz in carbonates (pp. 384, 388, 393), the carbon 
contents of iron and steel (pp*402 and 404), and the NO3 in 
nitrates (p. 456). : 

At this place a few more important determinations of the same 
nature will be described. 


Determination of Ammonia in Ammonium Salts. 


The following method, first proposed by Knop* and later 
modified by P. Wagner,j depends upon the fact that ammonia 
is oxidized by sodium hypobromite with evolution of nitrogen: 


2NH,+3Na0Br=3H,0+ 3NaBr+N,. 


The nitrogen is collected in an azotometer and measured. 

If the amount of the ammonia be calculated from the volume 
of the nitrogen, too low results will be obtained, and this fact 
was formerly explained by the assumption that a part of the 
nitrogen was absorbed as such by the alkaline bromine solution. 
To-day, however, we know that such is not the case. At ordinary 
temperatures all of the ammonia is not oxidized according to 
the above equation to water and nitrogen, but a small amount 
of ammonium hypobromite is formed; for this reason too little 
nitrogen is obtained in the azotometer. If, on the other hand, 
the decomposition takes place at 100° C., the reaction goes quanti-_ | 
tatively according to the equation. It is inconvenient to work 
at such a high temperature, so that it.is more practical to make 
a correction to the volume of nitrogen obtained at ordinary tem- 
peratures. 





* Chem. Centralbl., 1860, p. 243. 
+ Zeitschr. f. anal. Chem., XIII (1874), p. 383; XV (1876), p. 250. 


DETERMINATION OF ETHYLENE. 823 


Reagents and apparatus required: 

1, An ammonium chloride solution, obtained by dissolving 
8.3544 gms. of the pure sublimed salt in water and diluting to 
500 c.c. 

10 c.c. of this solution evolve at 0° C. and 760 mm. pressure 
35 ¢.c. of nitrogen if the reaction takes place according to the 
equation. 

2. Sodium hypobromite solution. 100 gms. of sodium hy- 
droxide are dissolved in water, diluted to 1250 ¢.c. and after 
cooling by placing the flask in cold water, 25 e.c. of bromine 
are added, the contents of the flask vigorously shaken and again 
cooled. 

This solution must be preserved in a stoppered bottle and 
protected from the action of light. 

3. An azotometer. Instead of the azotometer of Wagner,* 
Lunge’s Universal Apparatus (Fig. 61, b, p. 387), or any such meas- 
uring Instrument may be used. 

Procedure.—Ten c.c. of the standard ammonium chloride solu- 
tion are placed in the small Wagner decomposition-bottle (Fig. 
_ 61, a, p. 387) while 40 to 50 ¢c.c. of the hypobromite solution 
are poured into the glass L (which is fused to the bottom of 
the bottle H). The bottle is then connected with the measuring- 
tube A, which is entirely filled with mercury, b opened and the 
levelling-tube B lowered. The bottle H is inclined so that some 
of the hypobromite solution comes in contact with the solution 
of ammonium chloride and the two liquids are mixed by gentle 
shaking. A lively evolution of nitrogen at once takes place and 
the liquid becomes heated. As soon as the action ceases, more of 
the hypobromite solution is allowed to act upon the ammonium 
salt and the process is repeated until finally all of the hypobromite 
is in the outer part of H. As soon as no more gas is evolved 
by shaking, the decomposition-bottle is placed in water at the 
room temperature and after allowing it to stand ten minutes, 
the volume of the nitrogen is read under the conditions de- 
scribed on p. 389 The volume of nitrogen at 0° and 760 mm. 





* Loc. cit. 
+ The contents of the decomposition-bottle are previously cooled to the 
room temperature before the cock b is connected with it. 


824 GAS ANALYSIS. 


thus found will be smaller than the theoretical value of 35 c.c., 
but it corresponds to the amount of ammonia contained in 10 e.c., 
of the ammonium chloride solution, i.e., 0.05320 gm. NH,. 

A number of such determinations are carried out and the mean 
of the results obtained is taken for the correct value. 

After this, some of the ammonium salt to be analyzed is weighed 
out, dissolved in water, and diluted so that 10 c.c. of the solution 
will contain approximately the same amount of ammonia as in 
the case of the standard solution. Then if, for example, from 
a gms. of an ammonium salt, V e.c. of nitrogen at 0° C. and 760 
mm. pressure were found, we have: 


V: V=0.05320: x 


V1X 0.05320 
ae V 





and in per cent.: 


V1 X5.320 


Vig ber cent. NH3.* 


Remark.—The results obtained by this method agree exactly 
with those obtained by the distillation method described on p. 560. 
Only with substances containing sulphocyanates are the results 
obtained too high; in this case the sulphocyanate is decomposed 
by the alkaline hypobromite solution with evolution of nitrogen 
and carbon monoxide.t 

Consequently, the above method affords uncertain results in 
the analysis of the ammonia in gas liquors. 

Urea is decomposed by the alkaline hypobromite solution 
according to the equation: 





* Lunge (Lunge-Berl, Chem. techn. Untersuchungsmethoden, 6th edition, 
Vol. I, p. 155) does not standardize against the solution of ammonium chloride 
of known strength, but adds 2.2 per cent. more ammonia to correspond to 
the loss of nitrogen. Then V X0.001558=gm. ammonia. 

+ Donath and Pollak, Zeitschr. f. angew. Chem., 1897, p. 555. 


DETERMINATION OF NITROUS AND NITRIC ACIDS. 825 


CO(NH2)a3+3Na0Br=3NaBr+CO,+N2+2H20.* 


so that it can be determined in the same way as ammonium salts, 
the carbon dioxide produced by the decomposition being kept back 
by means of caustic soda solution. 


Determination of Nitrous and Nitric Acids. 


Principle.—If a solution of a nitrite or nitrate be shaken 
with mercury and an excess of sulphuric acid, all of the nitrogen 
is set free as nitric oxide: 


2HNO,+2Hg+ H,S0,=2H,0+ Hg,SO,+2NO, 
2HNO,+6Hg+3H,SO,=4H,0+ 3Hg,SO,+2NO. 


From the volume of the nitric oxide, the weight of the nitrate 
or nitrite is computed. 

The analysis is best performed in a Lunge nitrométer.t The 
latter is a Bunte burette, in which the lower stop-cock is lacking and 
the lower end of which is connected with a levelling-tube containing 
mercury. By raising the latter, the nitrometer (which need not 
be graduated) is entirely filled with mercury and the. two-way 
cock under the funnel is closed. Then a weighed amount of the 
substance dissolved in a little water is placed in ‘the funnel, the 
levelling-tube lowered, and the solution introduced into the 
nitrometer by carefully opening the cock, the funnel being finally 
washed out four times with two or three c.c. of concentrated 
sulphuric acid. The decomposition-tube is now taken out of 
the frame, it is placed several times in a nearly horizontal position 
and then quickly changed to a vertical position. By this means 
the mercury becomes intimately mixed with the acid and the 
decomposition at once ‘begins. The shaking is continued one or 
two minutes until there seems to be no further increase in the 
volume of the liberated gas. The decomposition vessel is then 





* This reaction does not take place as completely as with ammonium salts. 
Lunge finds in the determination of urea in urine that the nitrogen deficit is 
9 per cent. If, therefore, the volume of nitrogen after being reduced to 0° 
and 760 mm. is multiplied by 2.952, the correct urea value is obtained. 

+ Berichte, 1890, p. 440, and Zeitschr. f. angew. Chem., 1890, p. 139. 

t See also Lunge-Berl, Chem. techn. Untersuchungsmethoden, 6th edition, 

Vol. I, p. 156. 


826 GAS ANALYSIS. 


connected by means of a short piece of rubber tubing with the 
gas-burette filled with mercury, the nitric oxide is transferred 
to the latter, and its volume read after reducing it to the standard 
conditions by means of the gas-compensation tube. (Cf. p. 387, 
Fig. 61, 6.) 

If in an analysis a gms. of a nitrate were taken and Vg c.e. 
of NO were obtained, we have: 





(NOs) 
22,391 c.c.:62.01 = Vo: 
Vo X 62.01 
= 99901 ms. NO3 
and in per cent.: 
G20): Ne: s Kae: 
59391 <q = 0:2769 ate =per cent. NOs. 


Remark.—For the analysis of ‘‘nitrose,” * the author knows of 
no method which affords such exact results. 

For the determination of nitrous acid in the presence of 
nitric acid by a gas-volumetric method, P. Gerlinger+ treats 
the neutral solution of the two salts with a concentrated solution 
of ammonium chloride, whereby the following reaction takes 


place: 


Half of the nitrogen evolved, therefore, comes from the nitrous 
acid present. For the details of this determination, the original 
article should be consulted. 


Hydrogen Peroxide Methods. 


Hydrogen peroxide is oxidized by means of a number of 
different substances, giving up all of the oxygen; twice as much 
oxygen is evolved as is obtained from the oxidizing agent: 


a 
XO + H202= X+H20+4+ Orc. 





* Cf. Vol. I. 
+Z. angew. Chem., 1901, 1250. See also J. Gaihlot, J. Pharm, Chim., 


1900, 6th Series, Vol. XII, p. 9. 


STANDARDIZATION OF PERMANGANATE SOLUTIONS. © 827 — 


Since, however, hydrogen peroxide itself slowly decomposes 
on standing (the decomposition being measurable on shaking 
and quite considerable by shaking in the presence of solid sub- 
tances (sand, etc.)), it follows that in the following methods no 
great excess of hydrogen peroxide should be used, and long-con- 
tinued shaking must be avoided. 


Standardization of Permanganate Solutions. 


The determination is best made according to Lunge in a gas 

volumeter (p. 387, Fig. 61). In order to obtain correct results, 
however, it is absolutely necessary that no excess of hydrogen 
peroxide be present. Consequently it is necessary to determine 
by means of a preliminary experiment the exact value of the 
permanganate solution in terms of the HzO: solution used (cf. 
p. 626). Then a measured amount of the latter is placed in the 
outside part of the Wagner decomposition-bottle (Fig. 61a, p. 387), 
and 30 ¢.c. of dilute sulphuric acid (1:5) are added. After this, 
the exact amount of hydrogen peroxide required for the decom: © 
position of the permanganate is introduced into the inner part of 
of the bottle and the latter is connected with the measuring-tube, 
which is filled with mercury, the cock b being removed for the 
time being, but it is replaced at the end of two or three minutes 
and turned to the position shown in the figure. 
The two liquids are then mixed, taking care to hold the decom- 
position-flask so that its contents will not be warmed by the heat 
of the hand, inclining it to an angle of about 90° with the vertical, 
and shaking for exactly one minute. While the oxygen is being 
evolved, care must be taken that the gas in the eudiometer is 
under reduced pressure. At the end of the decomposition, the 
gas is placed under atmospheric pressure, b is closed, and by means 
of the compensation-tube, the volume of the gas is reduced to 
what it would be at 0° C. and 760 mm. pressure, as described on 
p. 388. 

One-half of the observed volume corresponds to the amount 
cf oxygen given up by the potassium permanganate. This number 
multiplied by 0.001429 gives the weight of the oxygen obtained 
from the permanganate. 


$28 / GAS ANALYSIS. 


Remark.—The amount of permanganate to be taken for the 
experiment is determined by the size of the measuring-tube. If 


¢ 


the latter has a capacity of 150 cc., 15 cc. of a = solution 


or 40 to 50 cc. of a a solution should be taken. : 

The hydrogen peroxide used should not be too concentrated : 
it should be about a 2 per cent. solution. The active oxygen 
present in a sample of pyrolusite * may be determined by the 
same procedure. 


Determination of Cerium in Soluble Ceric Salts. 


If hydrogen peroxide is added to an acid solution of a soluble 
ceric salt, the latter is reduced with evolution of oxygen: 


2CeO, + H,0, =Ce,0, + H,0+0,. 


The determination is effected in precisely the same way as 
was described above in the standardization of the permanganate 
solution. If half the volume of liberated oxygen is multiplied 
by 0.03077, the product represents the corresponding amount 
of CeOo.t 

Remark.—If a large excess of hydrogen peroxide:is avoided 
in the above analysis, satisfactory results will be obtained. 

For other determinations of this sort, consult “ Lunge’s Alkali 
Makers’ Handbook” and ‘‘ Hempel’s Gas-Analytical Methods.” 


Silicon Fluoride, Sify. Mol. Wt.=104.3. 


Density =3.605 (Air=1). Weight of one liter=4.660 gms. 
Molar volume = 22.40 liters. 





* Lunge’s Alkali Makers’ Handbook. 
ft Assuming that the atomic weight of Ce= 140.25. 


DETERMINATION OF FLUORINE AS SILICON FLUORIDE. - 829 


Determination of Fluorine as Silicon Fluoride _ (Hempel and 
Oettel) .* 


Principle.—If a mixture of calcium fluoride and powdered 
quartz is treated with concentrated sulphuric acid in a glass 
vessel, all of the fluorine will be expelled as silicon fluoride: 


2CaFs> + S102 + 2HsSO4 = 2CaSO,4 + 2H2O -+- Sik, 


and this gas can be collected and measured. 

One c.c. Sik’4 at 0° and 760 mm. pressure corresponds to 
0.006978 gms. CaFs, or 0.003395 gms. Fo. 

Procedure.—A weighed amount of the very finely-powdered 
substance, which must not contain any other acid that can be 
expelled by treatment with concentrated sulphuric acid,t is 
mixed with 3 gms. of ignited finely-powdered quartz and intro- 
duced into the dry decomposition flask K (Fig. 126). The latter 
is then evacuated somewhat by twice lowering the leveling-tube 
N with the stop-cock H open, closing the cock and expelling 
the air. At the beginning of the experiment, the burette H is 
not connected with the Orsat tube O. By raising the ground- 
glass tube R, about 30 c.c. of concentrated sulphuric acid are 
allowed to flow into the flask. This acid must have been pre- 
viously heated in a porcelain crucible for some time at a tem- 
perature near the boiling-point, in order to detsroy every trace 
of organic matter, and allowed to cool in a desiccator over 
phosphorus pentoxide. The acid in K is heated to boiling with 
the stop-cock H open and the flask is frequently shaken. During 
the entire experiment, the mercury level in the tube WN is kept 
a little lower than that of the mercury in the measuring-tube V.t 

At first the sulphuric acid foams considerably, but soon ceases, 
which is a sure sign that the decomposition is complete. The 
flame is then removed, the sulphuric acid allowed to cool and 
all the gas in K is expelled by introducing through V sulphuric 





* Gasanalytische Methoden, III, p. 342. 

t Cf. p. 479. 

{ In order to keep the inside of N perfectly dry, 2 or 3 ¢.c. of concen- 
trated sulphuric acid are placed on top of the mercury. 


830 GAS ANALYSIS. 


acid, which has been previously heated and cooled as described 
above. As soon as the sulphuric acid reaches the stop-cock H, 
this is closed. After waiting ten minutes more, the gas is placed 
under atmospheric pressure, by suitably raising N, and the 
volume and temperature are noted. 

The gas is now driven over into the Orsat tube O, containing 
caustic potash solution (1:2). Theysilicon tetrafluoride is imme- 
diately absorbed. The residual gas is carried back to the tube M, 
and after waiting fifteen minutes the volume is read. The differ- 


















Y fr) | 
NA Wu 
M XX 
- = =) 
ye AN 
a 
Fic. 126. 


ence between the two readings gives the volume of silicon tetra- 
fluoride. 

Remarks.—A. Koch tested this method in the author’s labor- 
atory and obtained results varying from 98.97 to 102.63 per cent. 
with pure calcium fluoride. To obtain this accuracy, however, - 
it is necessary to carry out the decomposition under approx- 
imately atmospheric pressure. When working under a vacuum 
the results were always too low. Thus in one case only 85.70 
per cent. of the theoretical value was obtained. — 

-- During these--experiments. with reduced pressure, a white 
sublimate forms atthe lower part of the condenser, which on : 
coming in contact with the sulphuric acid that is introduced at 


DETERMINATION OF VAPOR IN GAS MIXTURES. $31 


the last, causes a strong effervescence. Since, however, all the 
gas in the burette was replaced, we believed that the low results 
could be traced to the absorption of silicon fluoride by the sul- 
phuric acid. This idea proved to be false, for a measured volume 
of silicon fluoride does not change when allowed to stand for 
twenty-four hours over concentrated sulphuric acid. The error, 
therefore, must be caused by the strange deposit that has con- 
densed in the lower part of the condenser. If the work is carried 
out under atmospheric pressure as described above, the white 
deposit is never obtained. 

The method can be used for the estimation of fluorine in the 
presence of carbonates. In this case the silicon fluoride is 
absorbed by means of a little water and the carbon dioxide by 
means of caustic potash. As, however, a little of the carbon 
dioxide is dissolved by the water, the gas residue which has been 
freed from carbon dioxide is shaken with this water again, whereby 
this dissolved carbon dioxide is removed and can be absorbed 
by a further treatment with caustic potash solution. For further 
details, consult the original paper by Hempel and Scheffler.* 


Determination of Vapor in Gas Mixtures. 


-. It is often desired to estimate the weight, and from this the 
volume, of vapor present in a mixture of gases and vapors. This 
will be illustrated by one or two examples. Let it be assumed 
that in a unit of volume of a given gas mixture there are v’ parts 
of gas and v”’ of vapor. If the whole mixture is under the 
pressure P, then the partial pressures of the gas and vapor will 
be respectively v’P and v’’P’. The following equation then holds: 
Pow Pt olP. | 

The total pressure is therefore equal to the sum of the poe 
“pressures. 

If now the partial pressure of either constituent is lnown: the 
volume of this constituent can be found by dividing by the 
total pressure. If, for example, v’’P =w, then 


PPR | - 
Applications.—1. Reduction of Volumes of I Moist Gases toa 
Dry Condition at 0° C. and 760 mm. Pressure. 


* Z. anorg. Chem., 20, 1 (1897). 





832 GAS ANALYSIS. 


A gas saturated with water vapor occupies a volume 2; at 
i? C. and P mm. pressure. A unit of volume of the gas consists 
of v’ volumes of dry gas and v”’ volumes of water-vapor. Now 
the tension of aqueous vapor at f’=w, a value which can be 
obtained from the tables on p. 842. This value, w, represents 
in fact the partial pressure of the water-vapor. Consequently 
P—w is the partial pressure of the dry constituents and the 
volume of the latter, v’, is, as expined above: 


vans at t° and P mm. pressure. 





This volume reduced to 0° and 760 mm. pressure is 


oti (Pe wy Pa a Fimeap 
°” P-760-(1+at) 760(1+at)’ 





If the original volume of the gas and vapor is not 1; but V;, 
then, 
P-—w 


if 0” 760 Fat) 





¥ 5 


Similarly, the volume of the water-vapor at 0° and 760 mm, 
pressure is 


WwW 


. 0 760(1 Fat) 





V,.¥ 


2. Calculation of the Moisture in the Air at Normal Pressure 
(760 mm.) and Temperature t°. 

What is the volume of water-vapor contained in 100 e.c. 
of moist air at 0°, 25°, and 35° C.? According to the table: 
wo=4.6 mm.; we5=23.5 mm.; and w3;=41.8 mm. Hence the 
volume of water-vapor present in a unit of volume is 


a 8 Se ee 
760 760’ 760 760’ 760 760’ 





* In this formula is it assumed that the water-vapor also follows Boyle’s 
law, which is not strictly true. 


DETERMINATION OF VAPOR IN GAS MIXTURES. 833 
and the percentage of moisture is 


0.61 per cent at 0°; 
pe ea. Bie 
50: .<* oo. 


If the gas is not saturated with moisture, the relation of the 
dry gas to the moist one is not so easy to determine, unless the 
degree of saturation is known. The humidity of.a gas, or the 
degree of saturation, expresses the amount of moisture present 
as compared with the total amount which the gas can take up 
when perfectly saturated with moisture. Thus if the humidity 
is 50 per cent. the gas could take up as much again moisture 
at the prevailing temperature. 

If the degree of humidity of a gas is expressed by r, then the 
volume of the given gas at this temperature and 760 mm.pressure, 
‘ | 

V(P-r-w) 
fas | ees 





and the volume of the water-vapor present is 


V-r-w 
760 ~ 


3. Calculation of the Weight of Water-vapor in a Given Volume 
of Air which 1s Saturated with Moisture at the Temperature 
and the Pressure P mm. 

One c.c. of water-vapor weighs 0.000801 gm. at 0° C. and 
760 mm. pressure. 

One c.c. of water-vapor at ¢° and P mm. pressure occupies a 
volume of 








as ae Se 
o 760(1 +t)’ 
and weighs 
P 
CTT ahaa gm. 


If now w= the vapor pressure of water at /° and P mm. 


834 | GAS ANALYSIS. 


pressure, then the volume of water-vapor present in the volume 
V of the gas is, under these conditions, : 


w:V; 


P 7 
and the weight of the water-vapor amounts to 


Sa w:-v, 0.000801: w-r; 
Oita ea aD 








If the weight of the water-vapor is g, then the volume of 
the moist air is 
g:-760(1 +at) * 
5 ee 
0.000801 - w 





If the gas is not saturated with water vapor, but the degree of 
saturation (the humidity) is known, then the following formula — 
gives the weight of water-vapor present in the volume 2; of the 
gas, 

__ 0.000801 -r-w-v, 
~ -760(1 + at) 





Or, if the weight of vapor present in a given gas volume is 
known, then from the last equation the humidity, r, of gas may 
be computed: 

: _ g:760(1 +a) 
~ 0.000801 - w-v;' 





4, Calculation of the W eight of One Liter of Air at 0° and 760 mm. 
when Free from Moisture and Carbon Dioxide. 


1 c.c. of pure, dry air weighs 0.0012928 gm. at 0° and 760 mm. 
1 cc. of water-vapor weighs 0.000801 gm. at 0° and 760 mm. 

1 ee. of water-vapor is therefore 0.62 times as heavy as 1 c.c. of air. 
1 c.c. of COg weighs 0.001977 gm. at 0° and 760 mm. 





* Similarly, the volume of a gas saturated with any other vapor can 
be computed if the weight of vapor present and its density are known. 


DETERMINATION OF VAPOR IN GAS MIXTURES. 835 


Air contains,.on an average, 0.03 per cent of COy. One liter | 
of dry air, containing the average amount of COs consists of 


999.7 c.c.air and weighs 999.7 <0.001293 = 1.2924 gms. 
0.3 ‘° COs 0.3 0.001977 =0.0006 ‘‘ 


1000.0 ‘* . | P2000 “!.  =@ 


The corresponding volume of dry air and of water-vapor at 0° 
and 760° mm. pressure is 


760—w ow 
760 * 760 
and weighs 
760 —w w  0.38w 
As veabboa ees Os SS cart 
760 a+ 7e5 0.624 a(t 760 ), 


or, in other words, 1 liter of moist air weighs at 0° and 760 mm. 
pressure, 


5. Calculation of the Weights of One Liter of M oist Adr Cone 
taining Carbon Dioxide at t° and P mm. Pressure. 

If the tension of aqueous vapor=w:, then the volume of the 
_ moist gas is 
Pp 


= t= at © and P mm. pressure, 


Wt 
LER o 


P 
and at 0° and 760 mm. 

mi Wt 760 — w; 

760(1+at) 760(1+at)’ 





and, as shown under 4, this weight, 


ogee Fash Ww _a(P—0.38w) 
I= 760 ad) <** 7601 Fad) <9? 760(1+at) 8° 








or 








AS oe ole 8) 











838 APPENDIX. 


SPECIFIC GRAVITY OF STRONG ACIDS AT = IN VACUO. 


(According to Lunas, Ister, Narr, and MarcHLEWsky.)* 





Specific Srecific 














Gravit Per Cent. by Weight. Gravity | Per Cent. by Weight. 
at a : at = 
(Vacuo). HCl. HNO3 HSOx,. (Vacuo). HNOs. H2SO.. 
1.000 0.16 0.10 0.@9 1 235 37.5 31.70 
1.005 1.15 1.00 0.95 1.240 38.27 OZ ae 
1.010 2.14 1.90 1.57 1.245 39.03 32.86 
1.015 3.12 2.80 2.30 1.250 39.80 33.43 
1.020 4.13 3.70 3.03 1.255 40.56 34.00 
1.025 5:15 4.60 3.76 1.260 41.32 34.57 
1.030 6.15 5.50 4.49 1.265 42.08 35.14 
1.035 7.10 6.38 5:23 1.270 42.85 Sia8 
1.040 8.16 7.26 5.96 1.275 43 .62 36.29 
1.045 9.16 8.13 6.67 1.280 44.39 36.87 
1.050 10.17 8.99 tnd 1.285 45.16 37.45 
an, 1.055 11.18 9.84 8.07 1.290 45.93 38.03 
~ 1.060 12.19 10 67 8.77 1.295 46.70 38.61 
1.065 13.19 11.50 9.47 1.300 47 .47 39.19 
1.070 14.17 12,.32 10.19 1.305 48 .24 39.77 
1.075 15.16 13.14 10.90 1.310 49.05 40.35 
1.080 16.15 13.94 11.60 1.315 49.88 40.93 
1.085 Wile 14.73 12.30 1.320 59.69 41.50 
1.090 18.11 15.52 12.99 1.325 51.51 42.08 
1.095 19.06 16.31 13.67 1.330 52.34 42.66 
1.100 20.01 17.10 14.35 1.335 53.17 43 .20 
1.105 20.97 17.88 15.03 1.340 54.04 43.74 
1.110 21.92 18.66 15.71 1.345 54.90 44.28 
1.115 22.86 19.44 16.36 1.350 55.76 44.82 
bee oe 8 23 .82 20.22 17.01 1.355 56.63 45.35 
1.125 24.78 20.99 17.66 1.360 57.54 45.88 
1.130 25.75 21.76 18.31 1.365 58.45 46.41 
1.135 26.70 22 -Ba 18.96 1.370 59.36 46.94 
1.140 27.66 23.30 19.61 1.375 60.27 47.47 
1.145 28.61 24.07 20 . 26 1.380 61.24 48 .00 
1.150 29.57 24.83 20.91 1.385 62.21 48.53 
1.155 30.55 25.59 21.55 1.390 63 .20 49 .06 
1.160 31.52 26.35 22.19 1.395 64.22 49.59 
1.165 32.49 oF eS | 22.83 1.400 65.2/ 50.11 
1.170 33 .46 27 .87 23 .47 1.405 66.37 50.63 
1.175 34.42 28.62 24.12 1.410 67 .47 51.15 
1.180 35.39 29.37 24.76 1.415 68 .60 51.66 
1.185 36.31 30°12 25.40 1.420 69.77 52.15 
1.190 37.23 30.87 26 . 04 1.425 70.95 52.63 
1.195 38.16 31.60 26.68 1.430 72.14 53.11 
1.200 39.11 32.34 Give 1.435 13.00 53.59 
1.205 33.07 27.95 1.440 74.64 54.07 
1.210 33.80 28.58 1.445 75.94 54.55 
1.215 34.53 29.21 1.450 77.24 55.03 
1.220 35.26 29 . 84 1.455 78.56 55.50 
1.225 36.01 30.48 1.460 79.94 55.97 
1.230 36.76 31.11 1.465 81.38 ° 56.43 


























* Lunge-Berl, Chem. techn. Untersuchungsmethoden, 6th edition, Vol. I, 
Tables, 61, 39, and 49. : 


SPECIFIC GRAVITY OF STRONG ACIDS. 


839 


SPECIFIC GRAVITY OF STRONG ACIDS AT eee IN VACUO.—Cont. 




















4° 
(According to Isier, Narr, and MarcHLEWSKY.) 

Specific : Specific | Per Cent. Specific | FerCent- 
ery see Pee eden Weight. Gravity Weidht, 
at P at rc | at 4° 

(Vacuo). HNO, H.SO, (Vacuo). H.SO,. | (Vacuo). HoSO,, 
1.470 82.86 56.90 1.610 69.56 1.750 81.56 
1.475 84.41 ay gees if 1.615 70.00 on Te 82.00 
1.480 86.01 57.83 1.620 70.42 1.760 82.44 
1.485 87 . 66 58.28 1.625 70.85 1.765 83.01 
1.490 89 : 86 58.74 1.630 FB +: 1.770 83.51 
1.495 91.56 59.22 1.635 71.70 Lio 84.02 
1.500 94,04 59.70 1.640 72.12 1.780 84.50 
1.505 96. 34 60.18 1.645 4a200 1.785 85.10 
1.510 98.05 60.65 1.650 72.96 1.790 85.70 
545 99.02: 61.12 1.655 73.40 1.795 86.30 
1.520 99. 62 61.59 1.660 73.81 1.800 86 . 92 
£525 62.06 1.665 74,24 1.805 87 .60 
1.530 62.53 1.670 74.66 1.810 88 .30 
1.535 63 .00 1.675 75.08 1.815 89.16 
1.540 63.43. 1.680 75.50 ay. o20 90.05 
1.545 63.85 1.685 75.94 1.825 91.00 
1.550 64.26 1.690 76.38 1.830 92.10 
1.555 64.67 1.695 76.76 1.835 93 . 56 
1.560 65.20 1.700 77.17 1.840 95.60 
1.565 65.65 1.705 77.60 1.8405 | 95.95 
1.570 66.09 1.710 78 . 04 1.8410 | 96.38 
1.575 66.53 1.715 78 .48 1.8415 | 97.35 
1.580 66.95 1.720 78.92 1.8410 | 98.20 
1.585 67.40 1325 79.36 1.8405 | 98.52 
1.590 67.83 1.730 79.80 1.8400 | 98.72 
1.595 68 . 26 L735 80.24 1.8395 98.77 
1.600 68.70 ° 1.740 80.68 1.8390 | 99.12 
1.605 69.13 1.745 81.12 1.8385 | 99.31 



































840 APPENDIX. 


SPECIFIC GRAVITY OF POTASSIUM AND SODIUM HYDROXIDE 
SOLUTIONS AT 15° C. 








Specific Per Cent. Per Cent. Srecific Per Cent. Per Cent. 

Gravity. KOH. NaOH. Gravi.y. KOH. NaOH. 
1.007 0.9 0.59 ticoe 27.0 22.50 
1.014 1.7 1.20 1.263 Axce 23.50 
1.022 2.6 1.65 1.274 28.9 24.48 
1.029 3.5 2.00 ™ 1.2.6 29.8 25.50 
1.037 4.5 i 1.297 30.7 26.58 
1.045 5.6 3.79 1.308 31.8 27 .65 
1.052 6.4 4.50 1.320 32.7 28.83 
1.060 7.4 5.20 1,332 Sa8 30.00 
1.067 8.2- 5.86 1.345 34.9 - 31.20 
1.075 9.2 6.58 1.357 35:9 32.50 
1.083 ~ 10.1 7.30 1.370 36.9 33.40 
1.091 10.9 8.07 1.353 37.8 35.00 
1.100 12.0 8.78 1.397 38.9 36.36 
1.108 12.9 9.50 1.410 39.9 37 .65 
1.116 13.8 10.30 1.424 40.9 39.06 
1.125 14.8 11.06 1.4388 42.1 40 .47 
1.134 15.7 11.90 1.453 43 .4 42 .02 
1.142 16.5 12.69 1.463 44.6 43.58 
1.152 17.6 12.50 1.483 45.8 45.16 
1.162 18.6 14.35 1.498 47.1 47.73 
Late 19.5 15.15 1.514 48.3 48.41 
1.180 20.5 16.00 1.530 49.4 50.10 
1.190 21.4 16.91 1.546 50.6 roe 
1.200 22.4 17.81 1.563 51.9 arene 
1.210 23.3 18.71 1.580 &3:.2 — 
1.220 24.2 19.65 1.597 54.5 —- 
1.231 25.1 20 .69 1.615 55.9 — 
1.241 26.1 21.55 1.634 57.8 oo 























SPECIFIC GRAVITY OF AMMONIA SOLUTIONS. 841 


SPECIFIC GRAVITY OF AMMONIA SOLUTIONS AT 15° C. 


(According to LuNGE and WIERNIK.)* 














Specific Gravity. Per Cent. NH3. Specific Gravity. Per Cent. NH3. 
1.000 0.00 0.940 15.63 
0.998 30,45 0.938 16.22 
0.996 0.91 t= = 0.996 16.82 
0.994 2 Fiat 0.934 17.42 
0.992 é 1.84 0.932 18.03 
0.990 ; P2351 0.930 18.64 
0.988 2.80 0.928 19.25 
0.986 . 3.30 0.926 19.87 
0.984 3.80 0.924 20.49 
0.982 4.30 0.922 2f 12 
0.980 4.80 0.920 21.75 
0.978 5.30 0.918 22.39 
0.976 5.80 0.916 23 .03 
0.974 6.30 0.914 23 .68 
0.972 6.80 0.912 24 .33 
0.970 7qiol 0.910 ' 24.99 
0.968 7.82 0.908 25.65 
0.966 8.33 0.906 26.31 
0.964 8.84 0.904 26.98 
0.962 9.35 0.902 27 .65 
0.960 9.91 0.900 28 .33 
0.958 10.47 0.898 29.01 
0.956 11.03 0.896 29 .69 
0.954 11.60 0.894 30.37 
0.952 12.17 0.892 facta © yet 
0.950 12.74 0.890 31375 
0.948 ge oe | 0.888 32.50 
0.946 13.88 0.886 Ba.2D 
0.944 14.46 0.884 © 34.10 
0.942 15.04 0.882 34.95 











* Lunge-Berl, Chem, techn. Untersuchungsmetho den, 6th edition, p. 531. 


842 


APPENDIX. 


TENSION OF WATER VAPOR ACCORDING TO REGNAULT. 











2 mi % nsi 5 i i 
Degrees iiewes |G | neta ee so 
—2.0 3.9&5 +2.0 5.302 +6.0 6.998 
1.9 3.985 2.1 5.340 TER SS 7.047 
1.8 4.016 2.2 5.378 6.2 7.095 
1.7 4.047 zd @ £§.416 6.3 7.144 
1.6 4.073 2.4 5.454 6.4 7.193 
1.5 4.109 2.5 5.491 6.5 7.242 
LAs 4.140 2.6 5.530 6.6 7.292 
1.3 4.171 2.0 5.569 6.7 7.342 
1.2 4.203 2.8 5.608 6.8 7.392 
1.1 4.235 2.9 5.647 6.9 7.442 
1.0° 4.267 3.0 5.687 7.0 7.492 
0.9 4.299 3.1 5.727 fee Be 7.544 
0.8 4.331 3.2 5.767 7.2 7.595 
0.7 4.364 3.3 5.807 7.3 7.647 
0.6 4.397 3 4 5.848 7.4 7.699 
0.5 4.430 3.5 5.889 7.5 7. (1 
0.4 4.463 3.6 5.930 7.6 7.804 
0.3 4.497 3.7 5.972 a4 7.857 
0.2° 4.531 3.8 6.014 7.8 7.910 
0.1 4.565 3.9 6.055 7.9 7.964 
0.0 4.600 4.0 6.097 8.0 8.017 
+0.1 4.633 4.1 6.140 8.1 8.072 
0.2 4.667 4.2 6.183 8.2 8.126 
0.3 4.700 4.3 6.226 8.3 8.181 
0.4 - 4.733 4.4 6.270 8.4 8.236 
0.5 4.767 4.5 6.313 8.5 8.291 
0.6 4.801 4.6 6.357 8.6 8.347 
0.7 4.836 4.7 6.401 Pe 8.404 
0.8 4.871 4.8 6.445 8.8 8.461 
0.9 4.905 4.9 6.490 8.9 8.517 
1.0 4.940 5.0 6.534 9.0 8.574 
1.1 4.975 5.1 6.580 9.1 8.632 
1.2 5.011 §:2 6.625 9.2 8.690 
1.3 5.047 5.3 6.671 9.3 8.748 
1.4 5.082 5.4 6.717 9.4 8.807 
5 ee 5.118 5.5 6.763 9.5 8.865 
1.6 5.155 5.6 6.810 9.6 8.925 
ce 5.191 5.7 6.857 9.7 8.985 
1.8 §.228 5.8 6.904 9.8 9.045 
1.9 5.265 5.9 6.951 9.9 9.105 


























TENSION OF WATER VAPOR. 843 


TENSION OF WATER VAPOR.—Continued. 























i zs sion i 
Pt {waite eee | uiimen. oe | Malteeren 
+10.0 9.165 +14.0 11.908 +18.0 15.357 
10.1 9.227 14.1 11.986 18.1 15.454 
10.2 9.238 14.2 12.064 18.2 15.552 
10.3 9.350 14.3 12.142 18.3 15.650 
10.4 ~ 9.412 14.4 12.220 18.4 15.747 
10.5 9.474 14.5 12.298 18.5 15.845 
10.6 9-537 14.6 12.378 18.6 15.945 
10.7 9.601 14.7 12.45S 18.7 16.045 
10.8 9.665 14.8 12.538 18.8 16.145 
10.9 9.728 14.9 12.619 18.9 16.246 
11.0 9.792 15.0 12.699 19.0 16.346 
14 9.857 15.1 12.781 19.1 16.449 
11.2 9.923 15.2 12.864 19.2 16.552 
11.3 ~ 9.989 15.3 12.947 19.3 16.655 
11.4 10.054 15.4 13.029 19.4 16.758 
11.5 10.120 “15.5 13.112 19.5 16.861 
11.6 10.187 15.6 13.197 19.6 16.967 
4457 10.255 15.7 13.281 19.7 17.073 
11.8 10.322 15.8 13.366 19.8 17.179 
11.9 10.389 15.9 13.451 19.9 17.285 
12.0 10.457 16.0 13.536 20.0 17.391 
12.1 10.526 16.1 13.623 20.1 17.500 
12.2 10.596 16.2 13.710 20.2 17.608 
12.3 10.665 16.3 13.797 20.3 272717 
12.4 10.734 16.4 13.885 20.4 17.826 
-12.5 10.894 16.5 13.972 20.5 17.935 
12.6 10.875 16.6 14.062 20.6 18.047 
12.7 10.947 16.7 14.151 20.7 18.159 
12.8 11.019 16.8 14.241 20.8 18.271 
12.9 11.090 16.9 14.331 20.9 18.383 
13.0 11.162 17.0 14.42) 21.0 18.495 
13.1 11.235 i 14.513 2.1 18.610 
13.2 11.309 4252 14.605 21.2 18.724 
13.3 11.383 17.3 14.697 21.3 18.839 
13.4 11.456 17.4 14.790 21.4 18.954 
13.5 11.530 17.5 14.882 23.5 19.069 
13.6 11.605 17.6 14.977 21.6 19.187 
13.7 11° 681 sh Ser 15.072 $1.7 19.305 
13.8 11.757 17.8 15.167 21.8 19.423 
13.9 11.832 17.9 15.262 21.9 19.541 











844 


APPENDIX. 


TENSION OF WATER VAPOR.—Continued. 











Degrees Tension in Degrees, Tension in Degrees, Tension in 
or Millimeters. C. Millimeters. Cc. Millimeters. 

+ 22.0 19.659 + 26.0 24.988 +30.0 31.548 
234 19.780 26.1 25.138 30.1 31.729 
22.2 19.901 26.2 25.288 30.2 31.911 
22:3 20.022 26.3 * 25.438 30.3 32.094 
22.4 20.143 26.4 25.588 30.4 32.278 
22.5. 20. 265 26.5 25.738 30.5 32.463 
22.6 20.389 26.6 25.891 30.6 32.650 
22.7 20.514 26.7 26.045 30.7 32.837 
22.8 20.639 26.8 26.198 30.8 33 .026 
22.9 20. 763 26.9 26.351 30.9 33.215 
23.0 20.888 27.0 26.505 31.0 33.405 
234 21.016 27.1 26 . 663 31.1 33 .596 
23.2 21.144 27.2 26.820 31°52 33 . 787 
23.3 21.272 27:3 26.978 31.3 33 .980 
23.4 21.400 27.4 27.136 31.4 34.174 
23.5 21.528 27.5 27 . 294 31:5 34.368 
23 .6 21.659 27.6 27.455 31.6 34.564 
23.7 21.790 ry eg 27.617 Shia 34.761 
23.8 21.921 27.8 27.778 31.8 34.959 
23.9 22.053 27.9 27.939 31.9 35.159 
24.0 22.184 28.0 28.101 32.0 35.359 
24.1 22.319 28.1 28 . 267 32.1 35.559 
24.2 22.453 28.2 28 .433 32.2 35.760 
24.3 22.588 28.3 28 . 599 32:3 35.962 
24.4 22.723 28.4 28.765 32.4 36.165 
24.5 22.858 28.5 28.931 32.5 36.370 
24.6 22.996 28.6 29.101 32.6 36.576 
24.7 23.135 28.7 29.271 32.7 36.783 
24.8 23.273 28.8 29.441 32.8 36.991 
24.9 23.411 28.9 29.612 32.9 37.200 
25.0 23.550 29.0 29 .782 33.0 37.410 
25.1 23 .692 29.1 29 .956 33.1 37.621 
25:2 23 . 834 29.2 30.131 33.2 37.832 
25.3 23 .976 29.3 30.305 33.3 38.045 
25.4 24.119 29.4 30.479 33.4 38.258 
25.5 24.261 29.5 30.654 33.5 38.473 
25.6 24.406 29.6 30.833 33.6 38.689 
25.7 24.552 29.7 31.011 33.7 38.906 
25.8 24.697 29.8 31.190 33.8 39.124 
25.9 24.842 29.9 31.369 33.9 39.344 
































HEAT OF COMBUSTION OF GAS. 845 
TENSION OF WATER VAPOR.—Continued. 

i : 
Degrees, Tension, Degrees, Tension, Degrees, Tension, 

Cc. imeters. Cc; Millimeters. C. i eters. 
+34.0 39.565 +34.4 40.455 +34.8 41.364 
34.1 39.786 34.5 40.680 34.9 41.595 

34.2 40.007 34.6 40 .907 

34.3 40.230 34.7 41.135 35.0 41.827 


























HEATS OF COMBUSTION OF 1 LITER OF GAS MEASURED AT 0° 
AND 760 MM. BAROMETRIC PRESSURE. 














Referred to 
Gin Weight of One Z 
Liter. Gaseous Water Liquid Water 
Calories. Calories. 

Carbon monoxide.......... 1.25016 3,034 3,034 
Hiydrogen. . 0... ioc: sue 0.09004 2,595 3,077 
MENU i ae es oe ae 0.71488 8,505 9,469 
BANG i a va esc es as 1.25899 14,018 14,989 
PrOOy te 25 cos fect ees 1.93660 21,226 22,720 
Benzene-gas.:.... oe. se 3.48428 (?) 33,750 | (?) 35,198 
PNG ase a Sew 8 1.18080 13,582 14,073 
Generator-gas............. -—— about 900 | about 1,000 
Water-gas.......... Sire oe, — ‘f 3,386 33 Be AOO 
POW B08 6 ce — Ss 1,400 a 
Tlluminating-gas........... ao ee 5,000 hy 5,500 














The values in the above table are based upon Thomsen’s measurements 
and only in the case of benzene is the theoretical density used.* 
* Julius Thomsen, Thermochem. Untersuchungen (1882), Vol. II, pp. 56, 


85, 107, and Vol. IV, p. 254. 


CO + O =CO, +67,960 cals. 

H, + O =H,0 +68,357 cals. 

CH, +40 =2H,0+ CO,+211,930 cals. 
C,H,+60 =2H,0+2C0,+333,350 cals. 
C,H,+90 =3H,0+3CO,+ 492,740 cals. 
C,H, +150 =3H,0 + 6CO, + 787,950 cals. 
C,H,+50 = H,O+2C0,+310,450 cals. 


se 
Figs necesita 
post Rate iat 





es 


Oe 
» 








“TABLES FOR CALCULATING ANALYSES. 
DIRECTIONS FOR USING THE TABLES. 


Assume that it is desired to find the per cent. of arsenic present 
in an ore. For. this purpose a gms. of the ore are taken for the 
analysis and the arsenic is determined as Mg,As,O,; the ignited 
precipitate weighed p gms. The calculation of the arsenic ‘is 
made as follows: 


Mg,As,O,:As,=p:s 


Ze As, 
~ Mg,As,0O, 
and in per cent. 
As, 
a: <p=100:% 
Mg,As,O, 
_ 100-As, (p 
Mg,As,O,° a 


The last equation is solved for 2, after the proper values for 
p, a, etc., have been inserted, with the aid of logarithms. From 
the tables, the logarithm of os a is taken, to it the logarithm 
of p is added, and from the sum the logarithm of a is deducted. 
The number corresponding to this difference (found in the table 
of antilogarithms) represents the desired result in per cent. 

Example.—0.5 gm. of mispickel gave 0.4761 gm. Mg,As,O,. 
What is the per cent. of arsenic in the ore? 

In the following table under the heading Sought we find 
As, and under Found we look for Mg,As,.O,, the form in which 








* The tables in this book can be purchased separately in flexible cloth 
binding. Price, thirty-five cents. 
847 


848 TABLES FOR CALCULATION OF THE ANALYSIS. 


the arsenic was determined, and then from the column headed 
Log we take the logarithm of this factor multiplied by 100; 


log. factorxX 100= 1.6837 
+log. 0.4761 (p) =9.6777—10 


1.3614 
—log.0.5 (q)=9.6990—10 


1.6624 
Number = 45.96 per cent. arsenic. 








For most analytical computations the accompanying four- 
place logarithms are sufficiently accurate; where greater accuracy 
is desired tables of five- or even seven-place logarithms may be 
used. : 


INTERNATIONAL ATOMIC WEIGHTS. 


International Atomic Weights, 1915. 





Atomic 








Symbol.) Weight. Symbol.| Weight. 
Aluminium........ Al py ae | Molybdenum..... Mo 96.0 
Antimony......... Sb | 120.2 Neodymium...... Nd | 144.3 
SOR eed A 20.88 | Néon... 0 osc ee Ne 20.2 
APSORIO 26 Sn As (4,96: 4- Mieke): 4. cick ca: Ni 58.68 
Bara se Ba | 137.37 }| Nitrogen......... 1 N 14.01 
Pie es Bi 208 .0 amit rs, S: Os 190.9 
Mee es B 11.0 SURV WEI i ees sk: O 16.00 
merooune.. . so. Br 79.92 || Palladium........ Pd 106.7 
Cadmium. ........ Cd | 112.40 || Phosphorus....... rE 31.04 
Caeatiim. . 26éee: Cs 132.824): Piatinum: 3)))55 oS. Pt 195.2 
BOTS Ts ESE ie Ca 40.07 || Potassium........ K 39.10 
Ce 8 See ei C 12.00 || Praseodymium....} Pr 140.6 
os Fab grape Oe Dass Ce 140 .25:1) Radium... s...5. 6. Ra | 226.4 
Chiang: he: Cl 35.46. 1) Rhodium. . 33.05 2. Rh | 102.9 
Chromium......... Cr 52.0 Rubidium........ Rb 85.45 
CORR 6 is cc. Co 58.97 || Ruthenium....... Ru | 101.7 
Columbium........ » Cb 93.5 Samarium........ Sa 150.4 
Copper. ise. Cu 63.57 || Scandium........ Se 44.1 
Dysprosium....... Dy | 162.5 Selenium......... Se 79.2 
Me ics Er 167.7 EURO Ge ooh ent a Si 28.3 
Europium:.......,. Tu | 152.0 PUIVOR, <3. ease - 8 Ag 107 .88 
Mhuormess... foe... 0 19.0 SodwmM; . ves. . Na 23 .00 
Gadolinium........ Ge tbr38 Strontium........ Sr 87.63 
QemmUMt cs ees: Ga 69.9 PIMDNUS. ocak s < v S 32.07 
Germanium........ Ge 72.5 PARAM ES ses. G. Ta 181.5 
Glucinum *........ Gl 9.1 Tellurium........ Te 3275 
Ce ie er Sy 2 i Perot oes Tb | 159.2 
Helium :. isco. 6 He 3.99 || Thallium......... Tl 204.0 
Hydrogen. ........ H 1.008] Thorium......... Th | 232.4 
Pee Ce In 114.8 ‘Phaltum. 2305.4. Tm | 168.5 
|” SRR saiateiemtias fe I i Ro De RS go Aaa ee ae ae ae Sn 119.0 
Pee, 2. oes Ir 193.1 ‘EMORBUM es o0%.0. Ti 48.1 
i gies geese oor cea ee Fe 55.84 || Tungsten......... WwW 184.0 
Krypton. «0 ee ec. Kr 82.92 || Uranium........ Foe 238.5 
Lanthanum........ La 139.0 Vanadium........ V 51.0 
OY ee ee Pb -+ 207.10.}| Xenon. oo ec eres Xe .} 130.2 
Es eee 5 FO. Li 6.94 || Ytterbium........ 
Tatecttin. 220. Lu | 174.0 (Neoytterbium). | Yb | 172.0 
Magnesium........ Mg |? 24.32 |) ° Vttitain.. oes ees. Yt 89.0 
Manganese........ Mn | 54.93 || Zinc.............) Zn 65.37 | 

Hg | 200.6 Zirconium........ Zr 90.6 


MPOUEY oe ss es 




















* Also called Beryllium, Be. 


TABLE OF CHEMICAL FACTORS. 







































































Sought Found. Factor. Log.* Sought Found Factor. Log. 
Ag AgCl 0.75262) 1.87658); BaO BaSO, 0.65700} 1.81757 
AgBr 0.57444! 1.75925 BaCrO, 0.60532) 1.78199 
Agl 0.45946; 1.66224 BaSif, 0.54840) 1.73909 
Ag,O Ag(Cl 0.80843; 1.90764 Bi Bi,O, 0.89654! 1.95258 
BiAsO, 0.59942! 1.77773 
Al Al,O, 0.53033|} 1.72455 BeO, Bi 1.1154 | 2.04743 
AIPO, 0.22195) 1.34625 
Br Ag 0.74082! 1.86971 
ALO, AIPO, 0.41851} 1.62171 AgBr 0.42556) 1.62896 
AgCl 0.55755) 1.74629 
As AsS3 0.60911} 1.78470 
AsoSs 0.48319) 1.68412 C CO. 0.27273) 1.43573 
MgeAseO7 | 0.48269) 1.68371 CO; CC. 1.3636 | 2.13470 
MgeP:O7 | 0.67313) 1.82810 
Ca CaO 0:71464) 1.85409 
AsoCs As3Ss 0.80405; 1.90528 CaCQ; 0.40045! 1.60252 
AsoSs 0.63783) 1.80471 CaSO, 0.29440) 1.46893 
MgeAs,O;7 | 0.63724) 1.80430 CaF; 0.51326 1.71034 
CaO CaCQ; 0.56031 1.74843 
AsQ3 As.$3 0.99915) 1.99963 CaSQ, 0.41195 1.61485 
AsoSs 0.79260) 1.89905 CaF, 0.71820) 1.85625 
MgeAsQ; | 0.79186) 1.89865 
Mg2P20, | 1.1042 | 2.04304 Cd Cds 0.77801; 1.89099 
CdO 0.87539) 1.94220 
As,Ox AsoS3 0.93414) 1.97041 CdSO,4 0.53919) 1.73174 
AsoSs 0.74103). 1.86984 
MgeAs,O; | 0.74034) -1.86943 CdO Cds 0.88877, 1.94879 
MgeP2C7 | 1.0323 | 2.01382 Cd 1.14235} 2.05780 
CdSO, 0.61591, 1.78952 
AsO, AseSs3 1.1292 | 2.05276 
AsoSs 0.89574) 1.95218|}| CdS Cd 1.2853 | 2.10901 
MgeAs,O7 | 0.89490) 1.95178 CdO 1.1252 1) 2:O5%gE 
-MgeP2Q, | 1.2477 | 2.09616 CdSO, 0.69300; 1.84073 
B B,O, 0.31428} 1.49732 Cl AgCl 0.24738) 1.39337 
BO, B,O, 1.2286 | 2.08940 Ag 0.32870} 1.51680 
BO, B,O, 1.6857 | 2.22678 
B,O, B,O, 1.1143 | 2.04700)| ClIH AgCl 0.25442) 1.40555 
Ag 0.33804) 1.52898 
Ba BaSO, 0.58846) 1.76973 
BaCrO, 0.54217) 1.73414|| ClO, AgCl 0.58225) 1.76511 
BaSiF, 0.49119) 1.69125 KCl 1.1194 | 2.04897 
NaCl 1.4276 | 2.15462 






































* In this column the logarithm of the factor multiplied by 100 is given. 
The logarithms are given to five decimal places, but it should be borne in 


mind that the fourth decimal place is in most cases doubtful. 


Four-place 


logarithms are accurate enough for nearly all chemical analyses. The 
atomic weights for 1915 are used in these tables. 








































































































MneP207 








TABLE OF CHEMICAL FACTORS. 851 
Sought. Found Factor Log. Sought. Found Factor. Log. 
ClO,K| AgCl | 0.85503] 1.93198] SiFe | “CaF. | 0.60742|1.7834y 
KCl | 1.6438 | 2.21584 
Fe Fe,0; | 0.69940/1.84472 
ClO,Nal| AgCl | 0.74271) 1.87082|| FeO | FeO, | 0.89980|1.954is 
NaCl | 1.8211 | 2.26033 
) H H.O 0.111901 .04883 
ClO, | Agcl | 0.69388] 1.84128||—— 
| KCl | 1.3339 | 2.12514|| Hg | HeCk | 0.84979|1.92931 
NaCl | 1.7013 | 2.23079 Hes 0.86216|1.93559 
ClO,K| AgCl | 0.96665] 1.98527]| I Agl 0.54055 |1.73283 
KCl | 1.8584 | 2.26913 Pdi. | 0.70406|1.84761 
AgCl | 0.88545|1.94716 
ClO,Nal AgCl. | 0.85433] 1.93163 
NaCl | 2.0948 | 2.32114|| kK KCl 0.52441/1.71987 
K:SO, | 0.44873|1.65199 
ON | AgCN | 0.19426) 1.28839 KCIO, | 0.28219|1.45054 
Ag 0.24110) 1.38220 K2PtCl. | 0.16085|1.20643 
Pt 0.400611 60273 
CNS | AgONS | 0.34996] 1.54402 
CuCNS | 0.47744] 1.678911 KC] | - K.SO, | 0.85569/1.93231 
BaSO, | 0.24880] 1.39585 KCIO, | 0.53811{1.73087 
K:PtCl, | 0.30673|1.48676 
HONS| AgCNS | 0.35604] 1.55150 Pt 07639711 .88308 
CuCNS | 0.48572| 1.68639 
BaSO, | 0.25312] 1.40332! K,0 KCl 0.63170/1.80052 
K:SO, | 0.54054|1.73283 
Co | CoSO, | 0.38035] 1.58019 KCIO, | 0.33993/1.53138 
~ CoO Co 1.2713 | 2.10426 K2PtCl, | 0.19376|1.28727 
CoSO, | 0.48355] 1.68444 Pt 0. 4825811 67357 
Cr Cr,O, | 0.68421) 1.83519] Li LiSO, | 0.12624!1.10119 
PbCrO, | 0.16094] 1.20667 LiCl 0. 16368/1 .21399 
BaCrO, | 0.20523] 1.31248 | 
Li,O LiCl | 0.35236!1.54699 
Cr,0,| PbCrO, | 0.23522] 1.37148 LisSO, | 0.27176/1.43419 
BaCrO, | 0.29996] 1.47706 
Mg MgO | 0.60318/1.78044 
Cro, | _Cr,0, | 1.3158 | 2.11919 MgSO, | 0.20201|1.30537 
PbCrO, | 0.30950] 1.49066 MgP;0; | 0.21839|1.33924 
BaCrO, | 0.39468) 1.59625 | 
3 MgO | MgSO, | 0.33491/1.52493 
Cu CuO | 0.79892) 1.90250 Mg:P,07: | 0.36207|1.55879 
Cu,8 | 0.79857] 1.90231 | 
CuCNS | 0.52256] 1.71814|| Mn | MnSO, | 0.36378|1.56083 
: MnS __ | 0.63138/1.80029 
Cu0 | CuS_ | 0.99956] 1.99981 MnO, | 0.72030/1.85751- 
CuCNS | 0.65408] 1.81563 MnP:0; | 0.386721 .58740 
Cu 1.2517 | 2.09750 
MnO} MnSO, | 0.46973/1.67185 
F CaF, | 0.48662] 1.68719 MnS___| 0.81529/1.91131 
CaSO, | 0.27908| 1.44574 Mn;0, | 0.93011/1.96853 
0.49961/1.69863- 





852 


TABLE OF CHEMICAL FACTORS. 





_ Found. 
























































Sought. Found. Factor. Log. Sought Factor. Log. 
Mo MoO; — |_0.66667|1.82391)]| NO» NO 1.53832 |2.18559 
N NHs 0.82247|1 .91512]| N2Os NO 1.2666 |2.10263 
NH.Cl 0 -26187|1.41808 
(N H4)ePtCl¢|-0.06310/0. 80005 * MgeP2O7 |0.27874 |1.44519 
Pt 0.14854/1.15699 P,O;,24Mo0;/0.01725 |0.23688 
—|| w  |CNH4)sPO,, 
Na NaCl 0..39343/1 . 59487 12Mo00;/0.01654 |0.21842 
NazSO, 0.32378)1.51026 
PO, Mg.P20, {0.85345 |1.93118 
Na,O NaCl 0.53028)1.72450 P,05,24Mo03;/0.05283 |0.72287 
12Mo0;/0.05063 |0.70440 
NH; NH,Cl 0.31821}1 . 50271 
(NH,4)ePtCig} 0.07670/0.88483)]| P.O; MgeP,O7 |0.63793 |1.80477 
Pt 0.17449)1 .24177 P.O;,24Mo0;/0.03947 |0.59628 
(NH4);PO,, 
NH, NH; 1.0592 |2.02497 12Mo00O;|9.03785 |0.57800 
NH.Cl 0.33723)1.52793 
(NH,4)2PtCle| 0.08125/0.90985}| Pb PbO 0.92828 |1.96768 
Pt 0.18484)1 . 26679 PbO, 0.86616 |1.93760 
PbS 0.86591 |1.93747 
NHCl NH; 3.1409 |2.49705 PbSO, 0.68312 |1.83449 
(NH4)oPtCle| 0.24097)1.388196 PbCrO, |0.64098 |1.80684 
Pt 0.54815}1 . 73890 PbCl, 0.74491 |1.87210 
Ni NiO 0.78576/1 .89529]| PbO PbO, 0.93308 |1. 
NiCsHy4N,0,) 0.20316/1.30785 PbS 0.93281 {1.96979 
PbSO, 0.73589 |1.86681 
NiO Ni 1.2727 |2.10471 PbCrO, {0.69050 |1.83916 
NiCsHy4N,0,)} 0.25856/1. 41256 PbCl. 0.80246 |1.90442 
NO; NO 2.0663 |2.31520 Ss BaSO, (0.13738 |1.13792 
NH; 3.6404 |2.56115}| S$O2 BaSO, |0.27446 |1.43848 
NH,Cl 1.1591 |1.06411)) SOs. BaSO, {0.34300 |1.53530 
(NH,),P{Cl, 0.27930)1.44607)| SO, BaSO, 0.41154 {1.614417 
Pt 0.63535}1 .80301}| SO,H» BaSO, (0.42018 |1.62343 
CooHiz7N;O0;3 | 0.16528]1 .21823)}| HS BaSO,  |0.14602 |1.16443 
FeS, 4  |0.25700 |1. 
NO;3H NO 2.0999. |2.32220 
NH; 3.6995 {2.56815}; Sb Sb.0, |0.78975 |1.89749 
NH,Cl 1.1779 |2.07111 Sb.S8; 0.71418 {1.85381 
(NHY:PiCh 0.28385}1 . 45309 
0.64570}1.81003}, Si SiO. 0.46931 |1.67147 
CuHicO, 0. 16797)1 .22523)| SiO; SiO. 1.2653 |2.10221 
N20; NO 1.7997 |2.25521]| Sn SnO, 0.78808 |1.89657 
NH; 3.1707 |2.50116]} SnO, Sn 1.2689 /|2.10343 
NH,Cl 1.0095 |2.00412 
(NH,):PtCh 0.24327/1.38608)| Sr SrO 0.84560 |1.92717 
Pt 0.55338)}1. 74302 SrCO; 0.59358 |1.77348 
C.oHyzN;0;3 | 0.14396}1 . 15824 SrSO,; 0.47703 |1.67854 
Sr(NO;3)2 |0.41403 {1.61703 
































TABLE OF CHEMICAL. FACTORS. 


853 























Sought. Found. Factor. Log. Sought. | Found. Factor. Log. 

SrO SrCO; 0.70196/1.84631 W WO; 0.79310/1.89933 

SrSO, 0.56413/1.75138); WQ; W 1.2609 |2. 10067 
Sr(NO3)e | 0.48963!1.68987 

Zn ZnO 0 .80337/1.90491 

Th ' ThO, 0.87898) 1.94398 ZnS {) .67087/1.82664 

: ZnN H,4PO, | 0.36632/1.56386 

Ti TiO. 0.60051/1.77852 Zn2P2C7 0.42891)1.63237 

U U;Cs 0.84824/1.92852)| ZnO’ | - ZnS 0.83508) 1.92173 

UO, 0.88170)1 .94532 ZnNH,PO, | 0.45598/1.65895 

(UCs)2P207 | 0.66703/1.8241£ Zn2P.0; | 0.53390|1.72746 

Vv V205 0.56044|1.74853)| Zr ZrC2 0.73899) 1.86864 






































; 854 LOGARITHMS. 
Zz PROPORTIONAL PARTS. 
B10 Dace 4)5/6}7}81|9 
pis 
ZF 1/2|3|4]5/6 | 7|8|9 
10 |0000/0043) 0086 0170]0212/0253/0294/0334'0374] 4| s |12117]21/25 29'33/37 
11 |0414/0453)0492 0569|0607|0645|0682|0719 0755} 4 | 8 |11/15]19|23 26.30.34 
12 |0792:'0828| 0864 0934/0969) 1004/1038] 1072 1106} 3| 7 |10'14]17/21 24/28/31 
13 |1139)1173/1206 12711303) 1335)1367/1399 1430] 3 | 6 |10/13]16!19 23/26! 29 
14 |1461)1492) 1523 1584]1614 ene 1703 1732] 3| 6| 9|12]15/18 21 24/27 
15 |1761/1790|1818 1875}1903) 1931|1959| 1987 2014} 3/6! 8!11114'17 20 22/25 
16 |2041|2068/ 2095 2148]2175| 2201 | 2227 |2253 | 2279] 3| 5} 8!11]13/16 18 21 24 
17 |2304!2330)| 2355 2405}2430)2455|2480| 2504/2529] 2/5! 7/10!12'15 17.20 22 
18 |2553/2577/ 2601 2648}2672/2695| 2718) 2742 2765) 2| 5) 7) 9}12 14116119 21, 
19 |2788|2310|2833| 2856 2878]2900|2923 |2945|2967 2989] 2 | 4| 7| 9[11/13'1618 20 
20 {3010/3032/3054 3096/3118|3139|3160|3181/3201] 2| 4! 6} s}11 ne 17 19 
21 |3222/3243/3263 3304)3324 3345/3365|3325 3404] 2/| 4| 6) 8/10 12/14 16 18 
22 13424'3444/3464 3502/3522|3541|3560|/3579 3598] 2| 4| 6] 811012 14/15 17 
23 13617'3636'3655 3692]3711|3729|3747|3766 3784] 2| 4! 6| 7! 9'11'13/15 17 
24 {3802/3820 3838 3874/3892) 3909/3927 |3945 3962] 2/4) 5| 71 9 ae 14 16 
25 13979'3997/4014 4048/4065/4082/4099/4116/4133] 2| 3! 5] 7] 9 10,12 ice 
26° |4150/4166/4183 4216]4232/4249|4265/4281|4298) 2| 3/ 5) 7] 8/10.11/13 15 
27 |4314/4330/4346 4378]4393| 4409 4425 4440 4456 2/3] 5| 6] 8| 9:11 13 14 
28 |4472|4487/4502 4533/4548 | 4564'!4579/4594 4609] 2/3] 5] 6} 8 9'11/12 14 
29 4624/4639) 4654 4683]4698| 4713 4728 sie ses 1| 3] 4] 6] 7 ” 1213 
30 |4771|4786|4800 4829 4843]|4857/4871/4886/4900] 1/3] 4) 6] 7/ 9:10 Pe 
31 |4914/4928/ 4942/4955 4969/4983 |4997|5011)5024/5038) 1| 3) 4) 6) 7 g'10/11 12 
32 15051/5065) 5079) 5092 5105/5119/5132|5145|5159|5172) 1|; 3| 4) 5) 7| 8) 9 1112 
33 15185/5198/5211|5224!5237/5250|5263|5276|5289/5302) 1| 3| 4) 5] 6 8) 9/10 12 
34 |5315|5328/5340 5366/5378) 5391|5403/5416|5428] 1} 3| 4) 5] 6) 8| 91011 
35 15441/5453/5465 5490/5502|5514/5527/5539|5551) 1| 2| 4| 5] 6) 7| 91011 
36 |5563|5575 5587 5611}5623 5635) 5647)|5658/5670) 1 | 2| 4) 5) 6) 7| 8,10 11 
37 15632|5694'5705 5729]5740|5752)|5763|5775|5786) 1| 2! 3) 5) 6| 7) 8 9,10 
38 |5798| 5809/5821 5843/5855) 5866) 5877|5888|5899] 1} 2| 3) 5] 6| 7| 8) 910 
39 [5911/5922/5933 595515966/5977| 5988 |5999/6010} 1| 2/| 3) 41 5) 7} 8| 910 
40 |6021|6031/6042 6064|6075|6085|6096/6107|6117] 1) 2! 3) 4] 5) 6| 8} 910 
41 |6128)6138/6149 6170]6180/6191/6201/6212/6222] 1/2) 3) 41 5) 6| 7) 8 9 
42 |6232/6243|6253 627416284/6294|6304'6314/6325] 1/2] 3) 41 5!) 6| 7) 8| 9 
43 16335/6345)6355 6375]6385'6395 6405/6415/6425] 1/2| 3) 41 5) 6' 7) 8| 9 
44 |6435/6444/6454 647416484/6493|6503/6513/6522] 1| 2) 3) 4] 5) 6| 7| 8| 9 
45 16532/6542/6551 657116580/6590/ 6599/6609/6618] 1/2) 3) 4] 5) 6| 7| 8| 9 
46 |6628|6637)|6646 666516675/6684|6693/6702/6712] 1} 2| 3) 41 5) 6| 7| 7| S$ 
47 16721/6730/6739 6758|6767'6776|6785/6794/6803] 1| 2) 3) 41 5) 5) 6) 7! 3 
48 |6812/6821/6830 684816857/6866|6875/6884/6893] 1| 2| 3] 4] 4) 5] 6 7/3 
49 |6902/6911/6920 6937|6946|6955|6964/6972/6981] 1| 2| 3) 4] 4) 5) 6) 7| 8 
50 |6990|6998)7007 702417033) 7042) 7050/7059) 7067] 1| 2| 3] 3} 4! 5] 6) 7] c 
51 17076|7084|7093 7110]7118/7126|7135)|7143)}7152] 1} 2| 3} 3] 4| 5] 6| 7/ § 
52 |7160'7168/7177 7193]7202' 7210} 7218) 7226/7235} 1| 2} 2| 3} 4| 5] 6] 7) 7 
53 |7243|7251|7259 7275\7284!7292!7300)|7308/7316] 1} 2] 2} 3} 4| 5| 6) 6| 7 
§4 |7324)|7332)7340 735617364) 7372! 7380|7388/7396] 1| 2) 2} 3] 4) 5) 6 6 7 








—_ ————_— 













































































| 





LOGARITHMS. 


855 








7404 
7482 
7559 
7634 
7709 


7782 
7853 
7924 
7993 
8062 


8129 
8195 
8261 
8325 
8388 


8451 
8513 
8573 
8633 
8692 


8751 
8808 
8865 
8921 
8976 


9031 
9085 
9138 
9191 
9243 


9294 
9345 
9395 
9445 


QAO04 





vids 


9542 
9590 
9638 
9685 
9731 


9777 
9823 
9868 
9912 
9956 


7412 
7490 
7566 
7642 
7716 


7789 
7860 
7931 
8000 
8069 


8136 
8202 
8267 
8331 
8395 


8457 
8519 
8579 
8639 
8698 


8756 
8814 
8871 
8927 
8982 


9036 
9090 
9143 
9196 
9248 


9299 
9350 
9400 
9450 
9499 


9547 
9595 
9643 
9689 
9736 








9782 
9827 








8075, 


8142 
8209 
8274 
8338 
8401 


8463 
8525 
8585 
8645 
8704 


8762 
8820 
8876 
8232 
8987 


9042 
9096 
9149 
9201 
9253 


9304 
9355 


7419 
7497 
7574 
7649 
7723 


7796 
7868 
7938 
8007 


9405 
9455 
9504 


9552 
9600 
9647 
9694 
9741 


9786 
9832 





8014 
8082 


8149 
8215 
8280 
8344 
8407 


8470 
8531 
8591 
8651 
8710 


8768 
8825 
8882 
8938 
8993 


9047 
9101 
9154 
9206 
9258 


9309 
9360 


7427 
7505 
7582 
7657 
7731 


7803 
7875 
7945 


9410 
9460 
9509 


9557 
9605 
9652 
9699 
9745 


9791! 
9836 





9872 9877 9881! 

9917 9921 9926 9930 

9961 9965 eaedina 
| * 


8021 
8089 


8156 
8222 
8287 
8351 
8414 


8476 
8537 
8597 
8657 
8716 


8774 
8831 
8887 
8943 
8993 


9053 
9106 
9159 
9212 
9263 


9315 
9365 
9415 
9465 
9513 


7435 
7513 
7589 
7664 
7738 


7810 
7882 
7952 


9562 
9609 
9657 
9703 
9750 


9795 
9841 
9886 


8162 8169 
8228 8235, 
8293 8299 8306 
8357 8363 
8420 8426 


8482 8488 
8543) 
8603 
8663 
8722 


8779 
8837 
8893 
8949 
9004 


9058 
9112 
9165 
9217 
9269 


9320 
9370 
9420 


7443 
7520 
7597 
7672 
7745 


7818 
7889 
7959 
8028 
8096 


7451 
7528 
7604 
7679 
7752 


7825 
7896 
7966 
8035) 
8102 





7459 
7536 
7612 
7686 
7760 


7832 
7903 
7973 
8041 
8109 


8176 





8549, 
8609 
8669 
8727 


8785, 
8842, 
8899 
8954, 
9009 


9063 
9117 
9170 
9222 
9274 


9325, 
9375, 
9425 
9474 
9523 


9571 
9619 
9666 
9713 





9469 
9518 


9566 
9614 
9661 
9708 


8241 


8370 
8432 


8494 
8555 
8615 
8675 
8733 


8791 
8848 
'8904 
8960 
9015 


9069 
9122 
9175 
9227 
9279 


9330 
9380 
9430 
9479 
9528 


9576 
9624 
9671 
9717 





9754 


9800 
9845, 


9759 








9763 


$312 
8376 
8439 


8500 
8561 
8621 
8631 
8739 


8797 
8854 
8910 
8965 8971 
9020 9025 


7466 7474 
7543 7551 
7619 7627 
7694 7701 
7767|7774 


7839 | 7846 
7910/7917 
7980|7987 
8048/8055 
8116/8122 


8182 
8248 





8189 
8254 
8319 
8382 
8445 


8506 
8567 











8915 


| 
| 
| 


90749079 
9128/9133 


91809186 
9232 9238 
92849289 


9335 9340 
9385 9390 
9435 9440 
9484 9489 
got eem 
9531 9586 
9628 9633 
9675|9680 
9722|'9727 
9768/9773 





9805 9809/9814 
9850. 98549859 
9890 9894 9899 9903 
9934 9939 9943 9948 
9978 au neal ate 


9818 
9863 
9908 
9952 
9996 





PROPORTIONAL Parts. 





~ 
oO 








H 
i 























Nvyunyn 
ww ww ow 

Ake RD 

ree oOo | O 
Aoaaaca | «@ 
AAAAM]!] w 
Yarra [oe 


a> 
PDXRXXN 




















ll et ee 
SS ee RS 
LO Od Wo we) 
Www ww 
Www kh 
PRP Rh 
anna n 
Or Or & 





ell etl 
eS et ee pe 
NNN bN bd 
doh ww w& 
WwW Ww Ww 
Ga Gar a Gare 
Hw He Or Or Or 
ora tr or or 
AAA>AN 


ee eR Re 
Se Re et Re 
OR oD OM ow) 
NbyhyN bd” 
wWwwww 
Lk Pp 
LPP PO 
or ov or Gen 
Anan a 





ee ee 
Lal ell oat ell a 
NnwWw WN bv 
NNwNNWNWb 
WwW w Ww 
wWwww w 
ee 
ee PP Oo 
oor or or or 


a 
ee eR 
to wb Wb bs to 
toby Ww bd bo 
wwww ow 
ww www 
ee oR 
hh RP 
aoanwaa 


ooo fe 
ee 
Hee bo bo 
Nw h bw tb 
NNN Www 
wwwww 
www er »& 
Pe Pe 
we wm HB OT Or 


oooco 
Se 
el ee od 
NNNN WY 
NwN hw NY 
wWwwwww 
me He ee 
Pe 














8:03: o'O 
it ht 
ee i 
ww Ww WN b& 
or > > 
pp > 














NNN Ww WN 








856 


ANTILOGARITHMS., 





| Logarithms. 





PROPORTIONAL PARTS 





= 
.) 
ow 
on 
or 
for) 
a 
@o 





1000 
1023 
1047 
1072 
1096 


1122 
1148 
1175 
1202 
1230 


1259 
1288 
1318 
1349 
1380 


1413 
1445 
1479 
1514 
1549 


1585 
1622 
1660 
1698 
1738 


1778 
1820 
1862 
1905 
1950 


1995 
2042 
2089 
2138 
2188 


2239 
2291 
2344 
2399 


1002|1005 1007 
1026'1028 1030 
1050 1052 1054 
1074 1076 1079 
1099 1102 1104 


1125 1127,1130 
1151 1153 1156 
1178 1180 1183 
1205 1208/1211 
se weal 
1262 1265 1268 
1291 1294 1297 
1321|1324 1327 
1352 1355 1358 
1384/1387 1390 


1416 1419 1422 
1449 1452 1455 
1483 1486 1489, 
1517 1521 1524 
Ape Lab A 
1589 1592 1596 
1626 1629 1633 
1663 1667 1671 
1702 1706 1710, 
sia alae 1750 
1782 1786 1791 
1824 1828 1832 
1866 18711875, 


| 


1910 1914 1919, 








1009 
1033 
1057 
1081 
1107 


1132 
1159 
1186 
1213 
1242 


1271 
1300 
1330 
1361 
1393 


1426 
1459 
1493 
1528 
1563 


1600 
1637 
1675 
1714 
1754 


1795 
1837 
1879 
1923 


1954 


2000 
2046 
2094 
2143 
2193 


2244 
2296 
2350 
2404 


245512460 


2512/2518 
2570 2576 
2630 2636 


2692 
2754 


2818 
2884 
2951 
3020 
3090 


2698 
2761 


2825 
2891 
2958 
3027 
3007 








1959 1963 1968 


iu dood aise 
2051 2056, 2061 
2099 21042109 
2148) 2153|2158 
2198 2203 2208 


2249) 2254 
2301) 2307 
2355 2360 
2410/2415 


2259 


2366 
2421 


2312;2317 


2466 


2523 
2582 
2642 


2472 


2529 
2588 
2649 


2477 


2535 
2594 
2655 


2710 
2773 


2838 
2904 
2972 
3041 
3112 


2716 
2780 


2704 
2767 


2831 
2897 
2965 
3034 
3105 


2844 
2911 
2979 
3048 
3119 





1012 1014 
1035 1038 
1059 1062 
1084 1086 
possi, 
1135,1138 
1161/1164 
1189 1191 
1216 1219 
ed a 
1274 1276 
1303 1306 
1334 1337 
1365 1368 
1396 1400 


1429 1432 
1462 1466 
1496 1500 
1531/1535 
ile 


1603 1607 
1641 1644 
1679 1683 
1718/1722 
1758 1762 


1799'1803 
1841)1845 
1884) 1888 
1928} 1932 
1972|1977 


2018 2023 
2065 2070 
2113/2118 
21632168 
2213/2218 


2270 
2323 
2377 
2432 
2489 


2265 


2371 
2427 
2483 


2541 
2600 
2661 
2723 
2786 


2547 
2606 
2667 
2729 
2793 


2851/2858 
2917/2924 
2085) 2992 
3055 3062 

















1114 
i 





3126 3133 


1016 
1040 
1064 
1089 


1140 
1167 
1194 
1222 
1250 


1279 
1309 
1340 
1371 
1403 


1435 
1469 
1503 
1538 
1574 


1611 
1648 
1687 
1726 
1766 


1807 
1849 
1892 
1936 
1982 


2028 
2075 
2123 
2173 
2223 


2275 
2328 
2382 
2438 
2495 


2553 
2612 
2673 
2735 
2799 


2864 
2931 
2999 
3069 
3141 


1019 
1042 
1067 
1091 
1117 


1143 
1169 
1197 
1225 
1253 


1282 
1312 
1343 
1374 
1406 


1439 
1472 
1507 
1542 
1578 


1614/1618 
1652 1656 
1690 1694 
1730 1734 
17701774 


iy Dae 
1854 1858 
1897,1901 
1941 1945 
1986 1991 


2032 2037 
2080, 2034 
2128/2133 
2178/2183 
2228) 2234 


2286 
2339 
2393 
2449 
2506 


2564 
2624 
26-5 
2748 
2812 


2877 
2944 
3013 
3083 
3155 


1021 
1045 
1069 
1094 
1119 


1146 
1172 
1199 
1227 
1256 


1285 
1315 
1346 
1377 
1409 


1442 
1476 
1510 
1545 
1581 





2280 
2333 
2388 
2443 
2500 


2559 
2618 
2679 
2742 
2805 


2871 
2938 
3006 
3076 
3148 











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ANTILOGARITHMS. 


857 




















PROPORTIONAL PARTS. 





w 
oe 
on 
ir] 
2 
@ 





3162 
3236 3243 


3467/3475 


Sie 
3631/3639 
3715 3724 
3802/3811 





3981 
4074 
4169, 
4266, 
4365 


4467 
4571/4581 
4677/4688 
4786 4797 
eae 


5012 5023 
5129/5140 
5248|5260 
5370 5383 
5495 5508 


5623 5636, 
5754/5768 
5888 5902 
6026 6039 
6166/6180 


6310/6324 
6457|6471 
6607|6622 
6761|6776 
6918/6934 


7079|7096 
7244|7261 
7413/7430 
7586|7603 
7762/7780 


7943)7962 
8128/8147 
8318|8337 
8511/8531 
8710|8730 


8913/8933 
9120/9141 
9353/9354 
9550/9572 
9772\9795 




















3170 3177 


3311)3319 3327 
3388 3396 3404 
3483 


3565 
3648 
3733 
3819 
3890) 3899 3908 


3990 3999 
4083/4093 
41784188 
4276|4285 
4375 4385 


4477'4487 
4592 
4699 
4808 
4920 


5035 
5152 
5272 
5395 
5521 


5649 
5781 
5916 
6053 
6194 


6339 
6486 
6637 
6792 
6950 


7112 
7278 


3251 


7447 
7621 
7798 


7980 
8166 
8356 
8551 
8750 


8954 
9162 
9376 
9594 
9817 





3184 
3258 


3334 3342 
3412 3420 
3491 3499 


3573 3581 


3656 


3741 3750 
3828 3837 
3917 3926 


4009 4018 
4102 4111 
4198|4207 
4295 4305 
4395 4406 


4498|4508 
4603 4613 
4710.4721 
4819'4831 
4932 4943 


5047 5058 
5164 
5284 
5408 
5534 


5662 
5794 
5929 
6067 
6209 


6353 
6501 
6653 
6808 
6966 


7129 
7295 


7464 
7638 
7816 


7998 
8185 
8375 
8570 
8770 


8974 
9183 
9397 
9616 


3192 
3266 


3199 
3273 
3350 
3428 
3508 


3539 
3673 
3758 
3846 
3936 


4027 
4121 
4217 


3664 





4519 
4624 
4732 
4842 
4955 


5070 
5188 
5309 
5433 
5559 


5689 
5821 
5957 
6095 
6237 


6383 
6531 
6683 
6339 
6998 


7161 
7328 
7499 
7674 
7852 


8035 
8222 
8414 
8610 
8810 


5176 
5297 
5420 
5546 


5675 
5808 
5943 
6081 
6223 


6368 
6516 
6668 
6823 
6982 


7145 
7311 
7482 
7656 
7834 


8017 
$204 
8395 
8590 
8790 


8995/9016 
9204/9226 
9419'9441 
9638/9661 








9840 


9863/9886 





7015 


7178 
7345 


8054 
8241 
8433 


3206 


3281 3289 


3357 3365 


3436 
3516 


3597, 
3681 3690 
3767 3776 
3855 3864 
3945 3954 


4036 
4130 
4227 
4315 4325 
4416 4426 


4529 
4634 
4742 
4853 
4966 


5082 
5200 
5321 
5445 
5572 


5702 
5834 
5970 
6109 
6252 


6397 
6546 
6699 


6855 


7516 
7691 
7870 


8630 
8831 


9036 
9247 
9462 
9683 
9908 





3221'3228 
3296 3304 
3373 3381 
3451 3459 
ane eae 


3614 3622 
3698 3707 
3784 3793 
3873 3882 
3963 3972 


4046/4055 4064 
4140'4150 4159 
4236 4246 4256 
4335 4345 4355 
4436 4446 4457 


4539 4550 4560 
4645 4656 4667 
4753 4764 4775 
4864 4875 4887 
4977 pe en 
5093 5105 5117 
5212 5224 5236 
5333 5346 5358 
5458 5470 5483 
aie 5610 
5715 5728 5741 
5848 5861 5875 
5984 5998 6012 
6124 6138 6152 
ae 6295 
6412 6427 6442 
6561/6577 6592 
6714 6730 6745 
6871 6887 6902 
ee 7063 


7194 7211/7228 
7362 7379/7396 
7534 7551|7568 
7709 7727|7745 
7889 7907|7925 


ae 8110 
8299 


8260 8279 
8492 
8690 


8453'8472 
8650 8670 

8892 
9099 
9311 


8851/8872 
9528 


9057 9078 
9268 9290 

9750 
9977 


3214 


3443 
3524 


3606 





9484 9506 
9705 9727 


igen yaa 








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The Influence of Fine Grinding on Composition. 


THE rate at which a substance dissolves increases as the 
amount of surface exposed to the solvent is increased and for 
this reason solid substances always dissolve more quickly when 
reduced to a fine powder.. Moreover, when a material undergoes 
chemical attack an insoluble substance is often formed and, 
during the process of solution, the insoluble substance may form 
a protective coating over particles of material that have been 
unacted upon. This danger is diminished if the material is 
in the form of a fine powder. For these reasons the chemist 
usually prefers to grind a solid substance to an impalpable con- 
dition before attempting to analyze it. 

This practice, while desirable in most cases and absolutely 
necessary in others, is accompanied by certain disadvantages. 
If the material is hard there is always some contamination 
from the material of which the grinding apparatus is constructed. 
Thus when the sample is ground in a steel mortar or in a steel 
ball-mill, it will be contaminated with a little iron and if ground 
“in an agate mortar with a little silica.t Again, if the sample 
readily undergoes slight decomposition, such a chemical change 
is likely to take place during the operation of grinding. In 
this way the determination of moisture, of ferrous iron, and 
of sulphur may be influenced very appreciably. 





* The material in this appendix is added temporarily in this form at a 
time when the book is out of print and there is not time to get out a 
complete new edition. It will be introduced eventually into the main 
portion of the book. (Editor.) 

+ Hempel (Z. angew. Chem., 1901, 843) found that an agate mortar 


and pestle lost 0.052 gm. in grinding 10 gms. of glass to a fine powder. 
859 


860 APPENDIX I 


A number of investigators have pointed out the effect of 
grinding upon the moisture content of a sample. If the sample 
is practically dry, it is likely to absorb considerable moisture 
when dried in the air. Thus Hillebrand * found that a piece of 
unglazed porcelain contained no moisture originally, but showed 
0.62 per cent. of water when ground. If the substance is very 
hygroscopic, this danger becomes greater. On the other hand, 
grinding often causes loss of moisture. This is notably true 
in the case of substances containing water of crystallization or 
superficial moisture. Thus grinding can easily reduce the 
moisture content of a sample of gypsum from twenty to five 
per cent, and a sample of coal may show several per cent of 
moisture when large lumps of it are tested and very little 
moisture after it is reduced to a fine powder. 

The heat produced by grinding may not only serve to expel 
moisture from the sample, but it may cause chemical change. 
Thus Mauzeliust has shown, and the experiment has been 
repeated by Hillebrand,f that the ferrous iron content of a 
rock becomes smaller on account of grinding it to a fine powder. 
It has also been found that some sulphur may be lost by long 
grinding of a sample of pyrite. 

The effect of grinding, therefore, accounts for many cases 
of divergent results obtained in the hands of different chemists 
who have analyzed the same original material. 


The Use of Cupferron in Quantitative Analysis. 
Cupferron, CeHs—N—N—ONHg, is a short name for the 


O 
ammonium sali of nitrosophenyl hydroxylamine. O. Baudisch § 


first suggested its use as a reagent for the quantitative precip- 
itation of cupric and ferric ions. By means of cupferron, it is 





*The Analysis of Silicate and Carbonate Rocks, Bull. 422, U. 8S. Geol. 
Survey. 

+ Sveriger Geol. Undersékning, ‘hrebak 1 (1907). 

tJ. Am. Chem. Soc., 30, 1120 (1908). 

§ O. Baudisch, Chem.-Ztg., 38, 1298 (1909); Baudisch and King, J. 
Ind. Eng. Chem., 3, 629 (1911). 


#: 


USE OF CUPFERRON. 861 


possible to precipitate quantitatively copper, iron and titanium 
from strongly acid solutions and in this way a number of sep- 
arations may be accomplished which otherwise involve con- 
siderable difficulty. The copper salt is gray, the ferric salt 
red and the titanium salt yellow. 

As a precipitant for copper, the reagent apparently offers 
no special advantages, and when silver, lead, mercury, tin or 
bismuth are present, these elements contaminate the precipi- 
tate to some extent. On the other hand, in the case of iron 
and titanium it is very useful to possess a reagent which will 
precipitate these elements quantitatively from acid solutions 
without contamination from aluminium, chomium, manganese, 
nickel, cobalt, zine or alkaline earth metal. Zirconium is also 
precipitated by cupferron.* The advantages of cupferron for 
effecting separations has been pointed out by a number of 
chemists.f | 

Cupferron is readily soluble in water and the ammoniacal 
solution keeps well. The reagent is not very stable in acid 
solutions, particularly in hot solution or when an oxidizing 
agent is present. By oxidation, nitrosobenzene is formed and. 
its presence can be detected by the peculiar, sweetish odor in 
nearly every precipitation with cupferron. When much nitro- 
sobenzene is formed it separates out in the form of white needles. 
Precipitation with cupferron is always effected in cold, acid 
solution and an excess of the reagent must be used. The pre- 
cipitate is stable as long as an excess of cupferron is present. 

When iron and copper are precipitated together by means 


of cupferron, washing with ammonia serves to remove the copper 


and the excess of cupferron; it also serves to convert the iron 
precipitate into ferric hydroxide, in which form it is more readily 
converted into ferric oxide on ignition. In the ammoniacal 
solution of the copper precipitate, the copper is again precipitated 
upon the addition of acetic acid and by washing with one per 





* Schroeder, Z. anorg. Chem., 72, 89 (1911). 

+ Nissenson, Z. angew. Chem., 28, 969 (1910); Biltz and Hodtke, Z. 
anorg. Chem., 66, 426 (1910); Hanus and Soukup, zbid., 68, 52 (1910); R. 
Fresenius, Z. anal. Chem., 50, 35 (1911); Bellucci and Grassi, Gazz. chim. 
ital., 43, I, 570; Thornton, Am. J. Sci., 37, 173 and 407 (1914). 


862 APPENDIX I. 


cent. sodium carbonate solution the cupferron may be removed 
from the copper. 

The manner of preparing the reagent and four methods illus- 
trating its use will be described. 


Preparation of Cupferron. * 


Sixty gms. of nitrobenzene, 000 cc. of water and 30 gms. 
of. ammonium chloride are stirred rapidly to an emulsion with 
the aid of an efficient mechanical stirrer. To the emulsion 
(stirred constantly) 80 gms. of zine dust are added in small 
portions, while keeping the temperature of the liquid between 
15° and 18° by the occasional addition of shaved ice. The 
addition of the zine dust is continued until the odor of nitro- 
benzene disappears and the precipitate of zinc hydroxide appears 
gray. Usually about half an hour is required for the reduction. 
The solution is then filtered and the precipitate washed a few 
times with ice water. The filtrate is cooled to nearly 0° and 
saturated with sodium chloride, which causes the separation of 
phenylhydroxylamine crystals in the form of white needles. 
These are filtered immediately with the aid of suction and dried 
between filter paper. 

As phenylhydroxylamine solutions are active skin poisons, 
the hands and face should be washed with water and alcohol 
in case any of the solution spatters upon them. 

The crystals are dissolved in 300-500 c.c. of ether and the 
ethereal solution is filtered through a dry filter, cooled to 0°, 
and saturated with dry ammonia gas. While still at 0° some- 
what more than the theoretical quantity (one mole) of fresh 
amyl nitrate is added all at once. The solution will suddenly 
become hot and the whole beaker will be filled with snow-white 
crystals of cupferron. After filtration and washing with ether, the 
erystals are dried by filter paper and kept in a closely-stoppered 
bottle in which a small lump of ammonium carbonate is also 


placed. 





* The salt may be purchased from Eimer & Amend, New York, 


USE OF CUPFERRON. 863 


1. THE DETERMINATION OF IRON IN MANGANESE ORES. 


Principle.—After effecting the solution of the ore, the iron 
is precipitated in acid solution by cupferron. The filtrate then 
contains all the aluminium, chromium, manganese, nickel, cobalt, 
zinc and alkaline earths. After the removal of any of these 
elements that may be present, by washing the precipitate with 
cold water, the organic matter is removed and the iron con- 
verted into ferric hydroxide by washing the precipitate with 
ammonia, and the iron is weighed as Fe2O3. 

Procedure.—About 1 gm. of the finely pulverized ore is dis- 
solved in concentrated hydrochloric acid, sp. gr. 1.2, and the 
solution evaporated to dryness. The residue is moistened with 
strong hydrochloric acid, the solution diluted with water, boiled 
and filtered. The residue is fused with sodium carbonate in 
a covered platinum crucible, and after the fusion, the melt is 
taken up in water and hydrochloric acid. The solution is evap- 
orated to dryness, and after the removal of the silica in the 
usual manner, the filtrate is added to the main solution. The 
cold solution is then treated with a solution of about 3 gms. 
‘cupferron in 50 c.c. of cold water, added in a fine stream down 
the sides of the beaker while the solution is being vigorously 
stirred. A brownish red, partly amorphous and partly crystal- 
line precipitate of the ferric salt separates out. As soon as a 
drop of the reagent causes the formation of a snow-white pre- 
cipitate of nitrosophenylhydroxylamine, all the iron is precip- 
» itated. A slight excess of the reagent is added and the solution 
allowed to stand about 10 minutes, after which it is filtered 
through an ashless paper, using gentle suction. In case the 
last particles of the precipitate cling tenaciously to the beaker, 
a little ether is added to loosen them and the ether removed by 
adding a little boiling water. The precipitate is now washed 
with cold water until the washings are no longer acid to litmus 
and then with ammonia (sp. gr. 0.96) to remove the excess of 
the reagent. and form ferric hydroxide. The filter is finally 
washed once more with cold water. The precipitate is ignited 
carefully with the filter, avoiding reduction, and weighed as Fe2O3. 


864 APPENDIX I. 


Remark.—Following this procedure, R. Fresenius * obtained - 
the values 18.23. per cent., 18.56 per cent., and 18.03 per cent. 
Fe in the analysis of three manganese ores which when 
analyzed volumetrically for iron, showed respectively, 18.25 
per cent., 18.49 per cent., and 17.93 per cent. Fe. The manganese 
may be determined in the filtrate. 


; vw 
2. THE DETERMINATION OF MANGANESE IN FERRO-MANGANESE, 


About one gram of the material is dissolved in strong hydro- 
chloric acid and the residue is fused with sodium carbonate 
in a platinum crucible. The product of the fusion is extracted 
with water and alcohol is added to reduce the manganate, formed 
by the fusion, to hydrated manganese dioxide. The aqueous 
extract from the fusion is filtered, the residue is dissolved in 
hydrochloric acid, and the solution added to that obtained in 
the first place. The iron is precipitated with cupferron, as in 
the previous method, but it is not advisable to add the ammonia 
washings to the solution on account of the organic material they 
contain. In the filtrate the manganese is precipitated as described 
on page 122 and determined as sulphide according to page 125. 

Remark.—In testing this method R. Fresenius * obtained 
duplicate values of 81.04 and 81.01 per cent. Mn as compared with — 
81.01 and 81.17 by the basic acetate method. The cupferron 
method is much shorter and easier to carry out. 


3. THE DETERMINATION OF NICKEL AND COBALT IN ARSENICAL 
SULPHIDE ORES.T 


One gram of the sample is dissolved in concentrated hydro- 
chloric acid, which is saturated with bromine, and the solution 
evaporated somewhat to volatilize arsenic. Ten cubic centi- 
meters of sulphuric acid (1: 1) are added and the solution evap- 
orated until dense fumes are evolved. The solution is diluted, 
heated to boiling, and hydrogen sulphide is introduced to pre- 
cipitate the metals of the copper group and any remaining 
arsenic. In the filtrate the iron is precipitated by cupferron 





*Z. anal. Chem., 50, 35 (1911). 
+ H. Nissenson, Z. angew. Chem., 23, 969 (1910). 


USE OF CUPFERRON. 865 


as in Method 1, and after evaporating the filtrate until fumes 
of sulphuric acid are evolved, the nickel and cobalt are deter- 
mined electrolytically according to page 136. 

In case it is desired to determine the arsenic as well, the 
ore is dissolved in 15 c.c. of concentrated sulphuric acid, and the 
arsenic is precipitated as sulphide by passing hydrogen sul- 
phide into the hot solution. The arsenic is separated from cop- 
per according to page 235 by means of alkaline sulphide, the 
arsenic sulphide precipitated in the filtrate upon making it 
acid, the arsenic sulphide dissolved in concentrated hydrochloric 
acid and potassium chlorate, and the arsenic determined as 
magnesium pyroarsenate according to page 206. 


4. THE DETERMINATION OF TITANIUM AND ITS SEPARATION FROM 
IRON, ALUMINIUM AND PHOSPHORIC ACID.* 


Principle-—The iron may be reduced completely to ferrous 
salt by passing hydrogen sulphide into an acid solution, and 
then, in the presence of tartaric acid which prevents the pre- 
cipitation of titanium, precipitated as ferrous sulphide in ammoni- 
acal solution. After acidifying and boiling off the hydrogen 
sulphide, the titanium can be precipitated quantitatively by 
means of cupferron while the aluminium and phosphoric acid 
remain in the tartaric acid solution. It is unnecessary to remove 
the organic matter before igniting the yellow titanium precipitate. 

Procedure.—To the solution, which should have a volume 
not greater than 100 c¢.c., at least four times as much tartaric 
acid is added as corresponds to the weight of the oxides of iron, 
titanium, aluminium, aad phosphorus. The solution is neutral- 
ized with ammonia, acidified with 3 ¢.c. of sulphuric acid (1 : 1) 
and hydrogen sulphide is introduced until the solution becomes 
colorless. Unless the iron is all reduced, the subsequent pre- 
cipitate of iron sulphide will contain some titanium. Ammonium 
hydroxide is now added in considerable excess and the iron 
.is completely precipitated as ferrous sulphide by introducing 





*W. M. Thornton, Jr., Am. J. Science, 37, 407 (1914). 


866 APPENDIX 1. 


more hydrogen sulphide gas; the solution should remain alkaline 
to litmus paper. The ferrous sulphide is filtered off and washed - 
with water containing a little colorless ammonium sulphide. 
To the filtrate, 40 e.c. of HeSOq (1:1) are added and the lib- 
erated hydrogen sulphide is expelled by boiling. When this 
is accomplished, the solution is cooled to room temperature, di- 
luted to 400 c.c., and treated with ay excess of 6 per cent. cupferron 
solution, which is added slowly down the sides of the beaker 
while the solution is being well stirred. After the precipitate 
has subsided, the supernatant liquid is tested by adding more 
of the cupferron solution. A white precipitate of nitroso- 
phenylhydroxylamine indicates that an excess of the reagent 
is present but a yellow turbidity shows that the precipitation 
of the titanium is incomplete. It is also well to test the filtrate 
in the same way. The precipitate of the titanium precipitate 
is collected on filter paper, using gentle suction, and washed 
twenty times with normal hydrochloric acid solution. The 
precipitate is then ignited cautiously in a platinum or quartz 
crucible until the organic matter is all consumed. Finally, 
it is heated to constant weight over a Méker burner and weighed 
as TiOs. 


The Determination of Titanium Dioxide in Titanium-Iron Ore. 


Most titanium ores are very difficultly soluble in the usual 
solvents. The analysis of these ores is also complicated by the 
fact that aqueous solutions of titanium salts readily undergo 
hydrolysis, partly on account of the amphoteric character of 
titanium dioxide and partly because of the insolubility of this 
oxide. If sodium titanate, Naz2TiOs3, is prepared by fusion with 
sodium carbonate, it undergoes hydrolysis when treated with 
water and an acid titanate of sodium is precipitated, e.g., 
2Na2O-9TIO2-5HOeg. This acid titanate is soluble in hydrochloric 
acid, sp. gr. 1.12, but if the solution is diluted and heated to 
boiling, hydrolysis takes place, as with any other salt of titanium, 
and metatianie acid, HeTiO3, is precipitated. A method for 
determining titanium after fusing with sodium carbonate and dis- 


DETERMINATION OF TITANIUM. 867 


solving the sodium titanate in hydrochloric acid has already 
been given (p. 118). 

One of the best methods for attacking titanium dioxide, or 
an insoluble titanium ore, is based upon the fusion of the ore 
with potassium acid sulphate. This converts titanium into 
titanium sulphate,* (TiSO4)2, which is soluble in cold water. 
Potassium acid sulphate melts at about 200° and, on further 
heating, is easily decomposed into potassium pyrosulphate, 
K28207: 

2KHSO,4= K28207+ H20, 


which melts at a little over 300°. Upon raising the temperature 
sufficiently, it is easy to transform the potassium pyrosulphate 
into normal potassium sulphate, which melts at about 1050°. 
Between these last two temperatures, sulphuric acid is avail- 
able for attacking insoluble oxides of iron, aluminium and 


titanium: 
TiO2g + 2K28207 = Ti(SO4)2+2K2S0x. 


If the temperature is raised too high, however, TiO2 may be 
formed again, as is the case in heating the residue after the 
removal of silica with hydrofluoric and sulphuric acids: 


Ti(SO4)2 = Ti02+ 2803. 


Moreover, if the heating is continued long enough to convert 
all the pyrosulphate into molten potassium sulphate, there 
is some danger of the crucible bursting upon the solidification 
of the melt.t 

When potassium acid sulphate is decomposed into potas- 
sium pyrosulphate and water, the latter is liberated at a tem- 
perature far above its boiling-point. There is danger of the 
contents of the crucible boiling over if the reaction takes place 





* Titanium sulphate shows a tendency to form complex ions. Thus 
Warren (Pogg. Ann., 102, 449 (1857) obtained the salt K,Ti(SO,)3 after 
fusing with potassium acid sulphate. See also Schutte, Z. anorg. Chem., 
26, 239 (1901). , 

+ H. P. Talbot, Quantitative Chemical Analysis, 5th Edition, p. 4. 


868 APPENDIX I. 


too quickly. Before making the fusion, therefore, it is advisable 
to heat the potassium sulphate by itself until vapors of sul- 
phuric anhydride begin to be evolved. A low flame of the 
Bunsen burner is sufficient. 

The following procedure is given, not because it gives the 
most convenient or the most accurate method for determining 
titanium in an insoluble ore, but because it is an especially good 
method for teaching the art of fusing with potassium acid sul- 
phate to advanced students in quantitative chemical analysis. 
The method will give excellent results when properly carried 
out, but is likely to give too low results if the ore is not entirely 
decomposed by the acid sulphate fusion, or too high results if 
the titanic metatitanic acid is not purified sufficiently and washed 
free from alkali salts. 

Procedure.—From 0.4 to 0.6 gm. of the firiokye ground ore 
is weighed into a platinum crucible and fused with 6 to 8 times 
its weight of sodium carbonate for at least 30 minutes. The 
fused mass is extracted with hot water and, when the melt 
is all disintegrated, the solution is decanted through an ashless 
filter paper. The residue is boiled with 25 c.c. of normal sodium 
carbonate solution, transferred to the filter, and washed twice 
with dilute sodium carbonate solution and several times with 
hot water. This serves to remove the phosphoric acid as sodium 
phosphate,* and to convert titanium into acid sodium titanate; 
the iron is left with the titanium as insoluble ferric oxide. 

The residue and filter paper are ignited in a platinum cru- 
cible at a low temperature until the carbon of the filter paper 
is all consumed. From 12 to 15 parts of potassium pyrosul- 
phate, which has just been prepared by heating potassium acid 
sulphate until all the water is expelled, are added to the contents 
of the crucible and the latter is very carefully heated. The 
temperature should be kept low until all the potassium pyro- 
sulphate has melted, then the heat should be gradually increased 
until sulphuric acid fumes are noticed on removing the cover 





* Phosphoric acid interferes with the precipitation of metatitanic acid 
by hydrolysis. It tends to precipitate with the titanium to some extent 
and also to render the precipitation incomplete. 


DETERMINATION OF TITANIUM. 869 


of the crucible. At no time should dense fumes be given off 
from the covered crucible, but it will be necessary to raise the 
temperature from time to time in order to keep the flux melted. 
Occasionally the cover of the crucible should be raised to see 
if the residue has dissolved in the flux. Sometimes the fusion 
can be accomplished in half an hour, but more often nearly 
an hour is required. Finally, when the contents of the cruci- 
ble are at a dull red heat and there is no sign of undissolved 
material, a coil of platinum wire is suspended in the melt and 
it is allowed to cool. When cold, the fusion is then withdrawn 
by heating the sides of the crucible. 

The melt is suspended in 200 c.c. of cold water and 100 c.c. 
of saturated sulphurous acid solution, and allowed to stand in 
a cool place until solution is complete, stirring from time to - 
time. The solution is usually effected by allowing the dilute 
acid to stand over night with the melt suspended near the top 
of the liquid. The process may be hastened by stirring with 
a mechanical stirrer. 

If necessary the solution is filtered, and the residue treated 
with sulphuric and hydrofluoric acids to expel silica, fused 
with more potassium pyrosulphate, and the melt added to the 
solution previously obtained. Finally the solution is diluted 
to 800 c.c. in a large beaker, 125 c.c. of acetic acid, sp. gr. 1.04, 
and 20 gms. of ammonium acetate are added and the solution 
is boiled five minutes. Just before boiling begins, 25 c.c. more 
of sulphurous acid solution are added. When the precipitation 
is complete, the beaker is allowed to stand in a warm place for 
at least half an hour, and is then filtered through a 9-cm. filter 
paper. A siphon is meanwhile prepared which is long enough 
to extend to the bottom of the beaker, with the glass tubing 
bent upward so that the water will suck into the siphon from 
above without drawing much precipitate with it. The longer 
end of the siphon is connected by means of rubber tubing with 
a short piece of glass tubing which rests justs below the upper 
edge of a 9-cm. washed filter paper. When the precipitated 
metatitanic acid has settled to the bottom of the beaker, the 
supernatant solution is siphoned through the filter, regulat- 
ing the flow by means of a small screw clamp placed on 


870 APPENDIX I. 


the short piece of rubber tubing, a little way above the 
funnel. 

The precipitate is washed with 5 per cent. acetic acid until 
most of the sulphate has been removed. The filter and its 
contents are ignited at as low a temperature as possible and 
the fusion with potassium pyrosulphate is repeated. This 
time there should be no difficulty in getting a good fusion. The 
product of the fusion is treated exattly as before, but the residue 
is now washed more carefully, until all the sulphate has been 
removed. The filtered precipitate is ignited and weighed as TiO. 


Determination of Sulphur in Pyrite: Fusion with Sodium 
Peroxide. 


The Fresenius method (page 357) has long been accepted 
as a standard method for determining sulphur in insoluble sul- 
phides, but the procedure is long on account of the necessity 
of removing all the nitrate before precipitating the sulphuric 
acid ion. If the analysis is carried out in a platinum crucible 
the oxidizing mixture attacks the platinum quite badly, which 
is a serious objection, especially when the analysis is frequently 
made. If the Fresenius method is carried out in a nickel or 
iron crucible, it is difficult to raise the temperature sufficiently 
without using the blast lamp. 

It has been repeatedly shown* that equally satisfactory results 
can be obtained by using sodium peroxide as the oxidizing flux. 
Moreover, the analysis can be accomplished in about two hours. 

Procedure.—About 0.5 gm. of pyrite is mixed with 5 gms. of: 
pure sodium peroxide and 4 gms. of sodium carbonate in a nickel 
or iron crucible. An opening is cut in a piece of asbestos board 
(at least four inches square) sufficiently large to allow two-thirds 
of the crucible to project below the asbestos. The purpose of 
this shield is to keep the products formed by the combustion 





* W. Hempel, Z. anorg. Chem., 3, 193 (1893); J. Clark, J. Chera. Soc., 
63, 1079 (1893); Hohnel, Arch. Pharm., 282, 222; C. Glaser, Chem.-Ztg., 
18, 1448; Fournier, Revue générale de chimie, pure et appliquée, 1903, 
77; List, Z. angew. Chem., 19038, 414. 


DETERMINATION OF SULPHUR IN PYRITE. 871 


of the gas from reaching the mouth of the crucible. Illuminating 
gas often contains a little sulphur. The fusion mixture is heated 
gently for ten minutes so that the mass softens and bakes 
together and then the temperature is raised until the crucible 
is exposed to the full heat of the Tirrill burner for twenty 
minutes. 

The contents of the crucible are allowed to cool and are 
treated with 150 c.c. of hot water. When the sodium salts are 
all dissolved, the crucible is removed and the solution is treated 
with 5 c.c. of hydrochloric acid, sp. gr. 1.2, to which liquid 
bromine has been added till the acid iss aturated. The pur- 
pose of the bromine is to make sure that the oxidation of the 
sulphur is complete.* It is necessary to add acid as otherwise 
the hot, caustic soda solution is likely to destroy the filter paper. 
After heating to boiling, the solution is filtered and the residue 
of ferric hydroxide is washed free from sulphate. 

The filtrate is carefully neutralized with 6-normal hydro- 
chloric acid, sp. gr. 1.12, and 2 c.c. of this acid are added in 
excess. The solution is heated till all the bromine is expelled, 
diluted to 250 c.c. and the sulphuric acid precipitated by adding 
very slowly, while stirring, a slight excess of hot barium chloride 
solution (10 c.c. of normal barium chloride solution diluted 
to 100 c.c.). 

The barium sulphate precipitate is washed three times by 
decantation with hot water, then transferred to the filter and 
washed free from chloride, dried, ignited and weighed. 

If, in the above analysis, the aqueous extract of the sodium 
- peroxide fusion shows manganate or permanganate by the 
- color, these substances may be reduced by the addition of a 
few drops of alcohol and boiling. Often the reduction of the 
manganate to manganese dioxide causes a turbidity in the fil- 
trate. This does no harm, as it dissolves without difficulty on 
making the solution acid. 





* A black residue may denote ferrous sulphide or nickelic oxide. It 
may be tested for sulphur by dissolving in hydrochloric acid and bromine 
and adding barium chloride to the diluted solution. 


872 APPENDIX I. 


A Rapid Method for the Determination of Sulphur in Steel. 


Principle-—The method depends on the evolution of the 
sulphur as hydrogen sulphide and its absorption in an ammoniacal 
cadmium chloride solution, followed by an iodometric titration. 
As already stated,* it has been shown that in many cases some 
of the sulphur in iron and steel is ,not evolved as hydrogen sul- 
phide upon treatment of the sample with hydrochloric acid. 
This fact has received much attention in the literature and 
various attempts have been made to overcome the difficulty. 
Thus T. G. Elliot,t mixes 5 gms. of the iron or steel with 0.25 
gm. of anhydrous potassium ferrocyanide, wraps the mixture 
in filter paper, places it in a covered porcelain crucible, and 
anneals for twenty minutes in a muffle furnace heated to 
750°-850°. The sample is allowed to cool slowly, is broken 
up in a mortar and then introduced into the evolution flask. 
The fact that this annealing of the sample serves to convert 
practically all the sulphur into a form in which it is readily 
evolved as hydrogen sulphide has been confirmed by other 
chemists. 

It has been shown,{ however, that if concentrated hydro- 
chloric acid is used for dissolving the sample, and the sample 
is dissolved quickly without letting air enter the evolution 
flask, there is rarely any difficulty in obtaining accurate values 
by the rapid evolution process which is to be described. 

Besides hydrogen and hydrogen sulphide, various other 
gases are evolved to some extent when a sample of iron or steel 
is dissolved in hydrochloric acid. To prevent the influence 
of these gases upon the volumetric determination of the sulphur 
one of the most satisfactory expedients is to absorb the hydrogen 
sulphide in an ammoniacal solution of cadmium chloride, to 
filter off the resulting precipitate of cadmium sulphide, to dissolve 








* Cf. page 352. 

+ Chem. News, 104, 298 (1911). 

t Cf. C. Reinhardt, Stahl u. Eisen, 10, 480 (1890); W. Schindler, Z. 
angew. Chem., 1898, 11; W. Schulte, Stahl u. Eisen, 26, 985 (1906) and 
H. Kinder, zbid., 28, 249 (1908). 


DETERMINATION OF SULPHUR IN STEEL. 843 


it in acid in the presence of iodine, and to titrate the excess 
- of the iodine with sodium thiosulphate solution. The reactions 
involved are expressed by the following equations: 


FeS + 2HCI= FeCly +H. 
Mn$ + 2HCl= MnCly + H2S 
H»S + Cd(NH3)4Clo = CdS + 2NH,Cl+2NH3 
2KMnO, + 10KI + 8H2804— 6K 280, + 2MnSO,4+8H20 +52 
CdS + H.S0,4+12=CdS0,+2HI+S 
Ip+2NaoS203 = NaoS406-+ 2Nal. 


Requisite Solutions. 


1. Sodium Thiosulphate Solution, approximately 0.02 normal. 
This is prepared by dissolving 5 gms. of sodium thiosulphate 
crystals in one liter of freshly boiled water. 

2. Permanganate Solution, approximately 0.08 normal. It is 
prepared by dissolving 5 gms. of potassium permanganate in 
2 liters of distilled water, allowing the solution to stand some 

time, and then filtering through asbestos. 

3. Potassium Iodide Solution.—This is prepared by dissolving 
30 gms. of pure potassium iodide and 10 gms. of sodium bicar- 
bonate in a little water, filtering the solution if necessary and 
diluting to 1 liter. | 

4. Soluble Starch Indicator Solution.—About 0.5 gm. of 

«soluble starch is dissolved in 25 e.c. of boiling water. This 

solution is more stable than solutions of ordinary potato starch, 
and it is much easier to prepare. The solution is ready for use 
as soon as it is cold, It usually keeps a week or longer. | 

5. Ammoniacal Cadmium Chloride Solution.—20 gms. of 
cadmium chloride are dissolved in 400 e.c. of water, and 600 
c.c. of ammonia, sp. gr. 0.96, are added. 
_ The solution of potassium permanganate is standardized 
against sodium oxalate (p. 597) using about 0.2 gm. of the salt. 

The solution of sodium thiosulphate is standardized by 
taking 20 c.c. of the potassium iodide solution, adding 25 c.e. 


874 APPENDIX I. 


of sulphuric acid (1: 3) and allowing exactly 10 c.c. of the per- 
manganate to run in while stirring. The liberated iodine is 
then titrated with sodium thiosulphate, adding 2 c.c. of the 
starch solution toward the last. 

Apparatus.—A simple form of apparatus consists of a 250-c.c. 
round-bottomed flask connected in series with three 150-c.c. 
Erlenmeyer flasks. Each of the flasks is fitted with two-holed . 
rubber stoppers. The second hole in the stopper of the evolu- 
tion flask carries a dropping funnel, the end of which reaches 
nearly to the bottom of the flask. Each delivery tube is bent twice 
at right angles, starts just below the lower bottom of the rubber 
stopper in one flask and leads nearly to the bottom of the next. 

In the first Erlenmeyer flask is placed 50 c.c. of water to 
remove most of the hydrochloric acid which is distilled over. 
It does not absorb an appreciable amount of the hydrogen sul- 
phide gas because it becomes heated nearly to boiling toward 
the last. The second Erlenmeyer contains 50 c.c. of the cad- 
mium solution and the third 25 ¢.c. of the same. The hydrogen 
sulphide is usually absorbed completely in the second flask, 
but the third flask with its cadmium solution is added as a 
precaution. 

Procedure.—About 5 gms. of the iron or steel is weighed into 
the evolution flask. The apparatus is connected together and 
it is made sure that all the joints are tight. 50 ¢.c. of concen- 
trated hydrochloric acid, sp. gr. 1.19, are placed in the dropping 
funnel and about half of it is allowed to flow into the flask. If 
the action is not too vigorous, the remainder of the acid is at 
once added, but the stop-cock in the dropping funnel is closed 
while the tube is still full of acid. This is necessary, because 
if air enters the solution there is danger of some loss of sulphur 
owing to oxidation. The evolved hydrogen serves to prevent 
this oxidation, provided air does not enter through the funnel. 
The dropping funnel is again filled with 50 ¢.c. of HCl, and all 
of this, except enough to fill the stem of the funnel, is added. 

As soon as the rate of flow of the gas through the Erlenmeyer 
flasks becomes less than about 3 bubbles per second, a very low 
flame is placed below the flask and the flow of gas is maintained 
steadily at this rate. The burner should be furnished with a 


DETERMINATION OF SULPHUR IN STEEL. 875 


flame protector, as otherwise a sudden draft will cause trouble 
with the small flame. The steel sample should dissolve in about 
thirty minutes, but it is not advisable to heat too quickly as 
this will result in too much acid being carried over into the 
first flask. At first the flame beneath the evolution flask should 
be only about 0.7 em. high and it should not be much over 4 
cm. high until all the iron has dissolved. When this has taken 
place, the height of the flame is raised to about 7 cm. and the 
solution is boiled gently for ten minutes. When the solution 
begins to boil, the stop-cock in the separatory dropping funnel 
may be opened. This boiling serves to expel all the hydrogen 
sulphide from the evolution flask and from the first Erlenmenyer 
flask. It is unnecessary to pass hydrogen or carbon dioxide 
through the apparatus, as has been advocated by some chemists. 
Finally, making sure that the stop-cock in the dropping flask 
is open, the flame is removed and the apparatus promptly dis- 
connected. | | 

The precipitated cadmium sulphide is filtered off and washed 
once or twice with water. The filter and precipitate are placed > 
in a 250-c.c. Erlenmeyer flask containing 20 c.c. of the potas- 
sium iodide solution, 25 ¢.c. of sulphuric acid (1:3) and 10 
c.c. of the standardized permanganate solution. -When all the 
cadmium sulphide has dissolved, the excess of iodine is titrated 
with the sodium thiosulphate solution. 

Computation.—If 1 c.c. of the permanganate solution=a 
gm. of pure sodium oxalate, 7; ¢.c. of thiosulphate solution 
were used in titrating 10 c.c. of permanganate, and T2 c.c. for 

«titrating the same amount of permanganate used in the analysis 
of a sample of iron or steel weighing s gms., then the per cent. 
of sulphur is given by the equation of: 


S  _ (T,—T2) X100 _ 23.93a(T: —T2) _ 
a% NaeCo04 8 ie ¢ = e is 








Remark.—Massenez * has recommended the direct titration 
of the cadmium sulphide precipitate with potassium perman- 





* Stahl u. Eisen, 32, 2089 (1912). 


876 | APPENDIX I. 


ganate. The cadmium solution, without filtering off the pre- 
cipitate, is boiled thirty minutes to expel hydrocarbons and is then 
rinsed into a beaker containing 600 c.c. of water and enough 
permanganate to give a pink tint. Twenty-five c.c. of sul- 
phuric acid (1: 1) are added and the liberated hydrogen sulphide 
is titrated with permanganate. 


The Determination of Chromium in Steel. 


A small quantity of chromium is a common constituent of 
iron and steel and in some of the alloy steels it is present to 
the extent of several per cent. The simplest and most rapid 
method for determining this element consists in oxidizing it to 
chromate, adding a known quantity of ferrous salt and deter- 
mining the excess of the latter by titrating with standard potas- 
sium permanganate. A great many different methods have 
been proposed for carrying out the oxidation, two which are 
very rapid and give accurate results will be described, 


Oxidation by the Barba Method.* 


Principle-—The steel is dissolved in dilute sulphuric acid, 
the iron is oxidized to the ferric state by means of nitric acid, 
and the chromium is oxidized by the addition of strong per- 
manganate solution. The excess of the latter is destroyed 
by boiling in ammoniacal solution, the solution is acidified 
again, the precipitated manganese dioxide filtered off, a known 
volume of standard ferrous sulphate solution is added to an 
aliquot part of the filtrate, and the excess of the latter is titrated 
with permanganate. 

Procedure.—Exactly 1.25 gm. of steel is dissolved in 20 c.c. of 
6-normal sulphuric acid, sp. gr. 1.2, and when the solution is com- 
plete, nitrie acid is added drop by drop until the iron is all oxi- 
dized to the ferric state, five cubic centimeters of nitric acid, sp. gr. 
1.2, is sufficient in most cases. The solution is boiled to remove 





* J, Iron and Steel Institute, 18938, ii, 536; The Iron Age, 52, 153. 


DETERMINATION OF .CHROMIUM IN STEEL. 877 


nitrous fumes, diluted to 150 ¢c.c. and treated with_5 c.c. of a sat- 
urated solution of potassium permanganate (6 gms. per 100 c.c.). 
The solution is boiled briskly for 15 to 20 minutes. It ‘is then 
' removed from the hot plate, the sides of the beaker are washed 
down with water and 25 c.c. of strong ammonia are poured down 
the sides of the beaker. The liquid is stirred vigorously and 
then placed on the cooler part of the hot plate, for if it is heated 
too rapidly there is likely to be loss by ‘‘ bumping.”’ The diges- 
tion is continued with occasional stirring for fifteen minutes, 
or until the permanganate is all decomposed as shown. by the 
disappearance of the pink color. Then 20 c.c. of dilute sul- 
phuric acid (1: 1) are added cautiously and the solution is 
heated to boiling. It is transferred to a 250-c.c. calibrated 
flask, cooled to. room temperature, and diluted up to the mark. 
After thoroughly mixing by pouring back and forth several 
times into a dry beaker, the solution is filtered and exactly 
200 c.c. (corresponding to 1 gm. of metal) is taken for the rest 
of the analysis. Fifty c.c. of standard ferrous sulphate solution* 
is added, and the excess titrated with 0.08-normal perman- 
ganate. 3 

Computation.—When the ferrous sulphate solution is added, 
the chromium is reduced from chromic acid to chromic salt in 
accordance with the following equation: 


2Cr03 + 6FeSO, + 6H»S0,4 = Cro (S04) +3Fes(SOy)3 + 6H20. 


The ferrous sulphate solution is not very stable and should be 
titrated against the permanganate at the same time the analysis 
is made. If 50 c.c. of the ferrous sulphate solution are equiv- 
alent to 7, c.c. of permanganate, of which 1 ¢.c.=a gm. of pure 
sodium oxalate, and 7’, c.c. of permanganate were used in titrating 
the excess of ferrous sulphate in the analysis of 1 gm. of steel, 
then 





2Cr-a(7T'; —T2)-100 


3NaeC204 = 25.86a(T1 —T'2) = % Cr. 





* Page 617, footnote. This is sufficient in case the steel does not con- 
tain more than 2.5 per cent. of chromium. If more chromium than this 
is present, a smaller aliquot part of the solutions should be taken. 


878 APPENDIX I. 


Oxidation by Sodium Bismuthate. 
1000 c.c. of tenth-normal permanganate = 1.733 gms. Cr 


Principle.—Sodium bismuthate, NaBiO3, has already been 
used for oxidizing manganese from the bivalent to septavalent 
condition.* This oxidation takes place best in a cold solution 
containing 20 to 40 per cent. of “hitric acid (free from nitrous 
acid, in a volume of 50 to 100 c.c.t Moreover, an excess of 
sodium bismuthate should be present and the solution must 
not stand long. Under these conditions scarcely any chromium 
is oxidized so that the manganese may be determined by the 
bismuthate method even when chromium is present. If the 
solution is heated to boiling, the permanganate is decomposed 
and the manganese is precipitated as manganese dioxide. Chro- 
mium, on the other hand, is oxidized in hot solutions from the 
trivalent to the sexavalent condition and the chromic acid is — 
not decomposed by boiling, unless some reducing agent is present. 
In hot solutions, therefore, it is possible to oxidize the chromium 
by sodium bismuthate, to filter off the precipitated manganese 
dioxide and to determine the chromium in the filtrate by adding ~ 
a known volume of standard ferrous sulphate solution and 
titrating the excess of the latter with permanganate. 

Procedure.—One gram of the steel is dissolved in a 250-c.c. 
Erlenmeyer flask with 100 ¢.c. of 30 per cent. nitric acid (30 
c.c. of concentrated acid and 70 c.c. water). If there is any 
carbonaceous residue, as in the analysis of cast irons, it should 
be filtered off and examined for chromium by fusion with an 
alkaline oxidizing flux.t If the metal is difficultly soluble in 
nitric acid, it is sometimes necessary to add sulphuric acid to 
hasten the solution. 

_ When the sample is all dissolved, the solution is cooled to 
between 65° and 75° and 2 gms. of sodium bismuthate are 
added. The contents of the flask are agitated for a few minutes 





_ * Cf. p..617 of this book. 
+ William Blum, J. Am. Chem. Soc., 34, 1395. 
_ £Cf. p. 675. If the carbonaceous residue is not removed it will inter- 
ere with the oxidation of the chromium. 


DETERMINATION OF CHROMIUM IN STEEL. 879 


and then the sides are washed down with a little water. The 
solution is heated and gently boiled until the permanganate 
formed from the manganese in the steel is all decomposed as 
shown by the color. This usually requires about fifteen minutes... 
Fifty c.c. of nitric acid (8 per cent. by volume) are added and 
any precipitated manganese dioxide or undissolved sodium bis- 
muthate is filtered off on an asbestos filter. The residue is 
washed three times with 50 c.c. portions of the dilute nitric 
acid and then diluted with distilled water to 500 c.c.. A measured 
excess of ferrous sulphate solution is added and the excess titrated 
with permanganate. 

The ferrous sulphate is titrated against the permanganate 
on the same day that the analysis is made and the same quan- 
tities of ferrous sulphate and nitric acid are used as in the 
analysis. The computation is the same as in the previous 
analysis. The chemical reactions involved are the following: 


bo, 8: + 3HeSO4 = Cro (SO4)3 1s 3He 


Cre(SOx)3 + 3NaBi03 + 6HNO3= 2Cr03 + Bia(SO4)3 + Bi(NO3)3 
+3NaNO3 +3H20 


2CrO3 + 6FeSO4 + 1ZHNO3 = 2Fee(SO4)3 + 2Fe(NO3)3 
+2Cr(NO3)3+6H2O 


ae +5FeSO,4 + 9HNO3 = Fee (SO4)3 + 3Fe(NO3)3 + 
-+ KHS0O, + MnSO, +4H>.0O. 


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ARSENIC STANDARDS. See pages 211 and 212. 








INDEX OF AUTHORS, 





A, 
; PAGE 
wilefeld,.B.. Determimation of chlorine. 0... kee k ict ce tenes 708 
Allner, Determination of carbon monoxide....................-000-. 762 
Andrew, L. W., Use of potassium iodate............. 0c. cece ec ceeee 672 
Andrews, Volumetric estimation of nickel...................0000000. 720 
PUDUS BO ea ete, 716 
ener, TPO vermINaGION Of:OZ0NG 2... sk oSiers on ccc sc eb ccso en ga ones 677 
Meer, © OtUIBHION Of: NiMTIC OXIDE. |... Se Sone cp oe bbe ote wan apes 803 
Arthur, Volumetric estimation of nickel...................,800000- 720 
Augenot, Separation of tungsten from tin ..................eceeceeee 300 
Austin, M., Determination of magnesium. ..............0c00cceceeee 66 
NO Ee a lire UR fet nae are a 120, 126 
MIG rules ies ong ee ie Gack s es eee 140 

B. 
Bamber, Method for determining sulphur in iron and steel............ 354 
Barnebey, O. L., Determination of titanium......................... 118 
Baubigny, Determination of antimony.....................e00e000ee 222 
I Ne ge, ye 223, 224 
Beilstein, Electrolytic determination of cadmium..................... 189 
Belhoubek, Volumetric determination of uranium.................... 621 
Benz, Determination of thorium. ........ 5.2.2... ccc ccc ce wencecs 510 
Bergmann, Electrolysis of nickel solutions. ................000.00000- 131 
PirOreD FOP TMC POEUN occ ik cep ecwiaaceces 529 
Berthelot, Absorption of benzene. ....... 2... cee cee cee e eect ee cee 753 
Distribution of iodine between two solvents................ 658 
CUS ATIANGR UY BURCMRIS 50S edo ale ss sin voce ee sete che bee cy 499 
Mercurous tungstate precipitation... .. iO rove akivuce ce ved 289 
Silicic: AGI BYOCUCBLIOR oo sis C5 co nc dhe set ene 473, 489 
Vahadiim determinate sso 5 653 co asin ae bg bers ce ces ok 304 
Biltz, W., Separation of halides from sulphides....................... 329 
Black, Colorimetric determination of arsenic..,................0 0000: 208 


882 INDEX OF AUTHORS. 


PAGE 

Blair, A. A., Condition of sulphur in iron and steel. ..............005. 352 
Direct. combustion of pteel > oo. 25) oon cca ae hae 413 

Manganese in steel, iron, ores, CtC.. 2.6.0 sc cee ew eee eens 616 

Phosphorus in ironand steel, «35 oS 53 Fae oe ee 637 

Vanadium, molybdenum, chromium, and nickel in steel... 313 

Blasdale, W. C., Separation of calcium and magnesium ............. ps 
Bloxam, Electrolytic determination of arsenic. .............0.0eee eee 212 
Boéckmann, Sulphur in insoluble sulphides, ................-.+.0025. 358 
Bong, G., Decomposition of. silicates: oie. 6 os a ca ies es enw eee 490 
Borchers, W.,:-Sulphocyanite newbs. 265.645 6a ann oe a ee tee 341 
and hydrocyanic acids...............s.6. 342 

Borda, Method of weighing >: . 0502. 5. teh ea cae es een eee 9 
Boreli, Electrolytic determination of mercury. ...................200. 173 
Bormann, Expelling ammonia with magnesium oxide................. 59 
Borntrager, H., Determination of tungsten....................-2000- 300 
Recovery of molybdenum residues......... CE, ee 447 

Boudet, Error in measuring alcoholic soap solutions.................. 536 
Boutron, Error in measuring alcoholic soap solutions. ................ 536 
Braun, Effect of carbon disulphide on antimony sulphide.............. 242 
Brauner, B., Separation of selenium and tellurium............... 279, 280 
Brearley, Volumetric estimation of nickel....................-..000. 720 
Bretschger; M:., Density of acetylene (3.200) So So PI a 754 
ethylene. Sees oon se one ee 751 

Pipette for gan analysis. 30780) 6054.44955. 4a see 796 

Preparation of acetylene... 5 isis sone ee aed 754 

Brodie; B: C., Qeone 3656.05 eee Os ek a oe Cav a eile ee ee 676 
Bruhns, Volumetric estimation of sulphuric acid..................... 716 
Brunck, O., ‘Combustion of hydrogens 250". 2252S PAs Bi, ae. Se 772 
Determination of antimony 22:59. SV OS eas ee 222 

nickel 0 oP SEA Te ee 129 

Separation of iron and manganese... ..... 0.0. cece eee eee 153 

Niokek CPI TO PREG iia 166 

nickel and GoObalt <2) 2220. SAV 162 

mine. : APs AO Re eae 165 

Volumetric estimation of hydrogen sulphide.............. 688 

Brunner, Analysis of tungsten bronze. ......... 6... e cece eee eee . 299 
Determination ‘of arsenic . 0.30705. SOP OA 205 
Separation of zinc from nickel, cobalt, and manganese........ 158 

Brush, Water in ‘silicates... ie ii chee Sees, Dea a 512 
Bunsen, Alkalimneter. .. 000.00 bo case ee feeds sees seins be eewes wean 376 
Analysis of chromite 0. i co. eee eee ROUEN ed 5 ou 509 
Combustion of nitrogen. (370.566 0 as EAS eas 807 
Determination of antimony. ..............-se00. bbs ane a 

7 areenie 020 SIRE 0 eee 205 
Igniting precipitates... 6 666 c eect eaters eee eve cen ec cs cea pan 22 


INDEX OF AUTHORS. 883 


EAGE 
Bunsen, Iodimetric analysis of peroxides. ............c0ceeeeceeceees 661 
Separation of arsenic and antimony................00200008- 241 
Grate Gr te OMNI hes Oe chine wb de ak , i Ok 
ee SUN ee ee ree ca & HEA EKER ELS vis 514 
OP re a ey cay aie igig bo hb inte oka e wie 87, 98, 6C1 
Volumetric estimation of bromides..................+.02005. 659 
WME BOI 5b se es 692 
Bunte, Apparatus for gas analysis.................. eit a Keates, 798 
Burgstaller, A., Determination of chlorine.................2.. 000s ees 708 
ween, M., Determination of nitric acid. 5... os ck eile ci le eens 451 
Busvold, N. , Analysis of gases rich in chlorine....................05: 810 
Density of hydrogen chloride ............0..-e eee ceeee 814 
Experiments with chlorine..,,..... pits kk at + xe Ee PA 812 
C. 
\miperie, © cern ts EN i is hae cis ce eciveak odds ee bs 315 
Campbell, Absorption of hydrogen................2.0005. ieureee e - 771 
peeenntnennY We OROE eck Ge unieradt . Gis 720 
meres COMINEIOD OF SLO0R Fe VAs in eae A CH. 413 
Carius, Method for decomposing organic substances.............. 325, 370 
Casa A. teeceriinagon OF thniMm | oe oS cc icin bs VR ik odd 56 
Cavendish, Rare gases in commercial nitrogen.................0.00-. 807 
Chancel, Determination of aluminium. .................6c ccc e eee 83 
Separation of iron azid titanium. ..... 2. cece ee eee e ween. 115 
Coen, W. H., Boric acid im Sitbeates. . © c.ieck eck oe. oe vo ca 590 
Christensen, A., Metallic iron in the presence of oxide... .............. 612 
Christie, W. A. K., Computations in gas analysis.................... 782 | 
Density Of CATDON GIOSING. 4... os. Side. eee 750 
chiorine. .... =... sos EERE SIDR Vo b's bo 808 
Standardization of permanganate....... yy coho ee 98. 
Clarke, F. W., Electrolytic determination of mercury................. 172 
; Separation of antimony and tin....................... 248 
Classen, A., Determination of antimony.................... 224, 226, 227 
SUES CURIE 5s 5.0 ks ce miaewer evil. 380 
SO Bes in ei ciy eacalecds car's balou ne. 147 
POLY SIC MOP aa ee Ge ep h i votes Ce bee whe d vuNR ES 134 
; determination of mercury................... 172 
SUE ick bw Soaas ce Goveadt . 4 234 
Standardization of permanganate................6.. 92, 600 — 
Comment, Colorimetric determination of arsenic..................... 208 
ae ee sles Sip ee ee ede hoes poe es 98, 602 
Croke, J: 2, Ferrous itor Wi SUCRUeS. oS Soak cans ba cedewianiant. -503 
Corieis, Carbon in IO: Gnd ahomk 6. wie oan bihin ess, ies prot hivhn bwe'le pe 399 


Cormimboef, H., Nickel determination. ............0..ee eee eee cee e ee 138 


884 ‘INDEX OF AUTHORS. 


D, 
: PAGE 
Dakin, H. D.; Determination of sine ee ee eee 140 
Daniel, D., Determination: of tron}: 555 ee a eee as 89 
Davila, Density of- nitric oxide: oS 05 ee 802 
Deckert, Apparatus for chlorine determination. ...................... 813 
de'Haén, Titration of ferrocyanic acid 2 ro ee ee ee ee 632 
de Koninck, Determination of ferrous iron......................00-. 504 
Rte Bee @ ee a See eee 460 
Dennis, Technical gaeamatyaiss 3. ee ee ee ir eee 794 
Dennstedt, M., Combustion in open tube......................0..., 421 
Deussen, E., Fusion with potassium acid fluoride..................... 109 
Devada, Determination of nitric acid. ...................04.. AE pS 454 
Deventer, Preparation of nitric oxide. 2. icc ce ae seue 802 
Deville, St. Claire, Analysis of commercial platinum.................. 272 
Water-jacketed ‘tube. oo a 732 
Dexter, Determination of antimony...................... Pte pe Sa (224 
Diefenthiler, O., Volumetric estimation of ferricyanic acid............ 695 
Diehl, Volumetric analysis of lead peroxide. ..................5---0.. 675 
Diethelm, Burette float... 0.....0...0..... ee ee eee eee 529 
Diéts, .H., Analysis of iodides) eo ee Se ee 671 
Dittmar, Determination of potassium. ...........00.. 0c cee eee ee eee 45, 48 
Dits, H.; Analysis of chlerates 020 ee ae eee 669 
Divers, E., Absorption: of nitric oxide) a 803 
Donath, Determination-of ammonia: .:...)...0.0.0 0000, 824 
Separation of ‘tungsten and ti ae 301 
Dormaar, O. M. M., Determination of antimony..................... 227 
Drechsel, G., Volumetric estimation of chlorine...................... 707 
Drehschmidt, Analysis-of gas mixtures ............. 0. cece cece eee es 784 
Apparatus for gas dnalyms oo a ca eee 785 
Pipette... hagas fies os a Sa ee ee 743, 766 
Platinum capney 6 OS er ae 743, 766 
Drouin, Qualitative detection of carbon monoxide.................... 769 
Drown, T. M., Determination of silicon in iron and steel.............. 442 
Dudley, Determination of phosphorus in bronze..................... 239 
Duin, .Determination of eopper 2s ee so oes eee oe en eee 724 
Dumas, Method for determining nitrogen... ..............2..0-0 000 422 
Dupasquier, Volumetric estimation of sulphurous acid... . . pai 692 
Dupré, Determination of :potassium....................... 44, 45, 46, 48 
Durkes, Benzidine hydrochloride: )... 2... 2... ee cc cee ees 714 . 
Duschak, Determination of sulphuric acid...................... 465, 469 — 
Eddy, Determination: of magnesium.-....... 0.2.2.0. 020... eee eee 69 
Eggertz, Precipitation of ammonium phosphomolybdate.............. 436 
Emieh, Determination-of nitric oxide.-..2. 200 20. ee ce et cw ces 802 


INDEX OF AUTHORS. 885 


¢ PAGE 

Engel, Electrolytic determination of tin................ pebiicial sahsasiive 235 

Engels, Determination of potassium. sii. ce ccc ecw ens 48 

Dies, Tear Or tin i er ee ce 605 

Erdmann, Determination of mercury... .. 2... 0.0.00. eee eee beens 172 

Euler, Separation of vanadic and molybdie acids................. 666, 667 
F. 

Fairbanks, Volumetric estimation of molybdenum................ 667, 668 

Feldhaus, Gravimetric estimation of cyanogen.................00000 337 

Ferchland, P., Determination of chlorine...................0 eee eee 812 

Finch, G., Analysis of fuming sulphuric acid......................05. 577 

Finkener, Determination of antimony....... 2.0.6... 00.0. ee eee eeee 224 

, SOROS GUM 6 i525 os itd ain. vise bm 343 

TIMES. GONE ge 5 aos oe lg At aples 439 

Premmeatin. Of potassno: ... «cca sri bai ARG sa 47 

Volumetric estimation of sulphurous acid.................. 692 

Fischer, A., Determination of antimony..... Fe Se ro es ae 225, 227 

E., Separation of arsenic and antimony..... Linesavh «dds ues 246 

N. W., Separation of nickel and cobalt....................4. 162 

Fitzenkam, R., Determination of potassium........................4- 51 

Folin, O., Determination of sulphuric acid....................2.0.-. 469 

Follenius, Determination of cadmium. ................... 6. cece eens 191 

Bones, Determmation Gf boric acid. 5 0: . ea Sa i eta cae Wek. , cies 428 

Fordos, Volumetric estimation of sulphurous acid.................... 692 

Forster, F., Analysis of ferrum reductum.................02.. 0 eeu 612 

Electrolytic deposition of copper.................200 000% 187 

Determination of antimony eee ss). goatee cis... se. 227 

Testing of commercial platinum....................2..... 276 

Franzen, Use of hydrosulphite..... - (Gee... = <j) Ss 5b cay WER. Ss 760 

me ponensie Pima es Sey eae hae Dita. eaves Wied 376 

Determination of carbon dioxide..................0..-55- 380 

POLAR is a. vas GUEST ek bho 4 od 45 

sulphurin sulphides. .:.iiavc2d.i. 6.0.0... 357 

RIP PRCUIILALOBS eee sw ne oe ds ee es cee be 21 

Electrolysis of nickel solutions..................022 20 eee 131 

Separation of barium, calcium, and strontium.............. 79 

EEE ge Iara ane hr gas alee res Sage 80 

Dire Bi ION iss cei piiiciw’e Cooks os 198 

calcium and magnesium...................:. 76 

copper and cadmiuin is) 2. 25. 2. eee sae 202 

Solubility of barium chromate dicsnins ose iia fo. ee 75 

pinomieieh- Warbonate 0.6 ws hee ew ees os 73 

abe 6 sie ics ssa Bias Pics Hale 73 


Volumetric estimation of ferric iron... 2... 2... ee eee eee 697 


886 - INDEX OF ALTHORS. 


PAGE 

Fresenius, Volumetric estimation of iodides. ..............0.000000005 657 

nitrates 265205 5s SESE prea eee 634 

pyroluaite. 2256004 eee See 625 

Friedberger, O., lodates and periodates.................. 0002000000. 670 

Friedheim, Determination of sulphuric acid......................... 714 

VANOGIUIN . os 6. sad ed ee ee 666, 667 

Separation of arsenic and antimony...................... 245 

vanadic and molybdic acids.............. 666, 657 

Friend, J. A., Titration. with ohio fehid REED are ag ... 604, 607 

Funk, W., Separation of iron and molybdenum................ Pots 153 

es 4 : G. 

Gaihlot, Determination of nitrous and nitric acids.................... 826 

Gay-Lussac, Volumetric estimation of silver... ..............0200000: 702 

Geissler,. Alkalimetar. «-. ..-.:.:-:..9S7 Geo CURR os 4 oo be we oe 376 

Bulb for absorbing gases............... io eGwees pinapy Be Cie: 416 

Gelis, Volumetrie estimation of sulphurous acid. ..................4.. 692 

Gerlinger, Determination of nitrous.and nitric acids................-- 826 

Gerster, M., Preparation of fuming sulphuric acid mixtures............. 580 
Gibbs, W., Electrolytic determination of copper ....................- 187 . 

nickel i600) 35) ed. | 131 

Determination of magnesium. ...........0.000 0002 ccc eee eee 68 

manganese: 722507. 2. HUA GR i 

Gilbert, Determination of silica... . . 2.6. 528 Er oO. Pe 487 

vanes). 2 OR. ROE ae - 304 

Gladding, Modification of Kjeldahl’s method........................ 63 

Gladstone,. Density of .methane 545). 03. Gi Se ae 774 

Preparation of ethylene. s22iWiie. Siey aeeers 751 

Glaser, Determination of sulphur in sulphides. ....................4. 358 

Indicators... iiss cues & vee eens eee ae ae reesy oie ee 548 

Géckel, Screen for reading Durettes «oo 65. is0s base cee tidiestor sq aaiee 529 

Veale ys ones ccc dacreciy 6:5 ban DESO TIE ae ee 98, 602 

Gooch, F. A., Crucible for asbestos filters...............-....0.-. 24, 25 

Determination of: boric aemhiiiiid 6 erica F065 See ee 428 

magnesnim S97 evs SE a 66, 69 

manmelidees 6369'S. 2) BOOT Hes. « 120, 126 

HHOSDMONIG BOI. ks Dee te ee 434 

NGMMMANUTNE Sa sk hse 8 304 

Separation of aluminium and titanium................. 116 

10dine Hel ChIOYENB iO... os 331 

lithium from sodium and potassium....... 53 

Volumetric estimation of copper. .............0....-0-- 682 

molybdenum............. 667, 668 

Gott, Direct combustion: of steel : 37 /C55es ee bea iS ea ee 413 

Goutal, Determination of nickel...................- Pia... ieee 720 


INDEX OF AUTHORS. — fa 


PAGE 

Grandeau, Determination of nitric acid................... Peden Vigh he? 456 

pty Soeur oi ih nt ess os Vic bn wok 802 

Gressly, A., Separation of vanadic and phosphoric acids............... 307 

Griess, Peter, Colorimetric determination of nitrous acid.............. 344 

Grimm, Error in measuring instruments........................00.. 536 

DEMARIO 4s ies Bs i. eh aA ios 744 

Groger, M., Determination of sulphur in sulphides................... 367 

Grossmann, Determination of nickel... ... 2.0.0.0... 0. cc cc cece eee 720 

orang. 2 een cunele= rondo et GS As aia 259 

Gutbier, Determination of nitric acid. ...... 20... cece cc ena 451 

CR ici Bs Sacra Bon Oa dee Ge AK 279 

Separation of tellurium and antimony...................... 281 

Cee me ee Or -AbetViene 5 ooo 5. Leia Gita OS oes ean oo hoe bes 754 

MUO hs 6s SEBS i Le . 751 

nitric Oxide... 45 buck Hinweis y by peketeae <6 802 

Pvenspation Of acetylene .....1éi isda s San oesd Gs-mis-vbak bh « dds 755 

H. 

UORIGE. POTION OL ORVOOR 6 iiiiii 6 ia oo a vag ea ee i ce von 759 
Combustion of carbon monoxide................0.202.-+-+-. 766. 

CA LLCR 10k BAA GNBIVAIS occ is Haat ds. 2k cad 782 

Sietesmiination Of: Densene «. idk as asideiins ede oe nee ce eee 753 

DUR oles. Gaia ies WCE. ee 752, 818 

. Separation of benzene and ethylene......... PEARLY SORES Fa) 757 

Hackford, E., Electrolytic determination of arsenic................... 212 

Haén, Volumetric estimation of copper....................0.0e0 eee 682 

Haidlen, Separation of bismuth and copper........... eee ig Wks coon a 198 

GODDCE MAG MAMOIUID 2 i. de les ee ee 202 

TERING, LIOUETUMMAION OF CODDER. ici. Cg ss en ie te ecw 185 

TARUGOORG MA MUROL 2k i ke we ce es se 619 

Separation of antimony, arsenic, and tin...................... 257 

er I RIE Sis. ated ss oa Sec cas ves 252 

REO S00 OR oo iii ie aise wa ak Ree das 255 

Handy, J. O., Phosphorus in steel................... ee OnE pee 588 

Harbeck, Separation of benzene and acetylene.....................20. 756 

Harding, Hydrogen sulphide from insoluble sulphides................. 368 

BIBES, PRONE OF OV ONOMOR 6 os Si Os. 2 ok vse Ks ees 771 

Hauer, Determination of vanadium. ... . 2.6.2... ck cbse beeen 304 

Heath, Volumetric estimation of copper. ............-. 6600s se ee ee eee 682 

Heberlein, Determination of sulphocyanic acid. .................. 340, 341 

Hefti, F., Colorimetric estimation of arsenic..............-5....-.05. 208 

Determination of arsenic as arsine............... 2-2 eee eee 214 

Electrolytic determination of arsenic....................44. 212 

Results obtained in determining arsenic. ................... 218 


Helmer, Determination of sulphur in iron and steel.................... 364 


888 INDEX OF AUTHORS. 


PAGE 
Hempel, Absorption of oxygen. cu SPS Pee ee 759 
Analysis of illuminating gas............ Oe See ee Aa 775 
Apparatus for gas analysis); i400. i AER ae 743, 785 
Combustion of carbon monoxide.................0.0000000. 766. 
Detection of carbon monoxide............0........000..0. 768. 
Determination of carbon in iron and steel................... 404 
fluorine as silicon fluoride................. 829 
Rare gases in commercial nitrogen. ....................000. 807 
Solubility of acetylene in acetone.................. 0.40000. 754 
Technical gas :adnahysia:: 2 6. .cOR IS JE Pe ee 786. 
Henry, Non-explosibility of carbon monoxide and nitric oxide mixtures... 804 
Henz, Determination of antimony................. 218, 222, 225, 226, 227 
boric: acid ..3:.°. 2 (a SL Cee 432: 
Hos A AIRES a aa 233, 235. 
Separation-of antimony and tim... 2074. 91S... ae fen 248. 
Herold, Analysis of ferrum reductum...................4. aS ole oe 612 
Hibbert, Volumetric estimation of hydrogen peroxide................. 700 
iPOMLA. 0.5 BEN Sala wae ee eee 699 
persulphuric acid. .................. 701 
Hill, A. E., Volumetric estimation of chlorine. ....................... 707 
Hillebrand, W. F., Analysis of silicates.............0.0. 5.50000. 487, 491 
Dehydration.of silica. . 0/5 670.44 EU ee 493 
Determination of chlorine in minerals.............. 324 
ferrous iron in silicates........... 504 
iron in silicates. ....0/......0% ... 109 
sulphur and zirconium........... 505 
titemreein 251545 2 A a 100 
NA! 1 CBB Be 3) 2 aoa eS iat 106, 621 
vanadium and chromium......... 310 
Precipitation of vanadiumy 100... ke cece eens 304 
Removal of melts from crucibles ........... on eae 488. 
Testing silicates for barium si 0.06 0 ee 495. 
Hinder, .Decompeaition of silieates......-0 21), PO OUT Ries hasan ep ene 501 
Hinman, Volumetric’estimation of sulphuric acid..................4.. 716. 
Hinrichsen, Titration of hydrofluosilicic acid..................-..40-. 582° 
Hintz, Determination of sulphuric acid. .............. 6.5.00 eee eee, 469: 
Hofmann, A. W., Separation of copper and cadmium................. 200 
Hoitsema,. Determination of silver .i3 2c. ta ee ei. os Oe 706. 
Hollard, Determination of antimony s/s: = sl ced Siuies DORSAL oe 225 
Holliger, Volumetric estimation of sulphuric acid..................... 716. 
Holthof, Determination of. copper:: i024 scowls ar 186. 
Holverscheidt, Determination of vanadium.................. 304, 306, 666 
Hommel, W., Separation of molybdenum and Tungsten............... 294 
Hénig,. M., ‘Titration of -borie:aeid 230.05 sindase tae RSE a 589 
Hopkims, Permanence of permanganate solution. .........-...0.eeeees 90: 


INDEX OF AUTHORS? 889 


PAGE 

Huldschinsky, Separation of cobalt and nickel... .............000cees 165 

Hulett, Apparatus for distilling mercury..............cccceceecceees 748 

Determination of sulphuric acid. .........6...0....0006. 465, 469 

Hundeshagen, Precipitation of ammonium phosphomolybdate.. ....... 436 
I 

Ilinsky, Separation eS, MRR i aa ead cowed 165 

Tlosvay von Nagy. Ilosva, Determination of Acetylene................ 755 

INUTOUS MOI one. d 2 fens oa «as 345 

Inglis, Determination Ie i et a he ge ge 677 

Inhelder, A., Determination of BONING i inieir hes as pti hs ore dee napa wo 227 

Preparation of sodium sulphide reagent... ...,...<.-sc0is-- 225 

Isham, R. M. , Determination of titanium............. beings mht cate 118 

Isler, Specific gravity I ROMER 6 dines euiGe sh 204 46's s'0s 345 838, 839 
Jur cise J 

ST eee Decca et coe oes cheek ce Oo eee ts 797 

Jamieson, Volumetric estimation of copper................02eeeeeeee 672 

wes” erento OF Hint ee ce ee eee c cece No been 146 

ounneern, © ., Decomposition Of silitates: ect oe sce e cece oe 490 

Determination of alkalies in silicates.................... 501 

i Crore tl MOALIE. 323 

water in fluosilicates.................. 484 

ae WOM ce eee RAN Cooe nee 512 

_ Reparation of bismuth from lead........... eae agi OSE Ea orig 195 

Wore sey Cee INS  e eae 332 

selenium and tellurium.................. 279 

Jarvinen, K. K., Determination of phosphoric acid................... 434 

Jarvis, Volumetric air On DICE rs es i ie kk cee eae cs 720 

Jawein, Electrolytic determination of cadmium....................-. 189 

Jeffery, J. H.; Titration with permanganate. .................... 604, 607 

Johnson, Volumetric estimation of nickel.............. 0.2... eee ee eee 720 

Johnston; Determination of sulphur in iron and steel ................. 366 

SUN ee a ee eh ee acc ck seers e ve cehs 608, 637 

Jones, C. C., Titration with permanganate. Pt Ge Cpa eON en Cen ae 604, 607 

Jones, H., Titration ee es iv chp chor be aunecesdr ces 589 

Jones, W. A., Carbon monoxide-copper compound...................- 763 

a, OY ON BE PE ee as cbc vc ccccecececepeeet es 487 

Jorgensen, G., Determination of magnesium. ..................0000- 67 

DR pape B a oe ie ea ins fs 434 

Titration of boric acid... ... 2.6.52: doig gare is Rt ir 589 

Jungfleisch, Distribution of iodine between two solvents.............. 658 
K 

Kanter, Dehydration.of. silicic acid .) 0... piiscsine etiew ender scenes ee ae 


Kassner, Titration of barium peroxide, .....s..ceeee eee cece eee tenes 628 


890 © - INDEX OF AUTHORS. 


; PAGE 
Keen, W. H., Electric farnace. 20.5 268.5 2 Hi ss PR eS, 414 
Keller, E., Determination of selenium and tellurium in copper......... 284 
Separation of selentum.and tellurium..................... 282 
from copper...... 280, 282 
Kempf, R., ‘Titration of persulphates: <<... . 0... see ee 630 
Kerner, Separation of halogens and cyanogen................¢2+20+005 339 
Kessler, ‘Titration with permanganate... occ cs ec ensces eis cebu 604 
Kingzett, Determination of hydrogen pergxide nines ees? eo eert ae ee 680 
Kinnicutt, Oxidation of carbon monoxide. ........0..ccccsecereetves 767 
Kjeldahl, Method for determining nitrogen... ..................00-. 62 
Klapproth, Determination of antimony. .................0 eee eee eee 225 
Kling, Determination of ‘potassium 20°F So oes oc ests teen 48 
Knecht, Volumetric estimation of hydrogen peroxide................. 700 
WOR ess sy Oe ee 699 
péerauiphurie acid .. v6 5 cc cee ve eee 701 
Knerr, Electrolytic determination of mercury... ...........e0seeeeees 172 
Knop, Determination of ammonia... . 2.0.2.5. -2s er cr ever ecetececes 822 
Knorre, Separation of nickel and cobalt. ..........5+-cervceercseeess 165 
Koch, A. A., Determination of fluorine. ........0.2 2. eee eee eee 476, 830 
Separation of phosphoric and hydrofluoric acids.......... 474 
Kohlrausch, F., Reduction of weights to vacuoO.............-eee4ee 13, 14 
Testing of wweiebGiiss:.c dicen es es veeusua eee eee 15 
Koppeschaar, W., Determination of phenol. ...............eeeeeeeees 695 
Korbuly, M., Absorption of DERZeNE «0/46 £5 kek Ss eh ee kes ee 753 
Kramers, G. H., Precipitation of zinc sulphide. ..............eee0-0- 160 
Kraut, K., Determination of picked 43()25553 304 6009 GAS Crea be 129 
Kreider, Preparation of perchloric acid... 2... 20. ccesesouescvcs veges 51 
Kreitling, Use of burette floats... 5 sicicis, fs dua s ericeesenetebors dawns 529 
Krug, Determination of sulphur in iron and steel.............0---+--- 365 
Kiister, Determination of sulphuric acid... ...........seeececeeeeees 467 
Sodium hydroxide solution free from alkali carbonates......... 555 
Titration of alkali carbonates. .. 2.6.4... ceecceeccvesse 562, 564 
Kuzma, B., Separation of selenium and tellurium from metals.......... 280 
, L 
Ladenburg, R., Determination of ozone, ...........eseeeeeeecece 677, 680 
Lagutt, E., Determination of sthyvers. 0%. s tps esis «e's 0ae aa eee 318 
Langbeck, H. W., Ortho-nitrophenol as indicator.............6.-00+4: 543 
Leberie, Determination of I9@Rs 5 sss Kiss 9 2 5 0 Sass 6 0:5 2 eee 89 
Leeco, Calorimetric determination of iodine. .............6e eee ee eens 661 
Lecrenier, Determination of antimony......... shy 0 9.5: shee aes eae 225 
Ledue, Density of carbon dioxide ss oii w ok Peike ve ik een 750 
ebbing 50555220 hs as sa to ee 808 
hydrogen -obloride... .. Oe ak as i. eS 
hydrogen sulphide . . 2uisi9ee. .wliss. lo. caw 348, 816 


INDEX OF AUTHORS. 89x > 


PAGE 
Leduc, Density of sulphur dioxide: ...:......... 0. cscs secs cc cscceee 815 
Lefort, J., Aqua regia for dissolving sulphides...............00e+0e00- 362 
Lenssen, Titration with permanganate............ i eaeite seceded dad 604 
Volumetric estimation of ferricyanic acid................-... 694 
Leith, Rema RON ni Su iid fico eS etn ccvces 773 
Lavo; DIGter SURO OCICS no oi a slaicials 0 ves ols hae 206 
manganese in pyrolusite...........05.cce000 624 
aahabality of magnesium ammonium arsenate.............. ‘noe ae 
Levy, Volumetric estimation of copper. ......... snc ececceseccvceccies 672 
Lieben, Volumetric estimation of formic acid................02.0005- 626 
Liebig, J., Determination of carbon and hydrogen in organic compounds. 414 
sulphur in organic compounds.......... . 371 
Separation of nickel from cobalt...................... 163, 164 
NO FE CVBOORND As 2 cat Sine pice w os eek ek 711 
Depenei, Dreterounmetion Of nitric Acid «6 265 os ope osccae Siu oie hn ede kb oe ws 460 
Lindemann, Absorption of oxygen by phosphorus..................-. 759 
Luckemann, Determination of arsenic....... Riss Dis etic ee EE cin a 208 
Loose, R., Methyl red.......... OBR EAE Ue seh ee OM ice enon ali 543 
Eipekann, C., Determination Of ZING... ook vind Fs sch nen cha teiece dade 140 
SAG isi 41, BOON TIR CODDER ORCBEG 55555 5.) i KK Gian sire ore 8 725 
Determination of copper by iodide method............... 682 
potassium ecyanide........... 724 - 
ON sy a en ton eS ois to nee n ts 726 
fees, BeAr utiOn OE MISTANIED BNO FORE ooo nin kee: cic e coi cins wares oes 195 - 
Léwenthal, J., Precipitation of tin.............. Neer $ atahete yi a ce cs hea aS 232 
Titration with permanganate. ........-...-....++5+-- 604 
Luckow, Determination of antimony..............5...-s00se2eeeyee 224 
RE ae a a a a 147 
Electrolytic determination of mercury....................+- 172 
Sapec, G.; Analymie-of fuming, acide. «..:. 205 6 5 oe ae ce ees eee 579 
VON coy Ate haa Sin oho sD ee, +» ce Ob WB 624 
Dehydration of silicic acid <. oo2ok is bs vis eae dances Sas Pe 487 
Determination of carbon dioxide..................+6. 388, 391 . 
PN ROMs ae ok es a en ie 345, 626 - 
Ms WUNUREY MA TEVTIOE oats cece Cee nce win es nee 362 
SS 2) eae eae Pe are 575 
Separation of ethylene and benzene............ Wiens eet 756 
Soluble and insoluble silicic acid..................-..: ce ORE 
Standardization of permanganate..................-0-06: 92 
Specific gravities of ammonia solutions.................... 841 
ERO MONIN igs sass tna as 2 ks As x este 833 
EIOEVETIRE BOOP AIUS 65 shh. - oh vo do 3 Ors bt eo ey Bea 387, 823 
ER, RIOR ECONOMIES ee es Sie wk nee ses haere oy ny 669 
SC Sie ats ae a eh ee bv cee koe ecn eset. 677 


892 INDEX OF AUTHORS. 


| M 
PAGE 
Manchot, Titration with permanganate. ....5...... 0.0. cece cece eee: w05 
Marchand, Determination of mercury... ..........0.c0cceuceueeuees 172 
Marchlewski, Determination of carbon dioxide. .................0204.. 388 
Specific gravities of strong acids................. . 838, 839 
Margosches, B. M., Analysis of iodides... 2.0... 2... 0. ccc cee ee eee 671 
Margueritte, Volumetric estimation of iron................... 89, 99, 603 
Marquardt, Analysis of ferrum reductum. 6. .............2..0 ee eee 612 
Marshall, M., Colorimetric determination of manganese............... 128 
Martz, ‘E., Oxygen im pea water 02 28, ee aves 740 
Mascazzini, Determination of antimony.............. 0.0.0.0 cece eee 224 
Massaciu, Determination of chromium. ...).............c2 cence ees 103 
May, W.-C., Drying lead peroxide deposits... 2.2.0... 00.0000 0c cece eee 178 
Mayer, L., Determination of: hthrum:.*.) : 0. 2220 eos) i eee 56 
Purification of mereury «52 Oo OS Pa ae A ae 747 
Mayr, D. K., Determination of antimony... ...... 0000000000020, 227 
McArthur, Determination of potassium....................200000: 45, 48 
McKay, Determination of arsenic: 22.20... sc. s PEI 205 
Mensel, Volumetric analysis of iodides................ LISSA echt ae 656 
Merck, Determination of metallic iron in the presence of oxide......... 611 
Merling, Determination of lithiuii) 3.) ees eae as 56 
Metzl, Determination of antimony: ... 2... 0.52. ie cee beet ets eee 221 
Separation of antimony and tin......... WEP OO E GES ee 248, 250° 
Meyer, J., Determination of manganese....................00 sence 125 
Meyer, V., Nitrous oxide: <: 23.25 205. oe ae eee 800 
Michaelis, Separation of arsenic and antimony. ...................... 245 
Millberg, Dehydration of silicic acid. 6.6 ee a 487 
Separation of soluble and insoluble silicic acid. .............. 507 
Miller; Notes on assay. 3 oe ee orcs cae oe os Sane eee 264 
Miolati, A., Electrolytic determination of mereury................... 173 
Misteli, W.,; Preparation of etivylene. <s00 5 P es  e eee i 751 
Mitscherlich, Determination of ferrous iron in silicates. ............... 504 
Mittash, Separation of iron and manganese.................00000 eee: 154 
Mohr, ‘Definition of ‘liter. 625. Ae FE oeias satire 521 
Titration with pyroluaties soo ee ee ek 20S toe eee 625 
Mohr, C., Volumetric estimation of irom... ... 0.2 ee ee ee 681 
Terhicyanie Whig? 2) Pe Ries 694 
Mohr, F., Determination ‘of bromine. 7. oe eas os 709 
CHIONMHE ek oe Ee eas 708 
Preparation of lituvas solution; TO Aa er 545 
Moller, ‘Titration of fuosiligie seid | os 055 50 2ss ses a ee eee 583 
Moore, Volumetric determination of nickel. ..................2..0055 720 
Moore, C.J,’ Patification of mercury. 400.251.2776 Fs oe es ere 747 
Morse, Permanence of permanganate solutions..................0085 90 
Morton, Volumetric determination of sulphuric acid. ................. 716 


INDEX OF AUTHORS. 893 


PAGE 

Miiller, E., Analysis of iodates and periodates................--+40-- 670 
Determination of ferricyanic acid. ...............6.2-+.6- 695 
Miller, M., Separation of tellurium and selenium...................- 279 
EE ENE lie aera Perr mer oe 301 

Miiller, W., Use of benzidine hydrochloride.....................+---- 714 
Munroe, C. E., Crucible with platinized felt. .................--0200 27 
Muthmann, Separation of tellurium and antimony..................- 281 
Dayne. Lesune commercial PiAtIMUM : . . 6 io. a bie dee n eedwake Sewe$ 276 


N. 
Naef, Specific gravities of strong acids..................0.0..... 838, 839 
Pieper, ©, Determination of arsenic. soon 4 5 6 siw tps os einen be tenes 205 
Separation of arsenic and antimony....................++% 243 
ANS REED EEN eer Ren Ho RRC PEE 255 
Neubauer, Determination of cyanogen and halogens.................. 339 
BGIIOTIC BONE os oto oss pc ees so Vaden 434 
Precipitation of potassitim. «.. 4. < cds vs cca wiki $4 ames 47, 48 
MEET, AMINOE WONAICS 6 ha ok Na vb cee oe aw es 2 od 782 
Neuman, Electrolytic separation of copper and cadmium.............. 203 
Nicloux, Oxidation of caroon monoxide... . .< ...... joc ele ede ew es 767 
Noyes, W. A., Determination of sulphur in iron and steel............. 364 
Nydegger, Determination of sulphuric acid with benzidine............ 714 

O. 
Oberer, Separation of copper and cadmium..............-........--. 010 
Oechelhauser, Determination of ethylene...............:........-... 752 
Separation of benzene and ethylene..................-. 757 
Oettel, Determination of fluorine as silicon fluoride................... 829 
phosphorus in bronze................-...-.. 238 
Offerhaus, Determination of chlorine gas by titration................. 811 
NE MN Oe ee ee ice gu lg pla a hie ars hk oS 133 
Oppenheim, C., Absorption of nitric oxide..........................-. 803. 
Popa Pest RCN 100 RN URN ee, ce hinin a who chews 797 
Osann, Volumetric determination of silver..........-.--..-.-.----.-- 706 
cet Aocrenematiess Or arian 2. ek a 8 225, 227 
PTO se ERO a as s ob dle ee nres + oi 455 
PPI DIONE oo 5 vine t orcieinaete dy mane Meee 156 
UESty aI SRO ge ak he cae ws 18 

 ¢ 
Palmera, W., Iridium in commercial platinum..............-..---.-- 275 


Panting, Determination of carbon monoxide...........-...-.-..-..-- 762 


804 INDEX OF AUTHORS. 


, ‘ PAGE 
Parr, Volumetric determination of copper.............20.eeecceeeuee 673 
Parrodi, Electrolytic determination of antimony..................... 224 
Pattinson, J., Determination of manganese in iron and steel ......... 642 

SUINOVIG ONE. ee eee 467 

Paul; Drying OV@R aS ic oe ee oe ss ea 33, 34 
Heating antimony pentasulphide......... gt ds Caisse cea eee 220 
Pease, Determination of phosphorus in bronze....................... 239 

' Péchard, Analysis of wolframite.........: eo socks ae 296 
Pelouse, ‘Titration of sittrntess 25 ee ea ee sp ee ce aa eee 634 
Penfield, S. L., Determination of fluorine......................-0005. 476 

water in mhestes 55 ss eo 512 
Titration of hydrofluosilicie acid. ..................... 582 

Pennock, Volumetric determination of sulphuric acid................. 716 
Penny, Determination of iron by dichromate........................ 641 
stannous chloride / oo... 5.5.25 eae 697 
Perillon, Separation of tungsten and silicon. ............... tee See 302 
Pettenkofer, Determination of carbon dioxide in air.................. 593 
Pettersson, O., Determination of carbonic acid....................... 384 
carbon in steel os 00s ps ee 405 
Pfeiffer, Separation of benzene and ethylene.................... 756, 757 
Philipp, R.,; Analysis of tungsten bronzes... 0.3 D2. .s elec ee eee 298 
Determination of sulphocyanic acid..................... 340 
Colorimetric determination of cadmium.................. 189 
Separation of copper and cadmium...................... 203 
Phillips, Condition of sulphur in iron and steel....................... 352 
Determination of silicon in presence of silicic acid............ 513 

‘ Philosophoff; P., Analysis of chlorine gas..................00.000000s 812 
Piloty, O., Separation of arsenic and antimony...................... 245 
Pincus; Determination of phosphoric acid. ..............6..2..0.005, 718 
Poggiale, Titration. of pyfoniaites 70" 200 eT ose ee ~ 624 
Pollak, L., Explosibility of carbon monoxide—nitric oxide mixtures... .. 804 

Permanence of ammoniacal copper solution................ 756 
Solubility: of nitveun-oxnde O61 oo chi Sas a ast oa 801 

- Potain, Determination of carbon monoxide in air................605.. 769 
Preusser, Analysis of iron-tungsten alloys. ... 6.1.0... cece cece eeeeee 298 
Prince, Analysis:of iodide. 56032 O00 cis os ch vce veers teat caea es Cans 671 

Q. 
Quasig, Ozone determination.......... PESO TTTTPEET TT eee 
R. 
Rammage, Determination of manganese. .........++eeeceseee Nee ghet 616 
Rammelsberg, Analysis of wolframite....... Sd eee REE 2 C4 > 96 e* Fe 8e Og . 297 
Separation of lithium, sodium and potassium. ..... By siye 55 


INDEX OF AUTHORS. 895 

: PAGE 

Raschig, Titration of hydroxylamine...... SE its eat pus asia yaaa 631 
sulphuric acid by benzidine..................... 714 

WARIRSONIS WOME Cs ioo spo Vi hoo 8 POT A 693 

Rayleigh, Density of carbon dioxide. .................00000 000 ee 386, 750 
Io IS 655 5 8 60a 6 is is is. SO edgy 770 
Mr ONIN rt a ee ii i a 800 

Recoura, Determination of sulphuric acid............0...0..0..000000. 467 
Reddrop, Determination of manganese.......6..........0.0.0 000000 ue 616 
Regnault, Tension of aqueous vapor. .........00 00 cece eee ce ees 842-845 
Reich, Determination of nitric acid...... eee yg oe) Beene an rae eae ee 453 
WR CIORIGG 6. ee Oe a 815 

Reinhardt, Titration with permanganate........................ 607, 609 
Retgers, K., Determination of manganese.....................20..5. 125 
Reuter, M., Determination of sulphuric acid................... 0.0... 716 
Richards, T. W., Precipitation of calcium in presence of magnesium.... 76 
Solubility of calcium oxalate. ... 2. ..6.2.0. 000.0800. 70 

a WEN 5055S BPE PEPPERONI 15 

nts, Tenn ee MONI RS, Le Ce as 264 
Riegler, E., Standardization of permanganate. ................0000005 599 
Wess, Determination of antimony.......6...0....0006. 0880s. iis 224 
Mavhes, Determination of nitric.acid......... 20 2. eee a iaaiaa 460 
Rittener, Determination of carbonic acid. ............... cece eee ees 391 
Men NIN OL ORE NON k FO ees e788 
Determination of copper.......... sites SUA vga ‘ave 186 
Separation of copper from cadmium................ Trees 202 
Rohmer, M., Separation of arsenic and antimony................ 245, 246 
Romijn, Titration of formaldehyde....................0.4 Serer oa 694 
hydroxylamine salt a5 bic se sea s ny eae 581 

Rosanoff, M. A., Determination of chlorine.................63 eee ree 707 
Roscoe, Precipitation of vamnadium...............02 00.005. «+s+ 304, 305 
Rose, H., Determination of ammonium as chloroplatinate............. 58 
. bismuth. .... eee OOS RERIROU ER secsece 281 

CN a ee ae Veeseas 338 

ferricyanide 3: Po... ib oe cst Ho04 eer re . 848 

sulphur in sulphides............. erie 359 

water in fluosilicates.. 2.0.2... 6 0 cee ee eu 484 

Precipitation of vanadium... 2 ee i eC 8 304, 306 
Reduction of mercuric salts................0 040005 iG 6a Be 171 
Separation of antimony and tim... . 2.2... 0:6 eee eee ees 250 

>, aTdenic; aNd: tan cose asl. Acre 256 

barium, calcium, and strontium...... Se 79 

copper and cadmium. .............. (aa bee 202 

molybdenum and tungsten... .........6...005 296 

phosphoric and hydrofluoric acids. ............ 474 


Volumetric estimation of sulphurous acid... . 2.0... 0.0.0 0085 692 


896 INDEX OF AUTHORS. 


PAGE: 
Rosenbladt, Determination of boric acid. ..............00, Peet even es 428 
Rosenheim, Separation. of copper and nickel......................... 165 
Réssing, Determination of antunony © / eee ae eS 222: 
Separation of antimony. and tin....................... 248, 255 
Rothe, Solution of ferric eblaride im-ether...... 6 ).0200 00 oe 167 
Rothmund, Determination of chlorine. .......00.0000000. 0.0.0.2 e ee eee 708. 
Ruegenberg, M., Separation of molybdenum and tungsten............ 293. 
Riderff, Electrolytic determinaticn of mengury...................... 122 
Rupp, Methyl red.........- je senen ens SES TCL as oe. eee eee 543. 
.. Volumetric estimation of sulphurous acid..................... 692 
Rutter, Analysis of -chierates...: 305i Ais a 669° 
Ss. 
Sahlbom, Titration of hydrofluosilicie azid..........0.. 0.0000. ee ee 582° 
Sand, H. G.8., Electrolytic determination of arsenic................. 212 
Sanger, C. R., Colorimetric determination of arsenic............. 208, 210 
Sarnstrém, Determination of carbon in steel. ....................004. 399 
Schaffgottsche, Precipitation of magnesium ammonium carbonate...... 69 
Scheen, Determination of antimony » 3s) .¢s8605. Sci eee eee 227 
Schellbach, Burette. .:02004 260 fis 46a cee ee ee 529 
Sehiff, H.,.Asotometer .... io 6. cokes he A ee 423 
Schirm, E.,.Determination of aluminium. ...................20..005. 85 
as he ee ChYOMMIUM: 3 iio ds eS a Re 103. 
Schloetter, Examination of electrolytic chlorine. ..................... 812 
Sehloaser, W., Correction: tables: . inet won enw a va es 519, 533 
.o> Draining of ousetiesss finisui aise 3G ie iar 528 
wee. Exror in ‘measuring vessels. side oul. enti See 
Pe raey Meniscus.corrections ..iis2 aiguciennikds les ee 
Schléssing, Separation of potassium and sodium....................0. 50 
Schmidt, E., Analysis of ferrum reductum.........0...20. 00. e ce eeeeee 612: 
. .EL,. Separation of barium and strontium.................... 81 
Schmitz, B.,.Method of precipitating magnesium ..................... 67 
ccs ag ee plheapharnened 6... kaa 434 
Schneider,. Direct. combustion of steel.................. (Se eka oe eee 413. 
Manganese deternemiaiome 6 ing oo is ae 616 
Scholler, Determination. of chromium. ....:...6...0.0. 0000 c cece cecuce 103 
Schonhein, Eatimation of og0ne.. «io... sae eee a nein Seinedh oe 676. 
Schrauth, Determination of chromium.................000.0 cee eeeee 103. 
Schréder, Separation of tellurium and antimony..................... 281 
Schroter,. A., Dehydration af silieio seth ic 6 ei os ee 487 
Schritter, Alkalimetersii:i:ci%:neseeieles ci aethes iA 8 oe 376 
Schucht, Titration of hydrofluosilicic acid... ................2000008- 583 
Schudel, Determination of manganese... ............. 00 c cece ee ee ees 120: 
Schudl, Standardization of permanganate...............-.0:eeeeeeee 97 
Schulze, Determination of nitric acid. .................. pid yes ee 456: 


INDEX OF AUTHORS. 897 


PAGE. 

-pebnize, Titration of pyridine bases, 25. 2. oe ig coda veowacveecie 561 
Schweitzer, P., Solubility of barium chromate.....................0. 75 
Seeman, L., Precipitation of gold by hydrogen peroxide............... 258. 
Separation of gold and platinum........... oe Page Se 272: 

Shimer, P. W., Direct combustion of steel........................... 418 
Smith, Method for separating zinc from nickel, etc................... 158 
Smith, E. F., Electrolytic determination of cadmium................. 190: 
EERSTE lene TEES Bare 172 

| Separation of molybdenum and tungsten................ 293. 

Smith, J. L., Determination of alkalies in silicates.................... 496. 
Smitt, A., Be Misa iilens of catbonin-thedkis soi. Buk 405, 
Snelling, W. O., Use of a Munroe crucible. ..................0.000005 27 
Sonnenschein, Determination of phosphoric acid...................., 436. 
Sonnstidt, Separation of potassium and sodium....................., 50 
Sorensen, Standardization.of acids ..... isis beside siwcis dows 548. 
pertianganate..020i seh acn ti od. Stes ., 597 

Beret, Determination of ozone: 6-35 rt Cee BA ewe awed 680 
Sexhlet, Fat extraction apparatus... 0.020. Ssend noose yesiik ok es 236 
Spear, E. B., Electrolytic determination of zine.................2..... 146 
ney Ay. Lat WE OTIC BOING OEE i oe oe ie sc a vee 589° 
enter. Peterman Of Sine sis. cay eo ei. os oe 145. 
Stahrfoss, M., Density OP rt cas 22 i ia elo Fe eo 754 
IR sts ite PEGS ase gc ke es 751 

Preparation of acetylene.................. aes sce 755 

Stas, Analysis of commercial platinum.........................0005- 272 
Steen, .O.,.Separation of lead and bismuth........................... 197 
Steffan, Determination of phosphoric acid....................... 439, 440: 
Volumetric separation of vanadium and molybdenum..... 667, 668 
Stieglitz, J., Theory of indicators........ REE SSO SS, SR a aE 540 
Steinbeck, Determination of copper. 75.532... eect eee sees 724 
Steiner, O., Determination of chlorine by titration.................. . 811 
Stillman, T. B., Analysis of incandescent mantles.................... 512 
Stock,.A., Determination of aluminium............2.....-......20045. 84 
NNN is es SiS Cs ns oe 103. 

Separation of arsenic and antimony....................... 245. 

Stoffel; M., Determination oftead | 6... oie. es ce aie ee ead 179 
Shokan, Abenrpipan ol teer ee a scan edad $i bo ead. 753 
STINE a KS Sitka vilsd Ubaee aang ees. 752 

Determination of carbon Wiebe Oe iain dic wescld : Tecdvack 764 
POreCl WOM 6 cai So tiara) As on 5S 94RNS 504 

Separation of ethylene and benzene. ...............2.202000 756 
Stromayer, Separation of barium, calcium, and strontium............. 79 
ear? PRIN AMON ifs es oe See es be os 
Subech, A., Silicic eid: i OM er eh BN a sk bw ob kG 508 


Swett, O. D., Rie OE PO MUONS Sk nk os ho gore On ne RS 27 


898 INDEX OF AUTHORS. 


:¥ 
PAGE 
Talbot, H. P., Removal of melt from crucible. ...........0...0.0 0000s 488 
Tamm, H., Determination of manganese... 1.0.0.0... ck cc eee cee ees 121 
, BAN ies SES AE Oe A a ee 140 
Than, C., Standardization of sodium thiosulphate solution. ............ 647 
Thénard, Ozone. «3654 Hartke ig OO sds SAAS Pe ee 680 
Thiel, Determination of sulphuric acid. ...........0 00.00 ccc ew cee eas 467 
Thiele, J., Apparatus for chlorine determigation..................... 813 
Determination of arsenite (i56015 She. aia a 205 
QtOR Yi FS ca i ee 223 
Thomsen, J., Heats of combustion... . . t TT UE HIRE 845 
Thomson, W.,-Electrolytic determination of arsenic.................. 212 
Thorpe, T. E., Electrolytic determination of arsenic.................. 212 
Tiemann, Determination. of nitri¢ acid: )60 56 8h20. 4 ORG 456 
Tomicek, Determination. of argenic..i. 0... 2 2225S Ns aa 205 
Tootmann, S. R., Determination of arsenic. ..............0.2.00ee0ee 212 
Topf, Volumetric determination of lead peroxide..................... 675 
Toth, J.,. Titration of. phenol:.; 2x é455i5.2. 3r ieee ee 697 
Treadwell, F. P., Absorption of benzene. . 252.5... 0048 ae 753 
ethylene... lie. 028, Fe 752 
Apparatus for chlorine determination. .............. 808 
Collection :of.gas samples... . 2.0... 0... bie e cece eee 733 
Colorimetric determination of arsenic...... ere 208 
Computations in gas analysis... .......... 0. cece eee 782 
Density of carbon monoxide...............0.00000- 764 
oblorine ii as ine eines areata 808 
Determination of carbon monoxide................. 764 
ferrous iron in silicates............ 503 
hydrofluoric acid. 6) #20444 ..4.4453% 476 
OD .4340:77 6 Guise ee 89 
DickGhsiwis oi. a sehra d ah 137, 138 
CONOR HE IN aA Sia 5 See 677 
Gases from defibrinated blood. ..................... 740 
Separation of ethylene and benzene................. 756 
Titration of hydrofluosilicic acid. ................... 582 
Treubert, Determination of biamuth.......... 000.45 oot ease etd. al 181 
Tribe, Density of methane.................4% WE hh ne Aas ee 774 
Preparation: nf ethylene. iii. obs oie SRE ea es 5 cas © ee 751 
Tromsdorff, Determination of nitrous acid in water............... aor 
Tschugaeff, L., Determination: of nickels 240036 63. ss ee ei ee ee 129 
Separation of nickel and cobalt...................... 161 
MANZANEGE 36 6 Vas. HST 165 
RNG 8 NEN es ee 165 


INDEX OF AUTHORS. 899 


U. 

; PAGE 

Ukena, Determination of manganese in steel............4. Prise reais: 619 

Urech, W., Determination of bismuth....... vrececea EP viet ies ve 182 

V. 

Vanino, L., Determination of bismuth... .. SS Motives Vets hoEw lee 181 

Precipitation of gold by hydrogen peroxide............... 258 

Separation of gold and platinum..............ceceeeeeee, 272 

Van Name, R. G., Determination of copper.............e0ceeeeeeees 186 

van’t Kruys, M. J., Determination of sulphuric acid.................. 464 

Vernon, ft H., Anerems of fuming acids (2 ie ei ce cc eee es 577 

Viogili, J. F., Solubility of mangesium ammonium arsenate............ 208 

Vogel, Detection of carbon monoxide... ............. 00. cece ee eee 768 

MUNG rales oT SUH OE 2b SR OEE, 165 

Voit, Determmation of sine... 288. ree ei eo 140 
Volhard, Analysis of sulphocyanate and sadeesia' og Saye cee penumeery: 342 © 

Determination of manganese as sulphate.................... 120 

Precipitation of mercuric sulphide....................02000- 168 

Standardization of sodium thiosulphate..................... 648 

TN IGEMUMONL Ch ROUNIING 5 AS BER oe 8 AS 709 

os ey Og SANS chs oa sea Ag Ra ae a or ez 707 

OSE ee DOr OTA EAL APEC oP an Ge ape 710 

Re ee re ES 709 

Eo ham is Reerer cy Were: Rerimee ts ar ere rg ane 612 

ae a eT a 705 

WIRELINE es 692 

PIONS Sei SE SER ee 712 

WP OPIORROMIAIS ree. A Os 604 

Transformation of chloride into oxide..................0005 142 

von Girsewald, Electrolytic determination of cadmium................ 189 

Separation of copper and cadmium.................4. 202 

' -y. Jiiptner, H., Determination of phosphorus... ............ 0.000200. 436 

v. Knorre, Combustion of gases using copper oxide. .................. 797 

Mitre Re 67 PE PT at AAS OES 803 

Determination of manganese in steel...................0005 620 

euinhuric Bhs. 2. NPR es 714 

TNR RE SE OR eos 290, 291 

v. der Pfordten, O., Abeorption of ostyaei 2. eV 760 

Precipitation of mercurous tungstate............. 289 

v. Rath, G., Precipitation of mercuric sulphide...................... 194 

Vortmann, G., Electrolytic determination of mercury................. 172 

Determination of antimony ee he) 204A PO os eee 221 

Removal of sulphur from precipitates................. 169 


Separation of antimony and tin.................. 248, 250 


goo INDEX OF AUTHORS. 


FB 

PAGE 
Wade, Determination of carbon monoxide. ...............00. 000 eae 762 
Wagner, Determination of ammonia in ammonium salts.............. 822 
Draining of biirettess 3... be. Ss ss ee 528. 
Indicetens...¢ 6054.6. Wea Sis es ae gee ete eee 548. 
Standardization of sodium thiosulphate solution............. 647. 
Walker, Permanence of permanganate solutions...................... 90: 
Wallace, Volumetric estimation of iron. ¢.............2.060 0. ccc eees 697 
Walters, J. H., Jr., Determination of titanium....................... 101 
__H.E., Colorimetric determination of manganese............. 128. 

Warder, R. B., Behavior of sodium bicarbonate solutions toward 
phenolphthalein. : iiss Sivaac ban se A FE as 562. 
Titration of alkali carbonate and bicarbonate.......... 566. 
. carbonate and hydroxide................. 564 
Washburn, E. W., Iodimetric titration of arsenic. .................45. 650: 
Weber, H., Determination of sulphuric acid... ..............0...0.4. 469 
Weber, J., Ignition of calcium sulphate... .. 2.0.0. .0.0. 00.00.0000. 008 71 
Wegelin, A., Composition of sodium sulphide solution................ 226. 
Determination of irom. bis ofs Sot Ge ee 8° 
Witrie A000); .:i)034 &) sores OE Se . 458 
Weitnauer, H., Determination of manganese................-.....05 126 
Weller, A., Determination of titanium ....................... 100, 504 
Titration. of aptenOnY a es PD, a 687 
Wells, Determination of copper by potassium iodate.................. 672. 
Wense, Separation of potassium and sodium...................... 50, 51 
Wherry, E. T., Determination of boric acid in minerals............... 590: 
Whitby, G. S., Solubility of silver chloride... ......0............045. 317 
Wiernik, Specific gravity of ammonia solutions. ..... iss ti oo ee 841 
Wiborgh, J., Determination of carbon im steel....................000. 405. 
’ sulphur in iron and steel... ............ 354 
Wilfarth, Modification of Kjeldahl method.......................0.. 63 
Will, Alkalimeter ... 3), .:\ pep leow-beaotociish: Bs Cee es cee 376. 
Titration of pyrolusite........ MGS aba Tt EPSON ake, Sas Ra 625. 
Wilner, Determination of metallic iron in the presence of oxide........ 611 
Windelschmidt, A., Determination of nickel..................... 137, 138. 
Winkler, C., Absorption of benzene. 3.2.5.0... 00.8. e eee eile eens 753. 
Combustion. of hydrogen siacicws..6.. 00.6 oa eee 772 
Detection of carbon monoxide. ...............eeeeeeeeee 769 
Determination of chlorine by titration................... 811 
Titration of alkali carbonate and bicarbonate............ 565. 
hydroxide iiss .,.46G% 563. 
bicarbonate in the presence of carbonate...... 592 
Use of gause electrodes: i: ites ks cians aus -. cia 134 
Winkler-Dennis, Method of technical gas amalysis.................+4. 794 
Winkler, L. W., Absorption coefficient of hydrogen. .,..............4- 771 


INDEX OF AUTHORS. gor 


PAGE 

Winkler, L. W., Absorption coefficient of methane................... 774 

BUN OE ee ios Was Se sh ak 803 

MUON a ye 8c hese 8S 807 

Determination of absorbed oxygen................... 760 

ORION: WONORIAE Gs ee ie ie ee ee 762 

INE NONE ce wa na ne kb o a oe 757 

‘Winteler, Determination of mercury............ 00... c cece cece enees 172 

PE GIOH OF PVOTONUOTIC. ACID 22 si ic ce cee cca ee eenewen 581 

Wohl, A., Computations in gas analysis................ cc cece sceeees 782 

Wohler, Determination of carbon in steel... .............. 0s ec eeeees 406 

ET EN ors FU LVS Caer ace ek ota hare 298 

Wolff, O., Determination of antimony.....................0005. eos 227 

Wolfrum, L., Analysis of ferrum reductum...............0.0eceeeeee 612 

Wolter, L., Determination of tungsten in steel... ............ 0.00000. 292 

Woy, Determination of phosphorus... 60s i ee eee 436 

as phosphomolybdiec anhydride..... 440 

Method of precipitating ammonium atone keg Oe cee et 437 

Wynkoop, G., Determination of aluminium. ae SS EER EET 

eZ 

Young, S. W., Standardization of iodine solution..................04. 651 
Z. ‘ 

eek F POCHIIALION Of BUIDNIGES. 0... ee se cece tec ans 158 

Zimmermann, Determination of uraniuM...............00 eee e eee ees 106 

Precipitation of zinc as sulphide....................... 158 

BPOLI Gr CORTIC BOLLE, © ia so cree be ee ie Wg ees 609 

Titration with permanganate................. 604, 605, 609 


Volumetric determination of uranium.................. 621 





INDEX OF SUBJECTS. 





A. 
I os a5 ok vk Hee eo web ek a ha Cen beeen 605 
ay aa nes buss copie os Cees che beaks be 371,583 
I te ge Sh ed eee . 583 
ge as 54 CN Oe Many sb kk aus ec eee b seer 754 
determination in presence of ethylene...................... 821 
IIE sa et kal sg kw hee ued vats 755 
Nad as in Fo Ridin nse coe Rivinahenneaia sorgmMeOe® « 537, 571 
Be I IO Oe as wc ems cues less 838-389 
volumetric determination of. ............ces eee eee 571, 575, 583 
I Ee i iota Se aie sd ApS Shoe Mpehn aopee + 4 4 esd ests 18 
US ee i ees 6 ie bet RE oie pains «0 RS 27,32 
Air, determination of carbon dioxide in......................4-. 397, 593 
Alkali bicarbonates, volumetric determination of..................... 564 
in presence of carbonates........... PEs EN aay es 565 
Alkali carbonates, volumetric determination of...................2... 562 
in presence of bicarbonates....... hats Lil vce oped Bot es 565 
CU ONNN Sores a's oc bse lei oe eas 563 
era ce ie PME gi ee eid gin oe Sura a wieleseead ¢ » « 38 
repnU a Dt MEIN os see nae Weider « « 502 
WOGEOR OS Re. Se et ea. < dni 496, 499 
separation from metals of Group III..................... 107, 147 
WOO: fa, oo s chabca tsicn Sate cee 286 
(caustic), specific gravities of...... Rac 6 03 Oe ame vnehhabeka 840-841 
Waar in CLPIIIIRONON ON ia... 2k oe Se ek lee ean 558 
Alkali hydroxides, volumetric determination of...................... 558 
in presence of carbonates. . 563 
I geo Ps OF oan AEC Wi adie emi etek Es ome 0 oan 376 
PE ee cee ins ee iy CET EES ebm ae ord ibd oe ndeine 537, 558 
Alkaline earths....... OSE bebe a ody eee Bae cea ty 44Gb Se dos, po ee ee a EE | 
separation from the alkalies, and magnesium........... 78 
metals of Group III.......... 107, 147 
MOMENI FST oe vcs os 287 
CW AIICIOE on dso eh oe Ss eek es 79 
Alkaline earth bicarbonates, volumetric determination of.............. 568 
carbonates, volumetric determination of............ 566, 567 


993 


904 INDEX CF SUBJECTS. 


PAGE 

Alkaline earth: lrydroxidee <:. 055 8665 55 S53 oe ee oe 566 
galts, titration Of... oo eas Pita cee el eee 

Alkah sulphides, analysis of 3 5055 on Sl See Cae ee 689 
and hydrogen sulphide .: 3... suc Sg er eee ak ee 691 
sulphydrates, Wiratiot-Ofo So STS aes we ee 689 
Alumina 352655 Sods Fas kA ws eo ae eee 82 
determination in bronzes... 6.55... 04. 6.0. ae ee 236 

GOMER os ek i ek ok co Oe 492 

separation from alkalies and alkaline Oarthie. eee 107 

CHPOUAIUTN «acs Ok Ga ONS RE ee 114 

WOO 6.355 es a sae ees ke 107 

iron and phosphoric acid...... ENS gig ones 1il 

manganese, nickel, cobalt and zinc... .. 149, 152 

metals of Gritb Ls Fae 192, 235 

CHM SS Ose oe ong ee gee 116 

WARE os Seb areak eee ets Oe gee 119 

Ammonia (reagent), necessity for redistilling........................ 82 
preparation free from carbonate................. 149 

SHCCINC SYAVICY OF occ a ka enn Oe eee 841 

tubreitiotr OF: eo} OS Sk ee 560 

Ammobium . 2 sos es i pee ee ee 57 
colorimetric determination........... ik. ek Cw a ae 60 
gas-volumetric determination. ................0c0ceeeeees &22 

in Grinking WAt@P ooo fos se once Cha OE eel a eee 60 

yolumetric dotertaimAhtGn. SS. ns sc ss ee eee eee 560 
Ammonium molybdate, preparation of the reagent.............. 437, 638 
Pb yes oo BEES o 8s Be A ee a ee ee a oo ee 1 
avec. oct s reels SE CA ee Ge ea ee Rie 
gravimetric: 144). ses os wns eas Cea eee er ee eee 1, 38 

Indirect. oo vices ce ck ks Ce ee T pe OR a eee 2 
wolumeteh hh se a he ce eh as HEE ER REE oa ee 2 

Anions, gravimetric determination of. ............. 00. cece eee ence 320 
calculation: of «2.2 Seusieiho 3S GS eg eae 4 ee 737 
Ratimiony: 5 3) ht OE SS i MES ee ee ee eee 218 
determination in bearing metal. ...............0 cc cee eee 252 
elevtroly tie Gebeinma ation «i.e oe ee 224 

removal from: elec, 6 So oe wis 0s ee © ee 227 
separation from arsenic 62 es ea 241 

areeiic wed tet Oe oS oe ree 256 

mercury, lead, etec........ ches eee 235 

metas OF Cron dlls 65 pee ie 235 

selenram ‘and tellurnim. «so. 82555 haa 281 

Ge ee ais Ss See ee ee 248 
vyolumetrie GEtBANIMRON | cr he ea res get ie 685, 687— 
Apparent iron value of wire for standardization,.................. 98, 601 


INDEX OF SUBJECTS. 905 


PAG™ 

Arsenic..... PEM RORY ES ats Sop OLED EN POT os oes at ane Bee Sepa fang etek web EES 205 

colorimetric detemmination aiid. 46054. S935). Ce 208. 

determination as arsine................ Ce Oe wie gel mate Se py ee 214 

in commercial sulphuric acid................... 248. 

mispickel......... MoMA ee cea <b css eM 218 

NS ENG D8 ae ee ie a a a ea 309: 

elocteOlyinG GrertIaRLION | (2s eles... ck es oe 212 

Sormeremen Trot GOtmOUy 655555 ss ee eines 241 

SDF O see ocs eS ROR 256 

MINIIS PUNE ON Ps Eee ea ck a oe Pata oe 235 

mete of Group TL... holeaisesk sais oes. is 235 

EOIN sa eee a a 2 eae 287, 

selenium and tellurium.............. pak Gee 281 

eo es oe ee RR a ee 255 

MTN on... a a he, nh EAR PSL) GLE os kes 26. 

Ny, CONE NE nis ws canals ee a vee «0d BOAR Ee 259: 

I re. <A a oink oe Be bea tt CeO wee Gout 268. 

I SIRRNTIN SOR nas: 7, iw eta atts 6 poi. soa oso QoRa Pos 849 

Ei gS dirs ABS GEER PRRRE ED 7 SOON ID Gree C81 Sy Sale en ae a ae ee 423: 
B. 

nn Ny ea DE ISO Fp eT OA Re aaa a Pe areata 6. 

SE SEND REE ae aS | TT OE ea EE 7 

Baking powders, determination of carbonic acid in................... 377 

Ne ee i el Fhe a pk a Reis 2 CRD aces RBs ROEE 74, 

detection in calcium precipitates... .... 5 et ee bees 495 

determination in silicates and rocks....................--- 496, 507 

separation from calcium and strontium................5....... 79 

WRRROMOTD i's 5 ee. so oie ee ee 79 

_strontium...... AA. Soe a SLs os = 80: 

motais of Gro TF. ,..n.--5-ec200 ie ORNS 107, 147 

Wee Moglais OF (P00 60 ae ar OO ee 

Wonuneires Gelert atin. So ee. co tas Sea ee bos 566 

Barium carbonate method.......... Re eae es oe pda oe Pe peaerh oe 149 

DYGTORMS, OTIS! BOUMION OF is. os oes. ese cet OE 557 

Mawic heetate MChOd os Nl oe eleva inicd- Steaua: - . 152 

Da, ORG BERN OL 62. eo grag arnt sd «ld viol. ¢ + aa. 252 

Benzene, determination in gas mixtures... ...........0.00 000 e ee ceeee 752 

SODATAUON IPONS PUNTO oid 5. hJokeackind «WAS I@Rae so 58 756 

Benzidine hydrochloride, reagent... . 1. cs ceeee eee eee eeee 291, 714, 715 

Bicarbonates, in presence of carbonates. ..............2-2 eee eee eee 592 

ATOR MESA MNS ME ois Serta oS oe bp os bee te Ko we KA 591 

Bichromate, determination of chromium in.......... Pree wn aes 105- 

Pepa a iy ii va ccaeeerbeyigien so hia pi ce vss ee RS 344 


Bismuth........ LET ba NA AL ELE RPT ES SHOE NSU CS Ra eR NEN 179 


906 INDEX OF SUBJECTS. 


PAGE 

Bismuth, determination in bearing metal. ..... 0.0.05... 0550550000005. 252 
separation from arsenic, antimony, and tin................. 235 

copper ahd cadmium). 2. 6 See eee 198 
lena) 20905, PASE, ee ee 195 
MMCPOUrY 6.64 ae. A ee ee 194 
metals of Groups III, IV, and V............ 192 
molybdenum... 250 892 Oa ae ee 287 
selenium and,tellurium..................2. 280 

Bitter-almond wate 22 $3552 556 0 ORE re ee Ss ida oo 337 

Blick, the. ...:.. - 46sec SA Crean We aa eT heey ats yi 261 

Blood, gases from defibrinated «0.05 26.0 52 i oc cee eee 740 

Boric. acid... 0.2 26 doe ei fe el oe Pl We ODD We PUR Ts doen ee ee ne 428 
in mineral watete.-200 050 Re So rea i ee eee 431 

silicates-and enamels oo) 00 es s Ps ew en ace eee 431 
volumetric determination. of... . 6.6650 6660... ieee e eee ee 588. 
Brass, analysis of i. 35. S Feds re hee RAG oe ee ee 193 
Bromine, determination in mineral waters. ............. SNe Ge 3 ae eee 660 
non-eleotrolytes.. I ee ee 325, 329 
soluble bromides: :..5: 54 20s.40 250s coe e ee 659 
gravimetric determination Of ............ 2. cee eee eee eee 329 
separation from chlorine... 0: i... 63.5 ip sks see ee 334 
chlorine and iodine. ............ LUST ENS eae 336 
jodie fa 5 a ee Bae eee 335 
volumetric determination. ...................2004. 655, 660, 709 
Bronses, analysis Of ¢ 0. <0 Fe be oe eee oe SEN 236 
Risrittes. 2. es Fa 1 eee 514 
allowable error: in... 666. VES POR ae ee ee 530 
calibration 08 66s54 a Oe ie to re 527 
Graatimig OF soe ie eo ee AOR Ce eee a es, 527 
onte lOr....\6.e cee aeT eed salle oe We ae des Pe eee 528 
Feadne Of . 5 ..:026ach a cba aceee nent scede'saesun es eee 528 

C, 

Cadre ooo een eo nies 2 hos HEL SORT Bae) TERS Ce cee eee 189 
colorimetric determmation «6665 PO es ok es ok ee oe 189 
electrolytic Getermimapion : oe ee ws ee ek ee tea ee 189 
precipitation a% sulphide «2059 20 So. 5, ea ee 191 
separation from arsenic, antimony, and tin................. 235 

CODD io epee ius Can eh ee rane 200-204 
Lea cS eae Sc. 4a eee 200 
WOTOUTY : re res St oe ee ee 194 
metals of Groups III, IV, and V............ 192 
molyodenuM $500 it 66s ae ee ee 287 
selenium and tellurium.................... 280 


INDEX OF SUBJECTS. 907 


PAGS 
Caleitimn 352 es 2 Sa ae eg BG ee I ore a ky 70 
Cent nemtne MEI ee ee iia ws wei 494 
separation from barium and strontium...................... 79 
Ee, ee CRIMI ee a ee Ee me Dt 76 
ttt eat eee metals of Group TIT.) 2. ee 2 OF, 187 
er On SO Se ee od Ue cee 192 
a at ee er a okt aes 566, 623 
‘chloride, presence of free lime'in.......................00-- 377 
"* precipitates, testing for barium: in... ......0...0..00...00....- 495 
Saree ae ere OF ees oe iG 524 
epee oe eee rer ee ee nt EG: 527 
gas-measuring instruments: .:.. 52... ei elec ewes 743 
MR ee ieee oie eee totee bGaas Hi 522 
Se ee es So Pee ae 524 
Carbon, determination in iron and steel.................. 000 cee ences 398 
nitrogenous organic substances.............. 419 
organic substances. 0... 2. ce eee 414 
dioxide (see Carbonic acid), determination of........ 750, 778, 788 
ta-eleetrorytic chlorine: ec odes ees 808 
Carbonates, in presence of bicarbonate....................0e eee eeee 592 
Waneetrie Oatuetion OF... 3 ee IS ies 591 
ENE NE etn ss eS ee Ce a PS 375 
combaned, cnlewlation- Gf 722 eR Oe AS 736 ° 
OCT A Bs AS CST 397, 593 
Paki pOWaeNs CREP Pee. SS 377 
CRIMONMRR 8 POSES. PER IRS 591 
CMTS oes oe eR 397, 808 
illuminating -gas 2003.5 Sb. 778, 788 
mineral Waters... ESBS . 382, 736 
presence of bicarbonate............... 591 
cyanic and hydrocyanic acids 371 
m0; NO, and NN... 6.65. 806 
EPG, COMMABUOR ORO FE 6 ec as Ch ees Sh ce das wes 738 
determination of.......... eaten Ae = Soni die rn 590 
Carbon monoxide, determination of................204. 762, 779, 781, 789 
qualitative detection in air................0..000: 768 
CCEA, RE OE sie rr I a a ee es 136, 227 
Cations, calculation of those present in water. ..................0-6. : tt 
gravunethe determination of 6 Re es 38 
Ceric oxide, iodimetric titration of 3.5 2s an i EE PE OURS 665 
Cerium, determination in soluble salts... ...... 20... c ccc ee cence eee es 828 
Charcoal, testing the reducing power Of... ......... 00. c eee e eee eens 265 _ 
Chemical factand:: Sie Olek ee ic SR SI TS CPOE 850 
Chiorates; anahye@hoo9 ot or ice Oreos FP TPE ES 460, 633, 669 


Chloric acid, gravimetric determination. ................02 0. eee ee eee 460 


908 INDEX OF SUBJECTS. 


PAGE 
Chloric acid, in the presence of perchloric acid............0000. AS ae 463 
and hydrochloric acids....... 463. 
reduction of hua iipiansikc's poecak 1 ae isa 461 
volumetric determination of. ....,........e.eeecees 633, 669: 
Chlorine, free, determination by titration..............0..0cceeeeeee 809: 
gravimetric determination, 2. 6.0... 0c. bc cece vanes 324 
volumetric determination . . . .oeii-. ¢ oc ns kes oes 654 
gravimetric determination. << ooh gt os vse wae dee 320 

: : ‘ * 
2 BQWUECUS BOUMIONE: | so cu sack nabs ia sk ce 320: 
commercial tin chloride 3 <5 is 5015 Gs ts tees dew ee 321 
mmaolible chores eo ba a i isk ek ee 323. 
vetwenic substances... vis uias Make dual, shane ke 325. 
vanadinite..... Pe erry Cer re emg rr EE 308. 
separation from bromie. . 25s os oes 334 
carhom. dioxide «i544 \ieg cen epee eee ee 397 
chlorate and perchlorate. ...... Lise cee 463 
CVANOMOR 6) fi Siac ceo es pce 339, 711 
GOWN + seks oi Se el ges a 482 
WUING i ci khe Geena oe 331, 335 
iodine and brengine, »\5:i: ks :a< dau sew sos one 336. 
volumetric determination . ... 3.5 sccesisctie tye wid ear de 654, 707, 708. 
Chiocrine gas, examination: of io 6 SA sien Sas Sk os nageiaig 808. 
Chloroplatinates, conversion of chlorides into. ...........-........4-. 43: 
. Ghromates, gravimetric analysis of... 5 6.5 es sedis siren cke ee ee 105. 
WORIMCETIC ANALVONR OE oie iets sw oases 641, 649, 664, 675 

Chromic acid. See chromates. 

OopOUNGR ksi cs eae ee ol ae ee ee ee 102 
(Aercmite, snaiveis:obs jas weiguwiilies + iia ke 509: 
determination of ChromiUiiisisig.c os is ss oe ake oe 675. 
Chromium ; 32...2hc «cates b* Ge enews ooo ee 102 
determination in iron ores and rocks. .................+0:. 310: 
CRROMIEG | 6 8 a ee ee 510, 675 
Oi WOR .5 4... +5 des eabitienten eae oe 312 
ns Shee! ihe naan haeedn 1 eae oe 313 
separation from alkaline earths and magnesium............ 107 
SINR oy sidney sedate a 6 os os 3 ee 114 
Ly SNM Sth sia sy Sak otk hewn d's il ey ee en 113 
_ nickel, cobalt, manganese and zinc............. 149 
volumetric determination of... ......6.0.. scence eee 664, 675: 
Clay, soluble silicic acid iyo ces: ) 55 oe aes peat chine eee 508 
CHowet, Grying i. 25 5 a Se eee ee ee ee 24, 25 
CAA... 5 oe ios Cs adh 2) ad SS Be egeeehieaeny eeapel endian ae ee eae ee 138 
electrolytic determination. 4) 2.540405. <i sea e en cen ee 138: 
separation from alkaline earths and magnesium. ............... 147 
TUG DRI NG 55 is i aie esd ain a ae 16L 


INDEX OF SUBJECTS. 909 


PAGE 
Cobalt, separation frommetals of Group II].................006. 149, 152 
I te ONS Teo i al 232 192 
SR rks iia 5 Pest Bes EERE 161-164 
MURS Da retin s ocd so ook os AP pk Lee 156 
oil for henting erueiales Dy ates; oo a bce oe webs es 32 
Collection and confinement of gases... .......00. 00. cee eee eee e ee eee 730 
Combuntian (elemensany- ARVO) 6 oi ois ce eek. 414 
of organic substances containing halogen ................ 421 
Mit Ss a Ssofea ete fk... 422 
SUIDDNE Co cs yore 422 
i oy hana eee ube vindice 415 
ey Sb da Re hs hes bo Pde eh dee 416 
UM a ka eked kde yeh eee rs Mow r eee oot eee! 764 
ery ON SSK Care Dod vwe weiss overs 765 
nes, OF DreapnmidG 3: 832 ce ee. Le 766 
(c) by method of Winkler-Dennis....................... 794 
Re COMI 5S Si ek s De OE ed ce oo bees Ok oes 766 
Cone for holding crucible on water bath. .................2000eeeeee 31 
Ms ae rs ok ia eS. CEI CS ok ee kc aace reas 182 
analysis of for selenium and tellurium........................ 284 
aotermmation in’ bearing metal... |. 20 bas... eee eke 252 
ee rg rg eet eee TA Ee ere 193 
Re ee PRE. te ce ees 236 
OR tr i dOS oe e8 6 eae wes 673, 683, 725 
pe neerrareaey Ceerermintittt es en 2 a Fu Sa 187 
separation from arsenic, antimony and tin 235 
WN as os eae PO ARIE IEE ess SS 198 
SU se sg gh 8 ee ars Ra ee OS 200 
OR es co ee ak SW ed 198 
OE io a A ois ee sa A *. US 194 
metals of Groups III, IV and V............... 192 
. volumetric determination. 2.0.0... 60005000. ee es 672, 682, 724 
Re Mt a ss oe ok ea OR Aw Coit oe 24, 25 
CRON MIN OD bs ees oon ye KceNe cee dandberrccnecsensat bay ee dawe 36 
SN OP ia 1o wad eo ok eee ecient ews 259 
ON cee eo sci a big we a ne ees 371 
in the presence of carbonic and hydrocyanie acids......... 371 
ey te ae PUN re er ey Own yr wre Pie er epee Umm rey venta 337 
in the presence of halogens. .... 2.1.2.0... 000... s eevee 339, 711 
suinhocyalnlegeuee 6.58535 ELK. 342, 712 
chlorine and sulphocyanogen.............. 713 
volumetric determination): 060. HIROSE ie cl cece cc ee wes 710 

D. 
Pefibritinbee caer, ee TO a rs cre bit reas eterecevevcvbwes 740 


DeciCCROF ee eee PTV ORO PUENTE ERT Teer ec ke eaeeved 23 


gio INDEX OF SUBJECTS. 


PAGE 

Dichromate methods... 50.0550. ELSE i a 

Dimethyl glyoxime, reagent...) 02:52 FIG FH Ge ee 129 

Direct: methods: of analysis. : 5... 5. eS ere ee ee 2 

Distribution cootieient io i it PR ee eee 658 

Double: weighing 2.3.2.2. cena: A ees, GR ee ee eee 9 

Drying apparatuas i555. A Bea ee 416 
Closet. Sagas we a iS As See 24, 25, 33 . 

of precipitates. UO Res ie Bes PT Be oe eae ar aR gS Sg Gs 21 

of substances in currents of gases... . 2... 2... ee cee ee ees 33, 220 

OVONK S220, TATRES SATUS eat ee ee eS ae 21, 24, 25, 28, 33, 34 

E. 

Electric. furnaee .205 eo 6 one i's Fos 006 rete aed lc ee 28 

Electrodes 205 2. sabe eee eke aww p EU ee ee oe 93, 134 

chonmmngy ete). 2 SA Ps ee 136, 227 

Electrolytic determination of antimony.................. Livleinhne male 

BYOONIE 2.005. cs 6 le AO ee 212 

cadmium 2). Siete Fide 189 

e6bal6. 60. 1225035 Bee 138 

coppers aS y Se a 187 

lead 005 6 AOU Reed Ree 177 

MOTCUPY 56 CN SSA ee eee 172 

niekes 230002085 FS ee eee 131 

MIB ose Oa We Says aie a re ee 234 

BWMIC fos. boas SORTS RII ee eee 145 

iron, for standardizing permanganate solution............ 600 

preparation OL... i s6.cc6 oe eS pte et ae 93 

outhib, os cAccaeas » Se EG ss SNE, we ee 132, 178 

Elementary analysis..\.)..:: 0. cae. SRR as 8 SO eee .. 414 

Ethylene, determination of. 28: isi hey cea keer ee 751, 818 

separation from acetylene : wien 6. Flos od in ee ee 821 

heniiene. 35 cs eae ohio 757, 820 
Evaporation of liquids. ©. .i..00.00 0 ve ce ee de een tc euneaw ss ee W sien ee 30 — 

Explosion pipette........... eee eee eee eee eee ener e tenes eee teers 765 
F 

Factors, table of chemical... 6.6.6... 6c eee eee eee eee eee ener eenes 850 
Fahlerz, analysis of... 0600.0. 0s cede cee eect ce tee duncnnnnnne ice 359 

Ferric chloride, solution in ether... ... 1. se ce cece ee eee eee eee eaes , see 

iron, volumetric determination of. ......6+--s+5 99, 681, 697, 699 

Ferric salts, reduction of solutions Of... 2... .ee eee e eter renee eee -,.. 607 

by hydrogen sulphide. .......-64s6+e0eee> . 99, 112 

WHOtRIB 5 ek oo eo ee 608 

stannous chloride. .. 2... 6.0. 6s ee cece eee 609 

sulphurous acid... 2.1... cece eee eee eee 607 


INDEX OF SUBJECTS. QIr 


PAGE 

Ferricyanic acid, gravimetric estimation of.........0.....0000000000- 344 

volumetric estimation Of... 03.0500. 0. 0. cee eee 633, 694 

Ferrocyanic acid, gravimetric estimation...............20..cce0eeee 342 

volumetric estimation 3. fs. ie ei os Been 632 

Ferrous iron, volumetric determination of................ 89, 603, 607, 641 

Ferrum reductumn, aeeenenne fy i wt koe te es ons OLE, 612 

Filters, <a ee ee a ee ie eo Noe 18 

OE Ce i re POE PREECE ER 26 

TE ol a cigs. a De ae gehen As Cree iis al ave a pee Le ee 20 

Filtration and washing precipitates. ........... 0.0.0. cece ccc eececee 18 

Flasks, calibrated, permissible error in..................0..00.0eeeee 524 

ees el beak ae ae a eae Sid a a ee ea ee a Ee 522 

SM a go's Vg 8 PETE a RG ed be ced k Cb Bae 528 

Fluorine, determination as calcium fluoride. ...............02 0.2.0.0. 471 

HY GTORMOGHIO BOE 66S IER AS 476 

POON ONIN 5555 soe a Bees 475, 829 

ir ealemen : fmorise 5-5 533 58 is. Kee 472 

Speen ee Fe A ae eeindee 502 

PE OU ee es ee eet 480 

presence of phosphoric acid............... 474 

meraralaens Fee WO sc ee Pees a PER 482 

SOO ee eee I os ea se ee es 481 

Formaldehyde (formalin) volumetric determination of................ 694 

Formic acid, volumetric determination of................. eee Pe 626 

Fractional combustion of gases...... ee ey hae, oe ore ie 766 

IIE ONCE MEUM ONE G2 15 PEN ek cece See 6 Cine kee dics cans 575 

EE RRTOMEINIP ROI oe eR hs Ue WES 0 6 6 o'dlé cas o's « 577 

I REN 8 hs i he Ae tb Sk eae EUS SMES Oki 28 
G. 

Wee AAO so oe eee de pen va ee eee. iN SRS.) dy... 729 

ORG eG Va te i eK PEE aa uk 775 

SoCeiCa SEU I ee SSO ERE. oh Swed oss 786 

Ghmenomnuataon sinette 3. ee meee se cece er eters 792, 794 

Gases, collection and confinement of. .............026. eee ee eee eee 730 

Remmureremee (Of 15 ee are ee os vb ev ace ee Ce ews aces 742 

Gas measuring instruments, calibration of....................000005 743 

ES Teo a aererereme 2 ert pir Gare ccs rt Ge ME sees 786 

Cyan MINIOGR Se OE he a ee bans COk wees 822 

GRRUEE TEMES: CIBOETOOS oan Ce PR a gee cee 134 

ClETRET UPN, RID OL i see a ee oo Le Sle SERS 775, 783 

Gold; determination mi wONitns 8S SPORE. ck ees 257 

Re a I RSE onc eee oe 263 

precipitation by hydrogen peroxide. ..................5-. ics 258 


SOPRA Ne HONE URGE Fe cls occ ce eee stesces 271 


QgI2 INDEX OF SUBJECTS. 


PAGE 

Gold, separation from selenium and tellurfum..................0.05. 282 

BUREN. 5. ss kbs BELA aCe ee 

Gram-equivalent, definition of. . 2... #0 veel wine Sith sane eee «bas OS 530 

Graphite. in cestmon=. 06.666 deseo lee mihides Le ee 414 

Gravimetric analysis, methods of...........ccce cece eceesveveves .. 2, 38 

H. 

Halogens, determination by indirect analysis.................0e0000- 334 
gravimetric determination in*presence of cyanide .... .i4 kas 339 
separation from cyanide and sulphocyanide................ 342 

hydrofluoric a@idsci i ikon vis. Soba eee 482 
one. ahother so: 5.5 55s. > > caw 331 

Hordnens of water xo os ices cies Men nea nin a ee 568, 569 

Hematite, determination of irom in... 2.2.0.6 cee ee eens 610, 698 

Heats of .conabustion of games. « .)sise cadises Beles «0 cae ee 845 

HOOG Ei seid Vea hae ee PRG MOD is FO ee 30, 31 

Hydriodi« acid « . << x... ssc ee 4 eos ce tk 330 

separation from hydrobromic and hydrochloric acids, 331, 336 

hydrocyanie acide... 0.6 Ns bs ee 339 

volumetric determination .........055.....6 0000+ ceseeeen 709 

Hydrobromic. seid «20.3. Go Ei 5 ss, o.oo. s ae 6p 9 Blac SO ee 329 
determination in mineral waters................... 660 

separation from hydrochloric acid. ................ 334 

and hydriodic acids... .. 336 

hydro¢yanic acid & « «sis. en tee 339 

hydmiodic atid... 2s éissnsks dae 335 

volumetric estimation... oo. ois aks os Nee etewes 655, 659 

Wparooarbona, heavy... as > Se eS 751, 779, 788 
separation of... i... i. ys tees whee eee 756 

Hydrochloric acid . i p:540 5. cant 5 a oe a po ee as ee Sales ten .. 820 
gas; sletermination Of io 565i dl Seis ha ee . 814 

normal aamution Of}. o..6 3c isk ewe pve see eee 549 

separation from chloric and perchloric acid.......... 463 

hydricdic atid... . :sidsees nee veo 335 

hydrobromie aGisi.. fc. ecivvieiwees 334 

hydrogen sulphide. ................ 329 

hydrocyanic acid.............. 339, 711 

and sulphocyanic acids . 713 

sulphocyanic acid............. 342, 713 

volumetric determination................. 571, 707, 708 

Biydrocyanic acid. o:6 13.6028 ss is sas beak Ce a ee ae ee 337 
determination in bitter almond water............... 337 

separation from cyanic and carbonic acids........... 371 

halogen hydride............... 339, 711 


hydrochloric and sulphocyanic acids. . 713 


INDEX OF SUBJECTS. 913 


é PAGE 
Hydrocyanic acid, separation from sulphocyanic acid............. 342, 712 
volumetric determination of.................. 710, 711 

Hydroferricyanic acid... ..........eccevcrecces a ihren 160 oles 344, 633, 694 
ME PORCLEOCYANIO AGI...) asics yoni sek eis Ce ha enue 342, 632 
Hydrofluoric acid............... Oe RED POU ER URS COLA ee Dad SEA 471 
determination as calcium fluoride.................. 471 

hydrofluosilic acid. ............... 476 

MOON NOI oi) ee ae 475, 829 

ini calenun Thorkie. 66 eas eet. 472 

POMGONLG fC o ba’l Masiae ss baa cannes 502 

poimeral Waters. 45s ii ois Se ea 480 

separation from boric acid .....~ 0... csc cece cekecwes 483 

hydrochloric geid... ... 6.06085... . 482 

EDS GREE Pig Py OAR Pere Ltrs aairt 481 

POMDUONIO BOG 6 66 CO i EE 8 ek .. 474 

volumetric determination. .............ceeeeeeeeee 581 

mevarofluosilic ach. 05.2.0 ea Lak Hea Chee d arena Mak oe cae cates 483 
analyst OF Se OARS cos ead = dea be ehaeew 484 

determination as calcium fluoride................. 483 

potassium silicofluoride........... 484 

volumetric determination. .............cecceeeees 581 
PEPOTOECD .. csv ccs uke Lee emmeee eK ints TOE e cee dens «VERA HEH Y 770 
determination in nitrogenous organic substances............ 415 

CUEING HUIDTAMOOR 05 oi oon ok 5 Bee Sens 414 

Hydrogen peroxide, iodimetric titration..................... Retest 680 
methods........ iva ba eerie Deena omer: «a's 826 

titration by permanganate.................20.-- 626 

titanous chloride... 6s. bckdceceesec: 700 

sulphide, colorimetric determination.................0.0-6. 354 

expulsion from insoluble sulphides. ............... 367 

evolution and absorption... .............eeeceeee: 350 

determination in mineral waters............. 349, 688 

ss WAR PORTION i Siig edad s a's ,s 816 
: gravimetric determination... 0.6... ce cece eee ets 347 
MUP NCAINR CI re EG el eae uve bigtersi o'wlg SON 687 

separation from alkali sulphydrate................ 691 

GET DD 5 C chew oie Bienes 691 

Hydrosulphite of sodium. . 6... 2.2.6... sce cece eee eee e ees ee cease 760 
Hydrosulphuric acid (see Hydrogen sulphide). ...................-.--5 347 
Hydroxylamine...... esc see teense ete ce eselaeteecerescceses 581, 631 
Hypochlorous acid... e cece esse eee seer eet e een dene 344, 669, 701 
determination in the presence of chlorine........... 655 

Hypophosphorous acid...... 2.0.6 see eee e cece eee e eee eee eee e ners 372 


separation from phosphorous acid.............. 374 


O14 INDEX OF SUBJECTS. 


I 

PAGE 
Igniting precipitates, method of 2.2. cee cece ccc cece cence 21, 28 
Hiuminatiné waa analynie oe aS 0 Cas a ieee Pe ek 775, 786 
Incandescent mantles, analysis. of... 22.0... ..cccucccccceccceeceeces 512 
PRB COGORI Sos co pies ete lo ue ls neo couhas oto Aw 4 OO ee ee 538 
Talirect, SORIGGes ie oi.) 6 a Sa eae ea kd ew vs abe gene eee 2, 56 
determination of halogens by....................... 334 
SO; content of fuming sulphuric acid. 579 
OPO  JSce EN eh yetho ee i ee 6 
Inquarteons 65 6s Fe sa a is es Sc ee 259 
International atomic weieliteccc aso a yas. ss eos eee 849 
Fortic BIG: sss eisk RR is ee Une ee RE a ee eee 433 
determination in the presence of periodates................ 670 
Todides,. anelyaig of (ase Jadine) <2). 6s os eek cs es see 671 
Forimetyy oss ca es iV Ae SI lo ds Ste Spe kn ae 644 
Iodine, determination by gravimetric methods............. eis cates 330 
ID IBMCKAL WALLET bs i won cedelgsd sss SUR SRS 660 
. nen-electpoly tess 50.555 Soe ais ee 328 
eontitile lodades 62054 Pot ints ee 656 
free, Gtrnlaon Ob lice hii uldil ita 2 cela yeas sae 654 
PrOparation Of PIPE. Ad Gwe e AE ae eee ee ee 646 
separation from romine . 2.566 on ss ads oidetiele tae ee nee ee 335 
bramine:end chlorine : 0). a.602 5s ial eons 336 
COMING oo O32 ede secs ws ee ee 331 
solution, etandardimatian: Ob cic sisid Sis he ecelk oS as eee 649 
yolumetric determination. . 2. 06.5. cack ec ee cide ce caves 654, 709 
lodo-starch reaction, sensitiveness Of. .... 2.2... ccc cece ee ee cece eee 652 
WON ae. ee ey te et dig 2 peek Boe ed ea ee 87 
ClBCHRONV TG So. oa 2 Vee ENS res Wee ee Re eg 93, 603 
determination in bearing metal. .... 0.20.0... ccc cec ese pevesence 252 
DPBNE Sic eae eee ae eee Eee 193 
Terran @eactmt 5 a siz Ged ees a 611 
Herne: 64. te ae deey see oa en 610, 698 
PRESANCE OL OXIME hie sb eee oe 611 
BIMOOLOR es. Scan es ys eee ee LE Pas OPED 493, 502 

. separation from alkaline earths and magnesium................. me Ee 
SITS: SS cepitaiy tick Pan so aso ne seek oe ee 107 
and phosphoric acid........... a Siabstete 111 
COIORAIUIN . Gils gee ete ak on ee 113 
PORNAEDONS og ts ois nko ok as oak Sek eee 153 
nickel, cobalt and zine...... 149, 152, 155 
PIO 655k ve Ge eo Bar mie Hens > 0 166 
metals of Grotin 20 oso ce hae ari ee 192 
TitpMN ccs i GA ONSNS Gas cs sake 114 
UIPOTUNUD 05 Si alsintg ee tae cee eo wk oe bee 119 


INDEX OF SUBJECTS. 915 


PAGE 
Iron, volumetric determination by permanganate dichromate methoed..... 641 
iodimetric method ........... caule ano 684 
CPCIIRABONOKE . nw o.n. oS eisicinenls we 603, 607 
stannous chloride. .................. 697 
SARIN, COIONIIG 5 0655.6 ack cc cb ee 699 
ORG, GOUIDMIGIOR.OF VARAGIUM ID; .. 26 o)6 ie) 6/6 cds ence edede sbacn 310 
and chromium in............... 312 
wire, determination of apparent iron value................... 98, 601 
L. 
Ms ook cig ie dlb ac gf sc das bw ae bees dees s coves 544 
i ei io Wa Fe Nie ek ic baw bl wack vse bdeven 174 
Geuseenaoe sy beeraia metal. i 252 
EI eae add pill cle y bowie nicole calc ee 193 
ROEM occ iia Clee. puagh eablewue beste 236 
WUE er eke ee oN oer See 308 
electrolytic determination as peroxide........................-- 177 
separation from arsenic, antimony, and tin..................... 235 
Bust oie ad See o a, ae ge een, GRO 195 
Ni owas cise i os ee 200: 
DON as Be oa conic tie iaylar ol ected Motere neyain atte aes 198 
NN a ea Nes Nghia a o's 5 Gie: wD ee Meabenee he venask 194 
metals of Groups IT, IV, and V................. 192 
Ea Reg aS poe ae EPA S Mao nl 287 
RG EN eg ooo oon swt xs aiden ed's oan SA ROS A co 675 
sulphate, separation from barium sulphate and silica............ 176. 
volumetric determination by molybdate. ...............0....... 726 
INN ORIEN MIE CS 5 Siena da kn che so Nd anis a iwie's © Sieg 502 
Lime method for halogens in organic substances.................--.-. 329 
Re WA ee i a can dd nbletacie’ eae 
De EIS OE i to ke ee does oe Sa eee ee hpels wig Meee 516, 521 
IE CII Mas wis ag alieetra kde we aie hare oo win Oh a eeoehane ss 264 
MI ea ie a hh uh cen is in hs wince hb Rinse ae v alc wisis cabin aie does 53: 
SPE AAEEA GAOT 10h MONTIEL 5 isla de foc isis nie hw vic need ace cee 502: 
MAN COEUR Mg ca alia aa assim died b oe weal dia a 56. 
separation from sodium and potassium................-.2.-. 53- 
ee Ge rae tle oo on ek eon $a ce hs mae abe 6 Onin bys kien Oe Se 544 
TE NE Os Si ass sr ke oie ee ics pee eee nkercace 854, 855 
M. 
Magnesia mixture, preparation of... 2.2... ee cece sacccvccivcsccccces 206 
TUM Es rete he a ia'n 4s binas waste as oa nina sa bamep nine ars 65 
COLAPMAIG GIT 1 MANGRLOB So oso... nc.c hsb nwienyecc cceeciec 495 
SOOT AUIONT TEOED DERMIS . 's', 5 Sas abo s'c.e'e-d-nis.ccnbee ese 68 


916 INDEX OF SUBJECTS. 


PAGE 

Magnesium, separation from calcium. .............0eeceees Rae ones 
metals of Group Tho. cces ie ea dete eats 192 

Group EE. os os <apeaeees 107, 147 

“i StrOmbie 2 oo ee eae ee ee 78 
Manganese..... Wea le Fs Rega anes he) SLM ae a ee 120 
colorimetric determination. .................. Clieeu seers 127 
determination in bronee) oi Se hn kk Bea eee 237 


iron and steel: | 
by-bismuthate method .............. 616 


Vothard’s meghod. ... o...5.. esos ea 615 

v. Knorre’s method... ............ 620 

Pattinson’s method .............. 642- 

Williams’ method... ..........4 «. 619 

DYFOMGIVE 5-6 os ees ean 624, 663 

separation from alkaline earths and magnesium........... 147 

WO Ss. Ee CO Se et 2 ee ke oe 153 

motals-of Grown TE. ooo ia ee 192 

nickel and: cobelt()) i i.fs sO eee 161 

trivalent metals: 636. eee 149, 155 

SMU i Src a eee a oe va he aes 156 

volumetric determination.............. 612, 616, 619, 620, 663 

Measuring Gaska 5303s SURG 4 0s a ree en ce ree ms ms is 

CHUDERINR SUG a Coa wc! vee CR Ce eee ee 522 

insstyrimmente assis bs 5 CGS 5 4 oe os wk sone Vales obs ee ae 514 

{OP GAs ONAN Wi 6s 4. 4S GaAs a eC eee 743 

Melt, removal: from the eruieiiile | F520) set eh aes cei epee 488 

Meniscus Garrectaomia ie fies SUN LS a oe cea eae 745 

Mearoury 22s 55.55 SAR Ee Ses dtd ea tate le te a (oro wl ea er oe 168 

determination in organic substances. ................-0-000- 170 
electrolytic: debermimatiOn so... eo oes cea bees ewes 172 - 

purificatiqn oF) s Pos bbe c is chee as ber Vee eter ee wanes 747 

separation from arsenic, antimony and tin................5. 235 

lead, bismuth, copper, and cadmium......... 194 

metals of Groups II, IV, and V,............ 192 

selenium and re Pee ae date oath 281 

Metallic iron, determirmation in presence of oxide. .................05- 611 
Metalloids, gravimetric determination of... 2.2.0.6... 0c cece ee eee eee 320 
Metaphosphoric acid:.::3 scci¢ cade bi iipea eae ad teens es ieee nee 433. 

Methane noose tc kien peewee “eee Ee oe ec toe «10 plank emi 774 
determination in gas mixtures..............0..00- 780, 781, 790 

separation from: hydrogen; 3.5 decaecn ees wee we ches eee 790 

carbon monoxide and hydrogen.......... .. 781- 

Methods, gravimetric....... ULSAN Vue La etree oy a aura ciate ee sina 2 

VORMIICETEC 2) SRE 555 EEE eRe aes 514 

DA sthiyl Orange’. oi. 3 450045 8 en eee ee ek cae ee 539 


INDEX OF SUBJECTS. O17 


fers PAGE 
CUNO eee ich ohn war dew ads bdawaegn 543 
Minium (red lead), volumetric determination of................. 623, 675 
Mispickel, determination of arsenic in............... eee cece eee ees 218 
Mol, definition ofc 60 snes ie et ee es Rote codes Sawa ueate ae le 455 
PUDISTOO OI eK ahi oek sha s wc heveantadedytecsaeas eed 284, 666 
Muteremnaiaen in BLCEL 5s ee Se i Pe RE 313 
residues, recovery of molybdenum from...........: oe er Mee 
peparation from the alkalies... 22:22... 0026.00.000.4.. 286 
the alkaline earths. >. .........2.... Fania eee 
the metals of Groups II and III......... 287 
DNCRDNOTIG BING Fo iy ete 288 
TMU Se a pesca ee bate V i'n ed a medica Be 293 
we WENO Giclee tie ak) aGo aos 308, 667 
Molybdic acid, gravimetric determination.....................00004. 284 
Vourmmetric determination. . 2.6255... ec eee, 666 
So RR ee Oar i Da het sen Pa ego Samara Oy pers Raee pare base ogee 8 
Monazite, determination of thorium in; ...... 5.2020 ee ee 510 
ere rm a ee. Suet bei Pe ee ee 27 
N 

MON te ede g Fh a CW ERO wie Seis BU e cw s'ck lew dak 129 
IIIT Ain OI os Sas Se OTR Oo Se ce Rd wha 193 
MON pasha or ele ce OES. SS oiyig 166, 313, 723 
Gleeleoly tic GebenIMINALIOR 653. ea a 131, 136 
separation from alkaline earths and magnesium................ 147 
i Pag ie RSD Hees 2 An re gS ean 161, 162, 163, 164 
MO acer be. pea ee ei eres Sele eae 166 
, aluminium, titanium, and uranium...... . 149 

, aluminium, chromium, titanium, and 
gt DOD aE Gap kB Srpparare GREER grist haat bs 6le ie BEE DR 149 
ls RIB aves ite Pa Tee eee CN RIG. ai 161, 165 
PR Gases ee eres PES CEO aE wale 156, 165 
WEIS CTI oc acc ect eo he Pk a etdes bets ae 720 
Nickel-chromium alloy for crucible triangles..................0...... 29 
Niobium, determination in wolframite......... ee Reb attei em lath etate bles te ook 297 
Nitre, testing the oxidizing power of .: 0. 2.02. ee ee eee. 266 
NU ES Da es eR EIN co Se Cenk RA cis lnie wep VS ik eke Ole 451 
CLAUORIMRI GHD: BB AMKOOMIA EEN ois ove cas bees 453 
MINI OMB a is oe ce cee sore wuss 456, 825 
MEPOWEN PONLORICS. cs. ee os o's tie haw eae 453 
WUTOH Mirae. 2... SPO Re. Bonin 451 
. mr droning water...c5 2. bion Selo 460 
normal solution of.... : PeMAHUS Ne Te DECAY seus > suas Oe ee 552 
volumetric determination. ................0eeeeeecee 571, 634 


918 INDEX OF SUBJECTS. 


PAGE 

Witri¢. oxide... seis GcVUe echo eeeeb ine SERV EROS Pevaee pe ee 802 
separation from nitrous oxide. ..............c.cscse cece 804 

atid niftrogen 5 ee es 805 

nitrogen, and carbon dioxide. 806 

QUUOB Se ne 5 Oo Sed RL O EP LIE EUDICT Fe PAID ES IS Sie Pe PRR 451 
Nitrous acid, colorimetric determination... ..............0. 0.02 eee ee 344 
determination ae tithe oxide. 286i i a a 825 

volumetric determination: 2... @ oie ee ee ens 626 

OXIDE A e S OL a eS a ee ee 800 
determination in the presence of nitric oxide............. 804 


and nitrogen... 805 
nitrogen, and 
carbon dioxide 806 


Nitrogen, determination by Dumas method.......................5. 422 
Kjeldahl method ooo oes 3. Cai eee 62 
in organic substances............... au mtahes 422 
properties and method of preparation...................... 806 
separation from nitrous and nitric oxides................... 805 
oxide and carbon dioxide. . 806 
Nitrophenol as imdicator -...:. 5 is 6a se ae Oeleens ee eee 543 
Normal solintions. 3.62 ose eek 5 oa ee ee eee 530, 548 
of arvana hydroxide 20. a is Cer eee ee oe eee 557 
hydrochloric ated. (3 ACS ee eee 549 
nitric and ‘sulphuric acids; 6.3 20 05:2 SS eee 552 
oxalic aed SSS ses LRAT ee 552 
sodium hydrewee |e See Fs ee 553 
iCimitatiOn, Gy oS caste wey COs sb ea ew en cree 532 
standardization in acidimetry and alkalimetry...... . 548 
volume aud temmperatwre . 6. a ee ee 516 

O. 
Oil, removal from DOERR. © 6 a0) Gh: easel eee eee 236 
Oleum, analyaia OF... 2 5 seca ew oo othe ook eee ee 575 
Qperations.. oe a ess FPO rte te ns mere ak Gyat any Corer tor 6 
Organic acids; titration of 0. o.6. 3. ee ae a ee ae 583 
substances, determination of carbon in.................. 414, 419 
CUGRE EEE oe 66S Saas 325, 329 
hyarowen i cases 414, 419 
nibratem Meo; occas nthe es 62, 422 
WUNIOMIE SM eo oss. hai aka cee 370 
Orthoclase, Analysis OF oo 5 ESSE Se ieee ee pee eee 491 
Orthophosphore Bid | 64 es ee ee ae ee a eS oy a ene 434 
volumetric estimation of, 2.502.020. 6 a 718 
(yen for Arye os So eine a Fh ce ee 24, 25, 28, 33, 34, 220 


INDEX OF SUBJECTS. | 919 


PAGE 
AMIE reer re ASTI Pe os Ch eee sc nde ek cc ccunecebs 427 
WORIUAT GONMION OL > oo Foi a a a ec o's 552 
volumetric determination of... 2... ccc ewes 622 
CUKaG ONION WIC ie ee ie ee eee ee le ed ase 596 
SPI i ed eRe S Eee ede a cok bed ck ee bea ooo 757 
determination in illuminating gas................... ... @79, 789 
presence oF iyydrogen in: 2: se i oc i clics eee es 417 
Ozone, determination Of. ... 266 .6scccsccceccceeee ee Lest Lae 676 
Fr 

Partition, law of (see law of distribution). 
SME UENO oe rere EES eek cece ccd ceacaticese 33, 34 
MUNRO MUR VIB OE a os ee ca Cob ewes ce vadaceteebe 628 
UND ON eis es fs ec eG kee cee ceeded oh eek s chdeteee dene. 462 
determination in the presence of chloric acid........... 463 
hydrochloric acid. ..... 463 
MP OLMNY OE ee Tut waves se ees etc eka ee 51 
ORONO es ar ee ees oe eee he hhc bey 6 teh sid ane pea se meogees 230 
Periodic acid (and periodates)...............2.2008- BSB ey ys ay 670 
Permanence of ammoniacal copper solution..................2...005. 756 
permanganate solutions........................- 90, 603 
sodium thiosulphate solution. ........................ 649 
oxalic acid solutions.................; Te ae pr tea ed te 599 
OrreeMROALe MOLROES: .2 2. i ee ee ec: 596 
solution, permanence of..................2-eeeeeee 90, 603 
POLOEM BNO O55 28s cruia Sieleines \ebw hech sto + ntee 596 
standardization of. 2... .00.....0. 0600. 91, 597, 827 
Bee BOOP EE Gots oa ah) 0 SD aE PEA ta ay | Mepis a 603 
Tema, GRAIL VUE aoa ep dete re Se ee ae baa eee. 627, 680, 661 
Persulphuric acid (and persulphates), analysis by permanganate....... 629 
potassium hydroxide... 595 
titanous chloride... .. 701 
PHENOL, VOWMmMetriC -CstIMAGIOR Of... eee shige ceca eee 695 
BOOTING a 3a ane gc ei aah lete a 'oele  bieieie a whaiel b's ole Mass 545, 554 
POCO SAME Sy ie heer eh oe Hess Oe eepuleok coe ceu bank shee 434 
: determination in calcium phosphate.................. 720 
WRAL Waker 68 Ei A se 447 
EAN AAO Ey artes iy Wer ease 447 
WORN ae eu hie hewae Seek eek 309 
separation from alkaline earths and alkalies........... 449 
CHTOMIC- SOI fice ce ee beenesiaa es 499 
iron and aluminium................. 111 
metals of Groups I, IJ, and III....... 448 


EM TUMATIRRIR oo ch 3 crur chasansahah tea? Shane waa es 307 


920 + INDEX OF SUBJECTS. 


: : PAGE 
Phosphoric acid, volumetric determination. ..............05. 587, 588, 718 
FRORDROFOUS AUN sy oo, cas cre ca WR he wk EA CEA EURG Sr eee ee 374 

determination in the presence of hypophosphorous acid. 374 

Phosphorus, determination in bronze................00ceec bes 238, 239 

iron and steel......... 440, 443, 445, 588, 637 

organic substances. ...4. sedis dei cece 448 

Pinettes . oss aiase Ce MSR ORa UN Leelee ae eee) y Pe ea 514 

calibration Of 03 00k suc acu eee Geter cee eh es Cec eeeT se a 524 

permissible QrTOR GB i Visas Bust oe eee eee te bee x een eee 526 

Platinum 2 oo oc viene a pee 54. ces eR ER Ed cuss Vel ceCaeeeomes 268 

action of ferric chloride on: ........s cede ccceces hie Peon 110 

analysis of commercial platinum..............ccc cece eeces 272 

brags cone lined with»... +. occa eens 4co SOURS 31 

capillary for use in gas analysis... ........cscceeceaece ‘743, 766 

determination: In Alloys oi..6.5)4 is. 64.cn eee mass oe aaa are 270 

separation from gold and silver. ..............045- Sie 270, 271 

Poteet 2.) 5.05 bebe caidas co ebue ae Cs aee CaooNe eas ia curren 38 

determination in mineral water. ........0.ccccseececcceces 50 

indireet Geterminalion <6 635°. sess coho «he 9 Ceo ean een 56 

separation from lithium . 0). .!c0.6 5 cs eee AER Ra eee 53 

BOGHUID 3)o5 Gace 6 pale ELEC REDO RR ET 43, 49, 50 

Potassium bichromate, determination of chromium in................. 105 

potassium M 46d. eee ee es 40, 41 

bilodate SOW, . oa Sale CREWE Re as 0 deb anie hen Gb Coen 647 

dichfomate BOMMMOR Oasis oa dea wavs ose 4 eee RRR 532, 641, 649 

percarbonate, ANALYSIS. OF 62 9:0/035'6 058 Sass ob hele ole hb else oe 628 

permanganate solution. .....:.....0..ceeeeeee 90, 531, 597, 827 

Perswipnabe, MMR MI is oa Al be wi eis Sieg ble bs ey Ca eee 629 

Precipitates, drying And igniting OF 6.0. ys 0 hobs Son ee ceeeseweeenes 21 

filtration ‘and washing Of. ooo 6, <6 chee cnchnehkan ee ebanere 18 

method of igniting when wet. ........ccccccccceececeees 28 

Precipitation analyses, :(vOUMNOWIC). 4. sa sis bv ceeleecipasacansemaaaae 702 

Preparation cf the substance for analysis. ............eeseseceseeees 35 

Primary OxX0d08 0). 5s epistele thas baie ne ike ee a eee ae Wee teu ae ae 605 

Producer gas, analysis of........ in F MENA a ea nha wee ae 775, 783, 784 

Protoxides, separation from the sesquioxides...........,.eeeeeeeeeeee 149 

Prussian blue, analysis: OF fo ilcois 54 wis e's wb ere ois cis be hw ela aks a 06 wale aa 343 
Prussic acid (see Hydrocyanic acid). 

Pyridine bases; titration: of 554. s deeb Gan ws Woieical es eevehisne eee ene 561 

Pyrite, determination of sulin tt fv i5 seg’ 3's noe o's ove tk va den «yee ee 362 

Pyrogallol solution; preparatgon OF 5656665 044 G's 95d Sp ean eee ca ee cls 758 

Pyrolusite, analysin Oly oo 5. 35 ova tae ok bd baw > cos cite Meemieeraeeo 624, 663 

Q. 
Quartatian .) 5 FSP ESF ee a a ee eo he ee eee 259 


INDEX OF SUBJECTS. g21 


R. 
PAGE 
TEOMA OU en es ee ia cals OW Ghia aa (ok 05a Cacia eae 35 
FOG GGG CUMIN COIR OE odin sv ois a on occ wenpihe sds ostes 623, 676 
Reduction methods of volumetric analysis...................++------ 697 
ir Re Ie eae Visi Lists Ug ds) biaib: wleta's oad mes 607 
MOMMIES sg ao bib a telbie ev d dle ees 13 
PigAbeMiY Tle Oe NE a ek Pa Raha hy ok oe nes 544 
Rutile, determination of titanium in....... Cia ub Aes OR OTK Oe besoin 118 
Ss. 
Salting out method for precipitating zinc............. eres aku wabena ge 160 
I tee ee oe ee eae eae es os 277, 374 
determination in crude. COpper. os. cies ee le cece ees 284 
peneraton troin fold anid milver:.. oo. ob oa ieee ei ee 282 
metals of Groups IT, III, IV, and V..... 280, 281 
TRIN ee yes reek ee ble veatar 279, 282 
ce NEG SAE FORESE PRICES fel to eee ot 7a 2p aT ke NEGF Ae ea 277, 374 
Sensitiveness, or sensibility, of the balance..................2200000- 7 
mune OF CorV TEES Oe tire rce ee ee 82 
genaravion trom the protoxides.. .....0. cle eee 149 
eee WeArAtION, TrOIM CUNRSUEN Ce ke ee dv ca ceee ceeesy ews 302 
MEME IRIE GUS ee ee Sieh aie alco wate eo 4 487 
RRUe ENCE TOR PERCEAPATIENE CPUCIDICR; 6.655 ss eos eves vb enc ede eile we e's 29 
IT ME Bat a gtai eo sigalg oe 2 ots o's) po Gato ween ewe oo 491 
decomposable by acids..... Pane eed Orit nee men, Acer CMe acs, 485 
Geterummation of alkalies in... 265252000002 .o eo 496 
PORVOO NON SRE ty Sa cine ys a Sep anes 6s 502 
WRIT Og Ot es Soc oar ck ek ees 484, 512 
ast enctia Oe adie 83 is cise le eee 3 wile 491 
Silicon, determination in iron and steel.............cee cece eee 441, 442 
We OTIS OF GINO he cubec eis > soe ose o cste 513 
oe RE TEE SES BUR agi eM SS Ss A Shc da LCL el ay ie RR 317 
CRN EDDED RP ATO U Seiten) re ES oo etn ees ai 259, 706 
SY) ESE RAT Sana sn pe Anan 268 
BEDRIAGON, IFOMN OGRE INCUAIN isso cass wis cua be erieavensceavs 318 
selenium and tellurium. ..........-.-se0eceee: 282 
WOMINAEIIAG CCCEPOMINA LIOR S88 6 ees ee cake se recess 702, 705 
VO CORO, MINED CN cass a re ca hes duis cae cinady seek eeeuee 317 
Pd er te St eres Wows eed ob co owe sistent eo none baled 43 
MNT IONT S51 EONS 85 SG igs 6 ood oe Wine Gia NO o's evar eae 6 496 
SU LUT IOEROTL CE so 21Ge ss ss ok b ceshis nike e's 6 d's waelale's 5 56 
SPUN SEOTTE TNGTERRDIEN 595 oars es aS alae 6 cess alsin wees ins o's 53 


Q22 INDEX OF SUBJECTS. 


PAGE 
Sodium hydroxide, determination in commercial caustic soda.......... 558 
caustic soda solution............. 558 
Mégihal solution Of. oo. cs a ee 553 
preparation of a solution free from cnlbcnalen ae ae 555 
sulphide, reagent, preparation of...............0.c0ecececece 225 
succinate method of separation.0. 5.0... ok ks. cw aueidcan wees 155 
thiosulphate solution, normal solution of..................... 645 
PUTMAN Oso 2 ik ks.) on es See ee 649 
Solubility prdduct, definition of 3.2.6). osc ok ee ook ccc aed 156 
Solution of sulphides, explanation of the process...................0. 157 
Specific gravity tables of acid and alkali........................ 838-841 
Stannic chloride, analysis of................... BA a ee ee 321, 573 
Starch solution... 06 bras eta SESS. a ea en ee ae ae 652 
Statiehl moment:of a balance. =. 2)... WAS A 6 Sa eee 8 
Steel, determination of carbon in... 2.050) ecs o ainc ss © oes eae 398 
manganese In............-+ 615, 616, 619, 620, 642 
CRIN 5 60. 0 utes ye RE ne he I 166, 723 
nickel, manganese, chromium, and vanadium.... 313 
PROGHUOTUS 6 oe ee ee 440, 443, 445, 588, 637 
Stibnite, determination of antimony in.............. 0... cee eee ee eee 686 
Siroak-of pokd aiaye. 2.0. ee oe ae eta 261 
HOMER oh cis Baie eie 3S ORES sy eta eS wlee sseok ble aaa wens eee ree 72 
separation from barwim .-. 0665 ss. eee os bse oe 80 

ORMOND So 2 obs ok cee hn we pe fi ee 
MASNOSUNR 2.5 os eee eee eae ee 78 
metals of Group Il... 05 0s 5545 ets eee 192 
motels ot Group TIL i 5 6 Fines 107, 147 
Substitution, weighing BY 29600. Pes ns i ss ey eee 9 
Subpho-achde 5k Sa es SE SS eas wb eek eo wi hla depen 205 
separation from one another. ,............00esceceeecee 241 
Sulpho-bawee 635 Nets es es eet cers a mol Siaiaine se 168 
separation from one another... ...........cces eee ceeees 194 
Sulphides, determination in the presence of sulphates. ................ 470 
theory of their solubility in acid. ............0..0.. 020 eee 157 
tatratien OF ot is ESSA Ca os ee ee 689 
Bulphocyanid aod Ce os ae os ae oy ee oy Se 339, 712 
separation from halogen hydrides.................. 342 
hydrocyanic and hydrochloric acids.. 713 
Sulphydrates, analysis O81. 5 ie cs ai ete ae 689, 691 
Sulphur, colorimetric determination... ............00.00 eee e eee ee 354 
determination in insoluble sulphides............... 357, 367, 368 
Won aNd sbeels skews ea ee 352, 354, 364, 365 
mineral Wars. 6... fica oe see os eee 688 
organic substances. ............. oot aca 370 
SIVTUGE.. 655 5 wk KO Ss we ee 357, 362, 716 


INDEX OF SUBJECTS. 923 


PAGE 

Sulphur, determination in rocks. ...............0..4. Ree EU ogre re 505 

sulphides soluble in acids.................. 350 

Wane er oe eae, 349 

dioxide, gravimetric determination...................0.-... 373 

volumetric determination. ................ 587, 692, $15 

removal from precipitated sulphides............... 169, 180, 223 

volumetric determination in gas mixtures............... 687, 816 
Sulphuretted hydrogen (see Hydrogen sulphide and Sulphur). 

ee pe ek a ea eR Eee S, See dee ee aa ag ena net 464 

determination of arsenic in................... 2c eee eee 248 

in the presence of soluble sulphides. ...... 470 

preparation of concentrated acid of definite strength. .... 580 

ME ESI OE 65. o oe ee 6 aici ace siebia alte 551 

volumetric determination of............. 571, 577, 714, 716 

Sulphurous acid (see Sulphur dioxide)................... 0000 c ese eeee 373 

volumetric determination. ..................005. 587, 692 

Swings, weighing by the method of. .:........ 0... 0. cee cece cece eens 10 

{5 

Tene oe. AEE pile cou ees 517, 519, 520, 522, 533, 534, 535, 838-857 

Tantalum, determination of wolframite.....................e00c00-- 297 

Mr er ad ee tee GAA SC ye a + ok Cl Ge ta bw Saw ae 9 

Sa MRTG MIRE te ae Se icinin oa & wale eitlannre bap bes 685 

NN ne sy tlie SGA GA OS OW ee ReaD 433 

MII pe Oo cae cies foc a ie ss Saw aa wa Cheese la 94 

Men tne fate ler) ees a eek uw on oe 664 

Tellurium, determination in crude copper...............02.--200-00- 284 

precipitation with sulphurous acid..................2.00- 279 

separation from antimony, tin, and arsenic................ 281 

copper, bismuth, and cadmium............. 280 

WE AI MLW odie in nye ie imesh eke 282 

MORON er ee oh aS alee ie eek ohm ees 281 

PURI OL CIPD) EE rs oe Sees ce oon 280 

Groupe EET, FV; and Voc cco. 6 eo: 279 

URN or ark. sc cane cas pel dia sae ace 282 

CIPS NCR a od Siege Sw bb ese cake 664 

SPOUSES BENE HOO B OUINTUID Acces Po, Sse date dae Sle eck 374 

‘Temperature, taken as normal in volumetric work.................... 516 

OMAN OE WURERAB Oe Or Aoi ne S ley Sc wed eww a ae aoe aves 15 

SPetralareie, AemrRM GE co Pr Ul eA ci ch tick Oba ob wetes ha hpoieie re = 359 

“T PuA UPUREA Ce RSME ORE coos piv ws on wince ee wees Di etd Wie lela Ri 318 


‘ Thiocyanic acid (see Sulphocyanic acid). 





* The tables in this book can be purchased separately in flexible cloth binding. Price, 
thirty-five cents. ; 


924 INDEX OF SUBJECTS. 


é PAGE 
Thiosulphuric acid (thiosulphate)..........66. 0.0.03 oe eb dak 450, 645 
determination in presence of sulphide.............. 691 
Thorium, determination in monazite......... 2.0... 2. cece cece eee: 510 
BA 6 ee a ee FEE a Re 228 
determination in bearing metal ji 5.050. oo. SP a ee 252 
broOnee Se eee. eae FES is 236 
tin chiovide: = fa aes Pos Fee ee 321, 573 
separation from alkaline earths and magnesium............:..... 235 
GNEOODY 5 5A ee AN os laa k eee ee 248, 256 
SEROTUO 6505 S45 SRE Oe es Ce ee ee Giana 255 
mercury, lead, bismuth, copper, and cadmium..... 235 
metals of Group LH eaoiiees oo eae ae ee 235 
phosphorite acid... x07¢ srr Ee 238, 239 
silicic acid: ee bee eee ee PGR eae 298 
tungsten: cs (See ee 2. Ee 297, 300 
Titanium 3 ois sk Sia eS OG SO ee eee eee Rp et. 100 
Titanium, colorimetric determination. «. 0.0.2... 0 ccc ee dee eee cee eeee 100. 
determination in rocks. 3.3.6.6) 3 cb eaten nC ie eae 504 
rutile and iron ores. .e06 iS ee 118 
separation from alkaline earths... 2. 0.00366... Sb eee os 107 
ahaminngn 6335 6540556 3 oo ate ee eee 116 
WOODS 6.5 c ik ook ead > es ele Ba aha ele ee 114 
manganese, nickel, cobalt and zinc. ..... 149, 152 
Titanous chloride, as reagent in volumetric analysis.................. 700 
Total earbon in: rom Bid ateel | os oe ca ia is wise wie id oes bo ee 399 
Tungsten «sso 5.15 Seale lo oo ews Wb oo ss et gk eae 288 
determination iii: SteRI ee Oi 5 5 ecw 94 ee ge eee 291 
WOHTSINIGC. isi os cok se ee aoa Oe eee 296 
separation from molybdentm .. o.oo se hee pe ee 293 
: CGR oS SASS do he cite al oles Se 302 
121. Paar boar Ate Mads gh Sprabr va AE on aba et eS: 297, 300 
Tungsten bronzes, analysis Ol. 6 545fh6 cies Cs Fee as ashe ono wi See Caer ee 298 
: | U. 
Uranium..... ‘na ieidte Sp ERED OR REED SL ON FOUL haces oh ee 106 
separation from alkaline earths and magnesium.............. 107 
ites aluminium and tron... 2055.6. ce) se sh ae 119 
petals of Group Tis, oi ove Co es 192, 235 
nickel, cobalt, manganese and zinc........... 149° 
volumetric determination. ...... 6 05..400.09. «envi ve pe 621 
; : V. 
Vacuo,; reduetion: of weighings t0..:0 6. 0.0 cs cvve's cn cone couse cuaseun 13 
Valve, Bareen 5 is ee ee ee eee eae 87, 98, 601 
COntaiGGae) Fs a Fae ae eA he gee 304, 602 


Vanadic acid (see Vanadium). 


INDEX OF SUBJECTS. 925 


PAGE 

RGM GRIN ie Oy Pe ear ie ed hla Ge Sars hoe) 4 00:6 vas 6 bias 303 

determination in ores and rocks... .'.......s0ccsccsccceces 310 
Oa TRESS. Shichi E> VEG Pai ay OR ee er gre Rae 312 
| ET ioe tb eas A aA Nie aes eae es 313 
vanadinite.......... aad SES bays viv teers 309 
separation from arsenic..........+.eesee Fis AGU ae bane — 806 
molybdenum............ weecaen 308, 313, 667 
TOMIEIS BOIG, 6.5450 '9% 0:5 606 40S G6 tea ore 307 
volumetric determination. ..........ccececcssscsesess 636, 665 

Vanadinite, atalyais GF... .. . 6. 5k ce eh ce vee AEM eh a gar 308 

WOIOEE, COUMPIIIIRIIE TI OCS i wa ccc eee beesesteercceves 831 

We ss ie vic es aed bo uee Rep edce bys Cee beet see 516 

mR tic Ni ela nine ok CN Boe ea ee 6 UO ON ees 1, 514 

W. 
I IE ee phe eos ss be Poh es a eee ve VN ad wh ee wee 31 
Water, density at different temperatures... .......... cece cece eee ees 517 
determination of absorbed oxygen in..............00+eeee eee 760 
ee MRO onic 5s) vce die We olsle ce dees 484 
MN ee ig Phi AAs 4 0 0-35 bios we ee 502 
UREN rl a Fee ok ati ale'e'b4 SAG Sine boomed 512 
MII gion EG greets ccs Gch li sab bain 8 4440.0 WA OS 569, 570 
ee ee VLEs oo eda e dp be ewan 842 
WMI TE he eae canes tubcvcscbes et uae Uae 18 
ae a aie rk ak ch woe oft Ueis'ed se gh a bcc pease s 6 
Ne eae vaa a pan oi Coed ab bole eee ec 9 
Re NE ri ee ee ee hs bad mak bbe 9 
MMe a Cee eae g 8 be yk DE Bo 658 a wale 10 
I PU co. le pa ees apse veoh oe kM c 6 13 

EP I es ities eh ee sok EU aisles Ce Ua be eo bees y 15 

Wet precipitates, method of igniting. ............. cece eee e eee cee 28 

Wrens Serer, LI OF Re ink cv ce beh aes snlnie cess 512 

WY EINE ie eles ee ewe aS EPR e's VR ec eb ES eos 379 

wolframite (Wolfram) analysis of. .......... cece cceececececes: 296 
Z. 

PMS a ee ee eligi cae och ee « 6 pone kw wieibe De > ewes. 0s 140 
CRBs MOOTIRUI ie rea oaks Wh ohne aby on wioteawt 145 
separation from alkaline earths and magnesium.................. 147 

Timeunae OL CAND Be ora ss Syne sees os 192, 235 

nickel cobalt, and manganese... ..... .......0.2e0000: 156 

trivalent metals of Group III.................. 149-155 
determination in Dearing 10Gbal es. oo os oc ivese Sve gwen Ssietae o's 252 
WN ess trae key ane eas won See ee ae eee ees 193 

NI reer er ae ae a wha ia Sinie d bigae ne a Dies CORE 237 


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